4.4: Diatoms - Biology

4.4: Diatoms - Biology

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Diatoms, Phylum Bacillariophyta

Diatoms are another photosynthetic lineage of heterokonts that was derived from the secondary endosymbiosis of the red alga. Diatoms are an incredibly diverse group of unicellular organisms containing anywhere from 20,000 to 2 million species. These organisms are unicellular and surrounded by a frustule, a silica shell made from two distinct valves that enclose the plasma membrane. Frustules are amazingly intricate, covered with small pores in an arrangement specially adapted for capturing sunlight (Figure (PageIndex{1})). They have golden chloroplasts due to the carotenoid pigment fucoxanthin (Figure (PageIndex{2})).

Figure (PageIndex{1}): A diatom filled with golden organelles (chloroplasts). Photo by Vicente Franch Meneu, CC-BY-NC.

Figure (PageIndex{2}): An Isthmia nervosa frustule showing the intricate pattern of pores. There appear to be multiple layers with a different pattern of pores to each. Photos by Lama Mark Webber, CC-BY-NC.


We are still trying to figure out how to determine what a diatom "species" is and, so far, they have been classified based on the morphology of their frustules. Using this classification, historically there were two major groups of diatoms: centric (have radial symmetry) and pennate (have bilateral symmetry). These classifications have improved and increased in complexity, so here we will cover just the broad strokes. For a more in-depth look at current diatom morphological classification and fantastic images, check out this great website.

Figure (PageIndex{3}): These images show centric diatoms. You can draw several lines of symmetry through each of these organisms. Centric is still a morphological description used for diatom genera. First: Triceratium, photo by Ryan Watson, CC-BY. Second: Arachnoidiscus ehrenbergii found on Ulva, photo by Randall, CC0.

Figure (PageIndex{4}): These images show a variety of "pennate" diatoms. This has historically classified many varieties of bilaterally symmetric diatoms which have since been further divided into better-described groups (some even asymmetrical). When considered three-dimensionally, only one line of symmetry can be drawn through this type of diatom. First: Gomphonema acuminatum, scale bar = 10 µm. Photo by Enviroethan, CC-BY-NC. Second: Surirella undulata, photo by Lila_137, CC-BY-NC.

Figure (PageIndex{5}): Many diatoms live in colonies, where unicellular diatoms adhere together to make a more complex structure. This may make it easier to float in the water column (raft-formation) or make it more difficult to be engulfed by predators The examples above show colonies of diatoms in a variety of shapes. Left: Asterionella formosa, a bilaterally symmetric diatom, forming a star-shaped colony. Photo by Mindy Morales, CC-BY-NC. Right: Chaetoceros debilis, a centric diatom forming a spiral-shaped colony. Photo by Sarka Martinez, CC-BY-NC.


In addition to morphology, diatoms can also be classified by where they occur. Free-floating diatoms are planktonic. Diatoms attached to other organisms (like giant kelp) are epiphytic. Benthic diatoms tend to dwell toward the bottom of a body of water.

Figure (PageIndex{6}): This image shows many diatoms, though they tend to be clumped on either side of the picture. When they are clumped together, they appear much more golden in color. This is due to their golden chloroplasts, which contain the carotenoid fucoxanthin. Photo by Melissa Ha, CC-BY-NC .

Figure (PageIndex{7}): Epiphytic diatoms. These diatoms were photographed from a prepared slide of the red alga Polysiphonia. It is a fan-like colony of pennate diatoms that have attached to the surface of the red alga specimen used to make the slide. When the slide was made, it went through a staining bath. This turned the many golden chloroplasts within the diatoms blue. You can see the chloroplasts within the diatoms because the silica frustules are transparent, like glass. Photo by Maria Morrow, CC-BY-NC .


Diatoms primarily reproduce asexually by binary fission, similar to prokaryotes. During binary fission, the two valves of the frustule are separated and each new cell forms a new valve inside the old one. However, the new valve is always smaller. If diatoms only reproduce in this way, it results in a continual decrease in average size. When some minimal size is reached, this can trigger sexual reproduction. When diatoms sexually reproduce, they have a diplontic life cycle and produce a very large auxospore.

Figure (PageIndex{8}): When diatoms sexually reproduce, they make a large structure called an auxospore. In this picture, the auxospore is a lightbulb-shaped cell located at the end of the colony of epiphytic diatoms. There are many golden chloroplasts visible each diatom. Photo by Maria Morrow, CC-BY-NC .


Video (PageIndex{1}): This video shows some of the incredible diversity of diatom shapes and the amazing art Klaus Kemp makes with them. Sourced from YouTube.

Types of Chrysophytes: 2 Types | Protists

(i) Diatoms occur in all aquatic and moist terrestrial habitats.

(ii) They may be free floating or bottom dwellers. The free floating forms remain suspended on the surface of water by mucilage secretion and presence of light weight lipids.

(iii) Diatoms may show gliding type of movement with the help of mucilage.

(iv) The siliceous frustules of diatoms do not decay easily. They pile up at the bottom of water reservoirs and form big heaps called diatomite or diatomaceous earth. It may extend for several hundred metres in certain areas from where the same can be mined.

(i) The body is covered by a trans­parent siliceous shell (silica deposited in cell wall) known as frustule. The frustule is made of two valves, epitheca and hypotheca. The two valves fit together like a soap box. The frustule possesses very fine markings, pits, pores and ridges.

(ii) Diatoms are microscopic, variously coloured and of diverse forms protists which do not possess flagella except in the reproductive state.

(iii) They are basically unicellular but can form pseudo-filaments and colo­nies.

(iv) Depending upon the symmetry, the dia­toms are of two types namely, pennate and centric. The pennate diatoms show bilateral symmetry e.g. Navicula and the centric diatoms have radial sym­metry, e.g., Melosira.

(v) Each cell has a large central vacuole. The single large nucleus is com­monly suspended in the central vacuole by means of cytoplasmic strands.

(vi) Chloroplasts or chromatophores are yellowish brown to greenish brown. They contain chlorophyll a and chlorophyll c.

(vii) Dia­toms contain fucoxanthin (typical of brown algae) that provides brownish tinge.

The food is reserved in the form of oils and leucosin (polysaccharide). Volutin glob­ules (proteinaceous in nature) are also present.

(i) The common mode of mul­tiplication is by binary fission.

(ii) Resting spores or statospores are formed in some cases.

(iii) Meiosis is gametic. Sexual reproduction varies from isogamy to zoogamy. In the latter case, male gametes are motile and uniflagellate. Fertilization produces a zygote which grows in size and forms a rejuvenascent cell called auxospore in that their vegetative cells are typically diploid.

Triceratium, Pleurosigma, Navicula, Cymbella, Amphipleura.

iv. Economic Importance:

(i) Diatoms are very important photosynthesizes. About half of all the organic matter synthesized in the world is believed to be produced by them. Though microscopic, diatoms are an important source of food to aquatic animals. A 60 tonne blue whale may have 2 tonne of plankton in the gut which is mostly diatoms.

(ii) The oils extracted from some fishes and whales are actually the ones produced by diatoms.

(iii) Diatomite deposits are often accompanied by petroleum fields. Much of the petroleum of today is probably due to decayed bodies of the past diatoms.

(iv) Diatomite is porous and chemically inert. It is, therefore, used in filtration of sugar, alcohols oil, syrups and antibiotics.

(v) Diatomite is employed as a cleaning agent in tooth pastes and metal polishes.

(vi) Diatomite is added to paints for enhancing night visibility.

(vii) Diatomite is employed as insulation material in refrigerators, boilers and furnaces.

(viii) Diatomaceous earth is added to make sound proof rooms.

(ix) Diatomite is a good industrial catalyst.

(x) Diatomite is a source of water glass or sodium silicate,

(xi) Diatoms are very good pollution indicators.

Chrysophytes: Type # 2. Desmids:

Desmids are unicellular green algae. Like Spirogyra, they have an elaborate chloroplast. Their cells have two distinct halves. The outer wall of the cell has various protuberances covered with mucilaginous sheath which is thought to play a role in the cell’s slow gliding movement.

Sexual reproduction occurs by ‘conjugation’ similar to that of Spirogyra. They are mainly found in fresh water and are usually indication of clean (unpolluted) water.

Genome editing in diatoms: achievements and goals

Diatoms are major components of phytoplankton and play a key role in the ecology of aquatic ecosystems. These algae are of great scientific importance for a wide variety of research areas, ranging from marine ecology and oceanography to biotechnology. During the last 20 years, the availability of genomic information on selected diatom species and a substantial progress in genetic manipulation, strongly contributed to establishing diatoms as molecular model organisms for marine biology research. Recently, tailored TALEN endonucleases and the CRISPR/Cas9 system were utilized in diatoms, allowing targeted genetic modifications and the generation of knockout strains. These approaches are extremely valuable for diatom research because breeding, forward genetic screens by random insertion, and chemical mutagenesis are not applicable to the available model species Phaeodactylum tricornutum and Thalassiosira pseudonana, which do not cross sexually in the lab. Here, we provide an overview of the genetic toolbox that is currently available for performing stable genetic modifications in diatoms. We also discuss novel challenges that need to be addressed to fully exploit the potential of these technologies for the characterization of diatom biology and for metabolic engineering.

Keywords: CRISPR Conjugation Diatom Genome editing Mutant screening Promoter TALEN.

Diatom Molecular Research Comes of Age: Model Species for Studying Phytoplankton Biology and Diversity

Diatoms are the world's most diverse group of algae, comprising at least 100,000 species. Contributing ∼20% of annual global carbon fixation, they underpin major aquatic food webs and drive global biogeochemical cycles. Over the past two decades, Thalassiosira pseudonana and Phaeodactylum tricornutum have become the most important model systems for diatom molecular research, ranging from cell biology to ecophysiology, due to their rapid growth rates, small genomes, and the cumulative wealth of associated genetic resources. To explore the evolutionary divergence of diatoms, additional model species are emerging, such as Fragilariopsis cylindrus and Pseudo-nitzschia multistriata Here, we describe how functional genomics and reverse genetics have contributed to our understanding of this important class of microalgae in the context of evolution, cell biology, and metabolic adaptations. Our review will also highlight promising areas of investigation into the diversity of these photosynthetic organisms, including the discovery of new molecular pathways governing the life of secondary plastid-bearing organisms in aquatic environments.

© 2020 American Society of Plant Biologists. All rights reserved.


Model of the Global Distribution…

Model of the Global Distribution of Diatom Biomass. Model of the global distribution…

Diatom Characteristics and Morphological Diversity.…

Diatom Characteristics and Morphological Diversity. (A) Scanning electron micrograph of the model centric…

Simplified Scheme of the Major…

Simplified Scheme of the Major Events Leading to the Evolution of Diatoms through…

Regulators of Carbon, Nitrogen, and…

Regulators of Carbon, Nitrogen, and Iron Metabolism Characterized in the Diatom Model Species.…


Diatoms are generally 2 to 200 micrometers in size, [13] with a few larger species. Their yellowish-brown chloroplasts, the site of photosynthesis, are typical of heterokonts, having four membranes and containing pigments such as the carotenoid fucoxanthin. Individuals usually lack flagella, but they are present in male gametes of the centric diatoms and have the usual heterokont structure, including the hairs (mastigonemes) characteristic in other groups.

Diatoms are often referred as "jewels of the sea" or "living opals" due to their optical properties. [22] The biological function of this structural coloration is not clear, but it is speculated that it may be related to communication, camouflage, thermal exchange and/or UV protection. [23]

Diatoms build intricate hard but porous cell walls called frustules composed primarily of silica. [24] : 25–30 This siliceous wall [25] can be highly patterned with a variety of pores, ribs, minute spines, marginal ridges and elevations all of which can be used to delineate genera and species.

The cell itself consists of two halves, each containing an essentially flat plate, or valve and marginal connecting, or girdle band. One half, the hypotheca, is slightly smaller than the other half, the epitheca. Diatom morphology varies. Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical. Their distinguishing feature is a hard mineral shell or frustule composed of opal (hydrated, polymerized silicic acid).

Diatoms are divided into two groups that are distinguished by the shape of the frustule: the centric diatoms and the pennate diatoms.

Pennate diatoms are bilaterally symmetric. Each one of their valves have openings that are slits along the raphes and their shells are typically elongated parallel to these raphes. They generate cell movement through cytoplasm that streams along the raphes, always moving along solid surfaces.

Centric diatoms are radially symmetric. They are composed of upper and lower valves – epitheca and hypotheca – each consisting of a valve and a girdle band that can easily slide underneath each other and expand to increase cell content over the diatoms progression. The cytoplasm of the centric diatom is located along the inner surface of the shell and provides a hollow lining around the large vacuole located in the center of the cell. This large, central vacuole is filled by a fluid known as "cell sap" which is similar to seawater but varies with specific ion content. The cytoplasmic layer is home to several organelles, like the chloroplasts and mitochondria. Before the centric diatom begins to expand, its nucleus is at the center of one of the valves and begins to move towards the center of the cytoplasmic layer before division is complete. Centric diatoms have a variety of shapes and sizes, depending on from which axis the shell extends, and if spines are present.

Most centric and araphid pennate diatoms are nonmotile, and their relatively dense cell walls cause them to readily sink. Planktonic forms in open water usually rely on turbulent mixing of the upper layers of the oceanic waters by the wind to keep them suspended in sunlit surface waters. Many planktonic diatoms have also evolved features that slow their sinking rate, such as spines or the ability to grow in colonial chains. [29] These adaptations increase their surface area to volume ratio and drag, allowing them to stay suspended in the water column longer. Individual cells may regulate buoyancy via an ionic pump. [30]

Some pennate diatoms are capable of a type of locomotion called "gliding", which allows them to move across surfaces via adhesive mucilage secreted through the raphe (an elongated slit in the valve face). [31] [32] In order for a diatom cell to glide, it must have a solid substrate for the mucilage to adhere to.

Cells are solitary or united into colonies of various kinds, which may be linked by siliceous structures mucilage pads, stalks or tubes amorphous masses of mucilage or by threads of chitin (polysaccharide), which are secreted through strutted processes of the cell.

Reproduction and cell size Edit

Reproduction among these organisms is asexual by binary fission, during which the diatom divides into two parts, producing two "new" diatoms with identical genes. Each new organism receives one of the two frustules – one larger, the other smaller – possessed by the parent, which is now called the epitheca and is used to construct a second, smaller frustule, the hypotheca. The diatom that received the larger frustule becomes the same size as its parent, but the diatom that received the smaller frustule remains smaller than its parent. This causes the average cell size of this diatom population to decrease. [13] It has been observed, however, that certain taxa have the ability to divide without causing a reduction in cell size. [33] Nonetheless, in order to restore the cell size of a diatom population for those that do endure size reduction, sexual reproduction and auxospore formation must occur. [13]

Cell division Edit

Vegetative cells of diatoms are diploid (2N) and so meiosis can take place, producing male and female gametes which then fuse to form the zygote. The zygote sheds its silica theca and grows into a large sphere covered by an organic membrane, the auxospore. A new diatom cell of maximum size, the initial cell, forms within the auxospore thus beginning a new generation. Resting spores may also be formed as a response to unfavourable environmental conditions with germination occurring when conditions improve. [24]

Sperm motility Edit

Diatoms are mostly non-motile however, sperm found in some species can be flagellated, though motility is usually limited to a gliding motion. [24] In centric diatoms, the small male gametes have one flagellum while the female gametes are large and non-motile (oogamous). Conversely, in pennate diatoms both gametes lack flagella (isogamous). [13] Certain araphid species, that is pennate diatoms without a raphe (seam), have been documented as anisogamous and are, therefore, considered to represent a transitional stage between centric and raphid pennate diatoms, diatoms with a raphe. [33]

Degradation by microbes Edit

Certain species of bacteria in oceans and lakes can accelerate the rate of dissolution of silica in dead and living diatoms by using hydrolytic enzymes to break down the organic algal material. [34] [35]

Distribution Edit

Diatoms are a widespread group and can be found in the oceans, in fresh water, in soils, and on damp surfaces. They are one of the dominant components of phytoplankton in nutrient-rich coastal waters and during oceanic spring blooms, since they can divide more rapidly than other groups of phytoplankton. [39] Most live pelagically in open water, although some live as surface films at the water-sediment interface (benthic), or even under damp atmospheric conditions. They are especially important in oceans, where they contribute an estimated 45% of the total oceanic primary production of organic material. [40] Spatial distribution of marine phytoplankton species is restricted both horizontally and vertically. [41] [24]

Growth Edit

Planktonic diatoms in freshwater and marine environments typically exhibit a "boom and bust" (or "bloom and bust") lifestyle. When conditions in the upper mixed layer (nutrients and light) are favourable (as at the spring), their competitive edge and rapid growth rate [39] enables them to dominate phytoplankton communities ("boom" or "bloom"). As such they are often classed as opportunistic r-strategists (i.e. those organisms whose ecology is defined by a high growth rate, r).

Contribution to modern oceanic silicon cycle Edit

Diatoms contribute in a significant way to the modern oceanic silicon cycle: they are the source of the vast majority of biological production.

Impact Edit

The freshwater diatom Didymosphenia geminata, commonly known as Didymo, causes severe environmental degradation in water-courses where it blooms, producing large quantities of a brown jelly-like material called "brown snot" or "rock snot". This diatom is native to Europe and is an invasive species both in the antipodes and in parts of North America. [42] [43] The problem is most frequently recorded from Australia and New Zealand. [44]

When conditions turn unfavourable, usually upon depletion of nutrients, diatom cells typically increase in sinking rate and exit the upper mixed layer ("bust"). This sinking is induced by either a loss of buoyancy control, the synthesis of mucilage that sticks diatoms cells together, or the production of heavy resting spores. Sinking out of the upper mixed layer removes diatoms from conditions unfavourable to growth, including grazer populations and higher temperatures (which would otherwise increase cell metabolism). Cells reaching deeper water or the shallow seafloor can then rest until conditions become more favourable again. In the open ocean, many sinking cells are lost to the deep, but refuge populations can persist near the thermocline.

Ultimately, diatom cells in these resting populations re-enter the upper mixed layer when vertical mixing entrains them. In most circumstances, this mixing also replenishes nutrients in the upper mixed layer, setting the scene for the next round of diatom blooms. In the open ocean (away from areas of continuous upwelling [45] ), this cycle of bloom, bust, then return to pre-bloom conditions typically occurs over an annual cycle, with diatoms only being prevalent during the spring and early summer. In some locations, however, an autumn bloom may occur, caused by the breakdown of summer stratification and the entrainment of nutrients while light levels are still sufficient for growth. Since vertical mixing is increasing, and light levels are falling as winter approaches, these blooms are smaller and shorter-lived than their spring equivalents.

In the open ocean, the diatom (spring) bloom is typically ended by a shortage of silicon. Unlike other minerals, the requirement for silicon is unique to diatoms and it is not regenerated in the plankton ecosystem as efficiently as, for instance, nitrogen or phosphorus nutrients. This can be seen in maps of surface nutrient concentrations – as nutrients decline along gradients, silicon is usually the first to be exhausted (followed normally by nitrogen then phosphorus).

Because of this bloom-and-bust cycle, diatoms are believed to play a disproportionately important role in the export of carbon from oceanic surface waters [45] [46] (see also the biological pump). Significantly, they also play a key role in the regulation of the biogeochemical cycle of silicon in the modern ocean. [40] [36]

Reason for success Edit

Diatoms are ecologically successful, and occur in virtually every environment that contains water – not only oceans, seas, lakes, and streams, but also soil and wetlands. [ citation needed ] The use of silicon by diatoms is believed by many researchers to be the key to this ecological success. Raven (1983) [47] noted that, relative to organic cell walls, silica frustules require less energy to synthesize (approximately 8% of a comparable organic wall), potentially a significant saving on the overall cell energy budget. In a now classic study, Egge and Aksnes (1992) [38] found that diatom dominance of mesocosm communities was directly related to the availability of silicic acid – when concentrations were greater than 2 μmol m −3 , they found that diatoms typically represented more than 70% of the phytoplankton community. Other researchers [48] have suggested that the biogenic silica in diatom cell walls acts as an effective pH buffering agent, facilitating the conversion of bicarbonate to dissolved CO2 (which is more readily assimilated). More generally, notwithstanding these possible advantages conferred by their use of silicon, diatoms typically have higher growth rates than other algae of the same corresponding size. [39]

Sources for collection Edit

Diatoms can be obtained from multiple sources. [49] Marine diatoms can be collected by direct water sampling, and benthic forms can be secured by scraping barnacles, oyster and other shells. Diatoms are frequently present as a brown, slippery coating on submerged stones and sticks, and may be seen to "stream" with river current. The surface mud of a pond, ditch, or lagoon will almost always yield some diatoms. Living diatoms are often found clinging in great numbers to filamentous algae, or forming gelatinous masses on various submerged plants. Cladophora is frequently covered with Cocconeis, an elliptically shaped diatom Vaucheria is often covered with small forms. Since diatoms form an important part of the food of molluscs, tunicates, and fishes, the alimentary tracts of these animals often yield forms that are not easily secured in other ways. Diatoms can be made to emerge by filling a jar with water and mud, wrapping it in black paper and letting direct sunlight fall on the surface of the water. Within a day, the diatoms will come to the top in a scum and can be isolated. [49]

Energy source Edit

Diatoms are mainly photosynthetic however a few are obligate heterotrophs and can live in the absence of light provided an appropriate organic carbon source is available. [50] [51]

Silica metabolism Edit

Diatom cells are contained within a unique silica cell wall known as a frustule made up of two valves called thecae, that typically overlap one another. [52] The biogenic silica composing the cell wall is synthesised intracellularly by the polymerisation of silicic acid monomers. This material is then extruded to the cell exterior and added to the wall. In most species, when a diatom divides to produce two daughter cells, each cell keeps one of the two-halves and grows a smaller half within it. As a result, after each division cycle, the average size of diatom cells in the population gets smaller. Once such cells reach a certain minimum size, rather than simply divide, they reverse this decline by forming an auxospore. This expands in size to give rise to a much larger cell, which then returns to size-diminishing divisions. [ citation needed ] Auxospore production is almost always linked to meiosis and sexual reproduction.

The exact mechanism of transferring silica absorbed by the diatom to the cell wall is unknown. Much of the sequencing of diatom genes comes from the search for the mechanism of silica uptake and deposition in nano-scale patterns in the frustule. The most success in this area has come from two species, Thalassiosira pseudonana, which has become the model species, as the whole genome was sequenced and methods for genetic control were established, and Cylindrotheca fusiformis, in which the important silica deposition proteins silaffins were first discovered. [53] Silaffins, sets of polycationic peptides, were found in C. fusiformis cell walls and can generate intricate silica structures. These structures demonstrated pores of sizes characteristic to diatom patterns. When T. pseudonana underwent genome analysis it was found that it encoded a urea cycle, including a higher number of polyamines than most genomes, as well as three distinct silica transport genes. [54] In a phylogenetic study on silica transport genes from 8 diverse groups of diatoms, silica transport was found to generally group with species. [53] This study also found structural differences between the silica transporters of pennate (bilateral symmetry) and centric (radial symmetry) diatoms. The sequences compared in this study were used to create a diverse background in order to identify residues that differentiate function in the silica deposition process. Additionally, the same study found that a number of the regions were conserved within species, likely the base structure of silica transport.

These silica transport proteins are unique to diatoms, with no homologs found in other species, such as sponges or rice. The divergence of these silica transport genes is also indicative of the structure of the protein evolving from two repeated units composed of five membrane bound segments, which indicates either gene duplication or dimerization. [53] The silica deposition that takes place from the membrane bound vesicle in diatoms has been hypothesized to be a result of the activity of silaffins and long chain polyamines. This Silica Deposition Vesicle (SDV) has been characterized as an acidic compartment fused with Golgi-derived vesicles. [55] These two protein structures have been shown to create sheets of patterned silica in-vivo with irregular pores on the scale of diatom frustules. One hypothesis as to how these proteins work to create complex structure is that residues are conserved within the SDV's, which is unfortunately difficult to identify or observe due to the limited number of diverse sequences available. Though the exact mechanism of the highly uniform deposition of silica is as yet unknown, the Thalassiosira pseudonana genes linked to silaffins are being looked to as targets for genetic control of nanoscale silica deposition.

Urea cycle Edit

A feature of diatoms is the urea cycle, which links them evolutionarily to animals. This was discovered in research carried out by Andrew Allen, Chris Bowler and colleagues. Their findings, published in 2011, that diatoms have a functioning urea cycle was highly significant, since prior to this, the urea cycle was thought to have originated with the metazoans which appeared several hundreds of millions of years before the diatoms. Their study showed that while diatoms and animals use the urea cycle for different ends, they are seen to be evolutionarily linked in such a way that animals and plants are not. [56]

Pigments Edit

Major pigments of diatoms are chlorophylls a and c, beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin. [13]

Storage products Edit

Diatoms belong to a large group of protists, many of which contain plastids rich in chlorophylls a and c. The group has been variously referred to as heterokonts, chrysophytes, chromists or stramenopiles. Many are autotrophs such as golden algae and kelp and heterotrophs such as water moulds, opalinids, and actinophryid heliozoa. The classification of this area of protists is still unsettled. In terms of rank, they have been treated as a division, phylum, kingdom, or something intermediate to those. Consequently, diatoms are ranked anywhere from a class, usually called Diatomophyceae or Bacillariophyceae, to a division (=phylum), usually called Bacillariophyta, with corresponding changes in the ranks of their subgroups.

Genera and species Edit

An estimated 20,000 extant diatom species are believed to exist, of which around 12,000 have been named to date according to Guiry, 2012 [57] (other sources give a wider range of estimates [13] [58] [59] [60] ). Around 1,000–1,300 diatom genera have been described, both extant and fossil, [61] [62] of which some 250–300 exist only as fossils. [63]

Classes and orders Edit

For many years the diatoms—treated either as a class (Bacillariophyceae) or a phylum (Bacillariophyta)—were divided into just 2 orders, corresponding to the centric and the pennate diatoms (Centrales and Pennales). This classification was extensively overhauled by Round, Crawford and Mann in 1990 who treated the diatoms at a higher rank (division, corresponding to phylum in zoological classification), and promoted the major classification units to classes, maintaining the centric diatoms as a single class Coscinodiscophyceae, but splitting the former pennate diatoms into 2 separate classes, Fragilariophyceae and Bacillariophyceae (the latter older name retained but with an emended definition), between them encompassing 45 orders, the majority of them new.

Today (writing at mid 2020) it is recognised that the 1990 system of Round et al. is in need of revision with the advent of newer molecular work, however the best system to replace it is unclear, and current systems in widespread use such as AlgaeBase, the World Register of Marine Species and its contributing database DiatomBase, and the system for "all life" represented in Ruggiero et al., 2015, all retain the Round et al. treatment as their basis, albeit with diatoms as a whole treated as a class rather than division/phylum, and Round et al.'s classes reduced to subclasses, for better agreement with the treatment of phylogenetically adjacent groups and their containing taxa. (For references refer the individual sections below).

One proposal, by Linda Medlin and co-workers commencing in 2004, is for some of the centric diatom orders considered more closely related to the pennates to be split off as a new class, Mediophyceae, itself more closely aligned with the pennate diatoms than the remaining centrics. This hypothesis—later designated the Coscinodiscophyceae-Mediophyceae-Bacillariophyceae, or Coscinodiscophyceae+(Mediophyceae+Bacillariophyceae) (CMB) hypothesis—has been accepted by D.G. Mann among others, who uses it as the basis for the classification of diatoms as presented in Adl. et al.'s series of syntheses (2005, 2012, 2019), and also in the Bacillariophyta chapter of the 2017 Handbook of the Protists edited by Archibald et al., with some modifications reflecting the apparent non-monophyly of Medlin et al. original "Coscinodiscophyceae". Meanwhile, a group led by E.C. Theriot favours a different hypothesis of phylogeny, which has been termed the structural gradation hypothesis (SGH) and does not recognise the Mediophyceae as a monophyletic group, while another analysis, that of Parks et al., 2018, finds that the radial centric diatoms (Medlin et al.'s Coscinodiscophyceae) are not monophyletic, but supports the monophyly of Mediophyceae minus Attheya, which is an anomalous genus. Discussion of the relative merits of these conflicting schemes continues by the various parties involved. [64] [65] [66] [67]

Adl et al., 2019 treatment Edit

In 2019, Adl et al. [68] presented the following classification of diatoms, while noting: "This revision reflects numerous advances in the phylogeny of the diatoms over the last decade. Due to our poor taxon sampling outside of the Mediophyceae and pennate diatoms, and the known and anticipated diversity of all diatoms, many clades appear at a high classification level (and the higher level classification is rather flat)." This classification treats diatoms as a phylum (Diatomeae/Bacillariophyta), accepts the class Mediophyceae of Medlin and co-workers, introduces new subphyla and classes for a number of otherwise isolated genera, and re-ranks a number of previously established taxa as subclasses, but does not list orders or families. Inferred ranks have been added for clarity (Adl. et al. do not use ranks, but the intended ones in this portion of the classification are apparent from the choice of endings used, within the system of botanical nomenclature employed).

  • Clade Diatomista Derelle et al. 2016, emend. Cavalier-Smith 2017 (diatoms plus a subset of other ochrophyte groups)
  • Phylum Diatomeae Dumortier 1821 [= Bacillariophyta Haeckel 1878] (diatoms)
  • Subphylum Leptocylindrophytina D.G. Mann in Adl et al. 2019
  • Class Leptocylindrophyceae D.G. Mann in Adl et al. 2019 (Leptocylindrus, Tenuicylindrus)
  • Class Corethrophyceae D.G. Mann in Adl et al. 2019 (Corethron)
  • Subphylum Ellerbeckiophytina D.G. Mann in Adl et al. 2019 (Ellerbeckia)
  • Subphylum Probosciophytina D.G. Mann in Adl et al. 2019 (Proboscia)
  • Subphylum Melosirophytina D.G. Mann in Adl et al. 2019 (Aulacoseira, Melosira, Hyalodiscus, Stephanopyxis, Paralia, Endictya)
  • Subphylum Coscinodiscophytina Medlin & Kaczmarska 2004, emend. (Actinoptychus, Coscinodiscus, Actinocyclus, Asteromphalus, Aulacodiscus, Stellarima)
  • Subphylum Rhizosoleniophytina D.G. Mann in Adl et al. 2019 (Guinardia, Rhizosolenia, Pseudosolenia)
  • Subphylum Arachnoidiscophytina D.G. Mann in Adl et al. 2019 (Arachnoidiscus)
  • Subphylum Bacillariophytina Medlin & Kaczmarska 2004, emend.
  • Class Mediophyceae Jouse & Proshkina-Lavrenko in Medlin & Kaczmarska 2004
  • Subclass Chaetocerotophycidae Round & R.M. Crawford in Round et al. 1990, emend.
  • Subclass Lithodesmiophycidae Round & R.M. Crawford in Round et al. 1990, emend.
  • Subclass Thalassiosirophycidae Round & R.M. Crawford in Round et al. 1990
  • Subclass Cymatosirophycidae Round & R.M. Crawford in Round et al. 1990
  • Subclass Odontellophycidae D.G. Mann in Adl et al. 2019
  • Subclass Chrysanthemodiscophycidae D.G. Mann in Adl et al. 2019
  • Class Biddulphiophyceae D.G. Mann in Adl et al. 2019
  • Subclass Biddulphiophycidae Round and R.M. Crawford in Round et al. 1990, emend.
  • Biddulphiophyceae incertae sedis (Attheya)
  • Class Bacillariophyceae Haeckel 1878, emend.
  • Bacillariophyceae incertae sedis (Striatellaceae)
  • Subclass Urneidophycidae Medlin 2016
  • Subclass Fragilariophycidae Round in Round, Crawford & Mann 1990, emend.
  • Subclass Bacillariophycidae D.G. Mann in Round, Crawford & Mann 1990, emend.

Diatom Surirella spiralis

Diatoms Thalassiosira sp. on a membrane filter, pore size 0.4 μm.

Diatom Paralia sulcata.

Diatom Achanthes trinodis

Origin Edit

Heterokont chloroplasts appear to derive from those of red algae, rather than directly from prokaryotes as occurred in plants. This suggests they had a more recent origin than many other algae. However, fossil evidence is scant, and only with the evolution of the diatoms themselves do the heterokonts make a serious impression on the fossil record.

Earliest fossils Edit

The earliest known fossil diatoms date from the early Jurassic (

185 Ma ago), [69] although the molecular clock [69] and sedimentary [70] evidence suggests an earlier origin. It has been suggested that their origin may be related to the end-Permian mass extinction (

250 Ma), after which many marine niches were opened. [71] The gap between this event and the time that fossil diatoms first appear may indicate a period when diatoms were unsilicified and their evolution was cryptic. [72] Since the advent of silicification, diatoms have made a significant impression on the fossil record, with major fossil deposits found as far back as the early Cretaceous, and with some rocks such as diatomaceous earth, being composed almost entirely of them.

Relation to silicon cycle Edit

Although diatoms may have existed since the Triassic, the timing of their ascendancy and "take-over" of the silicon cycle occurred more recently. Prior to the Phanerozoic (before 544 Ma), it is believed that microbial or inorganic processes weakly regulated the ocean's silicon cycle. [73] [74] [75] Subsequently, the cycle appears dominated (and more strongly regulated) by the radiolarians and siliceous sponges, the former as zooplankton, the latter as sedentary filter-feeders primarily on the continental shelves. [76] Within the last 100 My, it is thought that the silicon cycle has come under even tighter control, and that this derives from the ecological ascendancy of the diatoms.

However, the precise timing of the "take-over" remains unclear, and different authors have conflicting interpretations of the fossil record. Some evidence, such as the displacement of siliceous sponges from the shelves, [77] suggests that this takeover began in the Cretaceous (146 Ma to 66 Ma), while evidence from radiolarians suggests "take-over" did not begin until the Cenozoic (66 Ma to present). [78]

Relation to grasslands Edit

The expansion of grassland biomes and the evolutionary radiation of grasses during the Miocene is believed to have increased the flux of soluble silicon to the oceans, and it has been argued that this promoted the diatoms during the Cenozoic era. [79] [80] Recent work suggests that diatom success is decoupled from the evolution of grasses, although both diatom and grassland diversity increased strongly from the middle Miocene. [81]

Relation to climate Edit

Diatom diversity over the Cenozoic has been very sensitive to global temperature, particularly to the equator-pole temperature gradient. Warmer oceans, particularly warmer polar regions, have in the past been shown to have had substantially lower diatom diversity. Future warm oceans with enhanced polar warming, as projected in global-warming scenarios, [82] could thus in theory result in a significant loss of diatom diversity, although from current knowledge it is impossible to say if this would occur rapidly or only over many tens of thousands of years. [81]

Method of investigation Edit

The fossil record of diatoms has largely been established through the recovery of their siliceous frustules in marine and non-marine sediments. Although diatoms have both a marine and non-marine stratigraphic record, diatom biostratigraphy, which is based on time-constrained evolutionary originations and extinctions of unique taxa, is only well developed and widely applicable in marine systems. The duration of diatom species ranges have been documented through the study of ocean cores and rock sequences exposed on land. [83] Where diatom biozones are well established and calibrated to the geomagnetic polarity time scale (e.g., Southern Ocean, North Pacific, eastern equatorial Pacific), diatom-based age estimates may be resolved to within <100,000 years, although typical age resolution for Cenozoic diatom assemblages is several hundred thousand years.

Diatoms preserved in lake sediments are widely used for paleoenvironmental reconstructions of Quaternary climate, especially for closed-basin lakes which experience fluctuations in water depth and salinity.

Diversification Edit

The Cretaceous record of diatoms is limited, but recent studies reveal a progressive diversification of diatom types. The Cretaceous–Paleogene extinction event, which in the oceans dramatically affected organisms with calcareous skeletons, appears to have had relatively little impact on diatom evolution. [84]

Turnover Edit

Although no mass extinctions of marine diatoms have been observed during the Cenozoic, times of relatively rapid evolutionary turnover in marine diatom species assemblages occurred near the Paleocene–Eocene boundary, [85] and at the Eocene–Oligocene boundary. [86] Further turnover of assemblages took place at various times between the middle Miocene and late Pliocene, [87] in response to progressive cooling of polar regions and the development of more endemic diatom assemblages.

A global trend toward more delicate diatom frustules has been noted from the Oligocene to the Quaternary. [83] This coincides with an increasingly more vigorous circulation of the ocean's surface and deep waters brought about by increasing latitudinal thermal gradients at the onset of major ice sheet expansion on Antarctica and progressive cooling through the Neogene and Quaternary towards a bipolar glaciated world. This caused diatoms to take in less silica for the formation of their frustules. Increased mixing of the oceans renews silica and other nutrients necessary for diatom growth in surface waters, especially in regions of coastal and oceanic upwelling.

Expressed sequence tagging Edit

In 2002, the first insights into the properties of the Phaeodactylum tricornutum gene repertoire were described using 1,000 expressed sequence tags (ESTs). [88] Subsequently, the number of ESTs was extended to 12,000 and the diatom EST database was constructed for functional analyses. [89] These sequences have been used to make a comparative analysis between P. tricornutum and the putative complete proteomes from the green alga Chlamydomonas reinhardtii, the red alga Cyanidioschyzon merolae, and the diatom Thalassiosira pseudonana. [90] The diatom EST database now consists of over 200,000 ESTs from P. tricornutum (16 libraries) and T. pseudonana (7 libraries) cells grown in a range of different conditions, many of which correspond to different abiotic stresses. [91]

Genome sequencing Edit

In 2004, the entire genome of the centric diatom, Thalassiosira pseudonana (32.4 Mb) was sequenced, [92] followed in 2008 with the sequencing of the pennate diatom, Phaeodactylum tricornutum (27.4 Mb). [93] Comparisons of the two reveal that the P. tricornutum genome includes fewer genes (10,402 opposed to 11,776) than T. pseudonana no major synteny (gene order) could be detected between the two genomes. T. pseudonana genes show an average of

1.52 introns per gene as opposed to 0.79 in P. tricornutum, suggesting recent widespread intron gain in the centric diatom. [93] [94] Despite relatively recent evolutionary divergence (90 million years), the extent of molecular divergence between centrics and pennates indicates rapid evolutionary rates within the Bacillariophyceae compared to other eukaryotic groups. [93] Comparative genomics also established that a specific class of transposable elements, the Diatom Copia-like retrotransposons (or CoDis), has been significantly amplified in the P. tricornutum genome with respect to T. pseudonana, constituting 5.8 and 1% of the respective genomes. [95]

Endosymbiotic gene transfer Edit

Diatom genomics brought much information about the extent and dynamics of the endosymbiotic gene transfer (EGT) process. Comparison of the T. pseudonana proteins with homologs in other organisms suggested that hundreds have their closest homologs in the Plantae lineage. EGT towards diatom genomes can be illustrated by the fact that the T. pseudonana genome encodes six proteins which are most closely related to genes encoded by the Guillardia theta (cryptomonad) nucleomorph genome. Four of these genes are also found in red algal plastid genomes, thus demonstrating successive EGT from red algal plastid to red algal nucleus (nucleomorph) to heterokont host nucleus. [92] More recent phylogenomic analyses of diatom proteomes provided evidence for a prasinophyte-like endosymbiont in the common ancestor of chromalveolates as supported by the fact the 70% of diatom genes of Plantae origin are of green lineage provenance and that such genes are also found in the genome of other stramenopiles. Therefore, it was proposed that chromalveolates are the product of serial secondary endosymbiosis first with a green algae, followed by a second one with a red algae that conserved the genomic footprints of the previous but displaced the green plastid. [96] However, phylogenomic analyses of diatom proteomes and chromalveolate evolutionary history will likely take advantage of complementary genomic data from under-sequenced lineages such as red algae.

Horizontal gene transfer Edit

In addition to EGT, horizontal gene transfer (HGT) can occur independently of an endosymbiotic event. The publication of the P. tricornutum genome reported that at least 587 P. tricornutum genes appear to be most closely related to bacterial genes, accounting for more than 5% of the P. tricornutum proteome. About half of these are also found in the T. pseudonana genome, attesting their ancient incorporation in the diatom lineage. [93]

Genetic Engineering Edit

To understand the biological mechanisms which underlie the great importance of diatoms in geochemical cycles, scientists have used the Phaeodactylum tricornutum and Thalassiosira spp. species as model organisms since the 90's. [97] Few molecular biology tools are currently available to generate mutants or transgenic lines : plasmids containing transgenes are inserted into the cells using the biolistic method [98] or transkingdom bacterial conjugation [99] (with 10 −6 and 10 −4 yield respectively [98] [99] ), and other classical transfection methods such as electroporation or use of PEG have been reported to provide results with lower efficiencies. [99]

Transfected plasmids can be either randomly integrated into the diatom's chromosomes or maintained as stable circular episomes (thanks to the CEN6-ARSH4-HIS3 yeast centromeric sequence [99] ). The phleomycin/zeocin resistance gene Sh Ble is commonly used as a selection marker, [97] [100] and various transgenes have been successfully introduced and expressed in diatoms with stable transmissions through generations, [99] [100] or with the possibility to remove it. [100]

Furthermore, these systems now allow the use of the CRISPR-Cas genome edition tool, leading to a fast production of functional knock-out mutants [100] [101] and a more accurate comprehension of the diatoms' cellular processes.

Paleontology Edit

Decomposition and decay of diatoms leads to organic and inorganic (in the form of silicates) sediment, the inorganic component of which can lead to a method of analyzing past marine environments by corings of ocean floors or bay muds, since the inorganic matter is embedded in deposition of clays and silts and forms a permanent geological record of such marine strata (see siliceous ooze).

Industrial Edit

Diatoms, and their shells (frustules) as diatomite or diatomaceous earth, are important industrial resources used for fine polishing and liquid filtration. The complex structure of their microscopic shells has been proposed as a material for nanotechnology. [102]

Diatomite is considered to be a natural nano material and has many uses and applications such as: production of various ceramic products, construction ceramics, refractory ceramics, special oxide ceramics, for production of humidity control materials, used as filtration material, material in the cement production industry, initial material for production of prolonged-release drug carriers, absorption material in an industrial scale, production of porous ceramics, glass industry, used as catalyst support, as a filler in plastics and paints, purification of industrial waters, pesticide holder, as well as for improving the physical and chemical characteristics of certain soils, and other uses. [103] [104] [105]

Diatoms are also used to help determine the origin of materials containing them, including seawater.

Forensic Edit

The main goal of diatom analysis in forensics is to differentiate a death by submersion from a post-mortem immersion of a body in water. Laboratory tests may reveal the presence of diatoms in the body. Since the silica-based skeletons of diatoms do not readily decay, they can sometimes be detected even in heavily decomposed bodies. As they do not occur naturally in the body, if laboratory tests show diatoms in the corpse that are of the same species found in the water where the body was recovered, then it may be good evidence of drowning as the cause of death. The blend of diatom species found in a corpse may be the same or different from the surrounding water, indicating whether the victim drowned in the same site in which the body was found. [106]

Nanotechnology Edit

The deposition of silica by diatoms may also prove to be of utility to nanotechnology. [107] Diatom cells repeatedly and reliably manufacture valves of various shapes and sizes, potentially allowing diatoms to manufacture micro- or nano-scale structures which may be of use in a range of devices, including: optical systems semiconductor nanolithography and even vehicles for drug delivery. With an appropriate artificial selection procedure, diatoms that produce valves of particular shapes and sizes might be evolved for cultivation in chemostat cultures to mass-produce nanoscale components. [108] It has also been proposed that diatoms could be used as a component of solar cells by substituting photosensitive titanium dioxide for the silicon dioxide that diatoms normally use to create their cell walls. [109] Diatom biofuel producing solar panels have also been proposed. [110]

The first diatom formally described in scientific literature, the colonial Bacillaria paradoxa, was discovered in 1783 by Danish naturalist Otto Friedrich Müller.

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". impressive in heft, magnificent in its production, encyclopedic in its coverage and elegant in its illustrations." Nature

". fills an enormous gap in our knowledge of the taxonomy of an environmentally significant group of autotrophic protists. The book is filled with picture perfect photomicrographics made through light microscopy, transmission electron microscopy and scanning electron microscopy, as well as finely executed illustrations." American Scientist

". an excellent reference on an important group of aquatic organisms." Journal of the North American Benthological Society

"An impressive encyclopedic treatise by three British experts on the biology and morphology of the diatoms. This outstanding volume, the first to summarize the genera of diatoms in more than 60 years, will be worth its weight in gold to professionals and graduate students. " Choice

"The book is magnificent and is needed." L. M. Van Valen, Evolutionary Theory & Review

". the most important book on the subject to come off of the presses in a long time. a must for every serious diatom worker." Robert B. McLaughlin, Microscope

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Diatoms are microscopic, single-celled, or colonial plant-like organisms, whose cell walls are composed of silicon dioxide (silica). There are numerous holes or areolae on their shells (or tests), which are visible under a microscope.

They are found on damp surfaces, in the oceans, rivers, lakes, streams, estuaries, puddles, on wet rocks, and in various soils. Most diatoms are microscopic, but some species are as long as 2 millimeters. They usually do not move, but few species use the flagella for locomotion.

Regarding their classification, diatoms belong to the class Bacillariophyceae, and there are more than 200 genera of this organism. They are classified as either protists or chromists. According to some estimates, there are approximately 100,000 existing species of this organism. Two types of diatoms are present: the round centrales, and the long or pen-shaped pennales.

They grow a silica shell that is preserved in underwater sediments after their death. This test is known as frustule, which is different for each species, so you can identify them by observing through a microscope. These frustules exhibit two asymmetrical sides with a split between them hence the name diatoms. They are mostly yellowish or brownish, and they have chlorophyll A, chlorophyll C, and carotenoid fucoxanthin that occurs in plastids. They produce food by photosynthesis, and are great suppliers of oxygen.

Diatoms undergo asexual reproduction as they reproduce by cell division. They becomes smaller with each round of replication. Very small species may follow a sexual mode of reproduction, which allows the growth of a relatively large zygote.

As they die, diatoms tests accumulate in the ooze, and form the material known as diatomaceous earth, which is also known as kieselguhr. It found in the form of a soft, chalky, and light-weight rock that is called diatomite, which is used as an insulating material to absorb both heat and sound. It is used to manufacture dynamite, other explosives, filters, abrasives, etc. It is sometimes used in gardening as a pest control.

Diatoms play an important role in the formation of the Earth’s structure, as the limestone layers are deposited by them, and occurrence of petroleum has partly been possible due to these organisms. Their communities provide a tool for monitoring past and present environmental conditions. They are useful in studies of water quality. Different species of this organism prefer different temperatures. Therefore, scientists can estimate the temperature of the water where they live. Diatoms are mostly present in great numbers, and their size helps to indicate lateral water movement, and how well the water is mixed.

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Aquatic environments have frequently been affected by different anthropic activities, resulting in negative impacts to river basins in developed [1–6] and wild regions [7–9], such as Pantanal of Mato Grosso, the largest continuous floodplain in South America, located in Brazil. Compared to the six Brazilian continental biomes, the Pantanal Biome accounts for only 1.76% of the country territory. However, it is of outstanding importance due to the complexity of habitats and high diversity of plant and animal species, and therefore is considered a World Natural Heritage and Biosphere Reserves by Unesco [10].

Moreover, it is well known for its annual flood pulse, a river-plain interaction which affects the entire biota of the system [11]. For example, the natural eutrophication phenomenon locally called Decoada, which occurs during the beginning of the flood phase, causes a series of changes in water quality that are of great importance to the processes of decomposition and chemosynthesis [12].

Another important impact is the anthropogenic eutrophication, aggravated in wetlands by the annual floods [13,14], which harms the structure and dynamics of the communities of aquatic organisms [12]. The increase in nutrient concentration during the eutrophication process, drastically changes the microorganism biomass populations [15]. Robust conclusion of environmental condition may be drawn from the presence of bioindicators that have intense relationships with stressors, such as the diatoms [16]. Nevertheless, the interpretation of individual dominant taxa, needs to be addressed when making environmental inferences [17–19].

Diatoms are a group of silicified microalgae, considered as one of the most sensitive groups to environmental changes [20]. In the last decades, the study of diatom assemblages, linked to any single substratum, has received increasing attention [21–26], because it provides relevant information about the ecosystem stratification, allowing a correlation of ecological information with time and space [27–29], as well as the assessment of the ecological status of rivers, streams and lakes in temperate zones [30–32]. However, there is still an urgent need to expand the information to the wetland regions of the globe, where studies of diatom community are scarce [33,34]. So far, in South America, namely in Brazil, studies have focused on the planktonic and epilithic diatoms in rivers and streams, mainly related to the evaluation of water quality [35–37] and periphytic diatoms in floodplain [38]. Recent studies have focused on the role of eutrophication in environmental reorganization, with diatom assemblages and land-use records used as a tool to infer the trophic state history of the water body [39,40] as well as a record of biotic homogenization of diatom diversity in sedimentary samples [41]. Furthermore, some advances were also made in the auto-ecology of tropical species, the influence of environmental and spatial factors on diatom biodiversity, and its distribution [42,43].

In spite of these advances, there are a lack of studies on diatoms in surface sediment. Taxonomic studies contribute to the knowledge of biodiversity and provide the basis for the advancement of other approaches such as bioindication, environmental reconstruction, research on conservation and definition of priority areas such as the Pantanal, among many others. The more relevant works in the region are on the distribution of two species of diatoms and their association with the historical variation in water levels in the Paraná River [44] the diatom flora in the Pantanal of Mato Grosso do Sul [45] the history of the salinity in the southern Pantanal [46] and records of flood pulse dynamics [47]. Other studies, not related to diatoms, refer to the wetland carbon storage [48] and the influence that hydroclimatic variables exert on limnogeological processes [49]. However, the diatom biodiversity of this region remains understudied. This may be due to the difficulty of sampling in flooded areas as a result of a lack of suitable transportation through the wetland, as well as people specialized in diving in these areas. Thus, to better understand the biodiversity of diatoms in sediments of tropical wetland areas, the present study aimed to evaluate the influence of environmental factors on the distribution of these organisms in surface sediments in three different lakes of the Brazilian Pantanal (wetland) of Mato Grosso State. Our approach merges an assessment of diatom species with presence/absence and relative abundance data.

This study brings a contribution to the understanding of flooded tropical regions and intends to increase the knowledge of diatom biodiversity in Pantanal, using the structure and composition of species as a limnological bioindicator in tropical wetlands that are still poorly explored. Furthermore, the results of this study in the current context of the environmental destruction of Pantanal (fire/forest 2020) and the influence that it may have had on limnological processes, currently and also in the future, are extremely relevant to raise new comparative studies for the area, to help with decisions that may be of socioeconomic reasons, environmental management and also of biodiversity issues.

Diatom Discoveries and History

An unknown 18 th century scientific observer from England is credited as the first person to discover diatoms. This observation was published by the Royal Society of London. The observer was looking at the roots of a pond weed under a simple microscope, remarking that “”adhering to them (and sometimes separate in the water) many pretty branches, composed of rectangular oblongs and exact squares.” These oblongs and squares were diatoms.

During the 19 th century, interest in diatoms surged among early microbe hunters, who competed amongst themselves to describe them. In the late 19 th century, diatoms were some of the first cells for which cell division was described and diagramed. During this fruitful period in the history of microscopy, many early 19 th century observers produced detailed drawings and schematics of the diatoms they observed under a microscope. Then, in the later 19 th century and early 20 th century, fossil records of diatoms began to be studied in detail.

Hustedt produced detailed taxonomic references in the early 20 th century that is still referenced today. In addition, Round’s diatom reference from the 1990s is considered the most complete taxonomic diatom record. More recently, new discoveries in diatom biology continue to be made. Genomic analyses have shown that diatoms contain a mosaic of genes evolved from plant, animal, and bacterial lineages. Many diatom genes have been acquired from ancient symbiont cyanobacteria, highly prevalent oceanic bacteria.

While it is still not fully understood how diatoms glide by secreting raphe mucus, in the 1990s it was determined that an actin motility system plays a role in this process. In addition, it has been recently shown that oceanic diatoms require less iron to live than coastal diatoms, most likely due to their differences in the cellular machinery necessary to conduct photosynthesis.

Most recently, it was shown that diatom phytoplankton like have their own microbiome consortia, and examples of these can be stably assembled in the laboratory. There is a known interplay between bacteria, which outnumber diatoms by orders of magnitude in the world’s oceans, and diatoms. This involves sharing nutrients and metabolites within an ecological niche. Some current researchers are interested in diatom biology from a bioengineering perspective, hoping that unique facets of diatom biology might be useful in human manufacturing or medicine.

It has been proposed that the oil droplets stored within diatoms may be engineered to produce oil for human use. In addition, it has been proposed that the unique biosilica that comprises diatom cell walls might be engineered for use as a drug delivery vehicle with medical and pharmaceutical applications.

Development and evaluation of a diatom-based Index of Biotic Integrity for the Interior Plateau Ecoregion, USA

We developed an Index of Biotic Integrity (IBI) for the Interior Plateau Ecoregion (IPE), USA, which assessed effects of human disturbance on the biotic condition of stream diatom communities. We selected 7 metrics from 59 diatom attributes at reference and impaired sites (training sites), based on significant differences between site groups using the Mann–Whitney U test, high separation power, and a low coefficient of variation. We calculated IBIs by summing metrics for a site after transforming them to a discrete 1, 3, 5 scale or a continuous 0–10 scale. Both discrete and continuous scaling systems successfully separated reference and impaired sites, and IBI scores were significantly related to agricultural land use in IPE watersheds. We then tested the diatom IBI using a 2 nd data set from IPE streams (test sites), processed by slightly different sampling methods and taxonomic references, and classified sites as reference or impaired based on the same criteria used for the training-site data set. Diatom IBI scores differed significantly between reference and impaired streams and correctly classified 80% of sites in the test-site data set. Compared with other diatom IBIs, our measure showed higher separation power among sites and provided an accurate characterization of stream impairment in study watersheds. The developed diatom IBI can be a useful tool for stream and watershed management.

Watch the video: Notes for IB Biology (May 2022).


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