Kalina Manoylov1* and Mohamed Ghobara2
1Dept. of Biological & Environmental Sciences, Georgia College and State University, Milledgeville, GA, United States
2Department of Physics, Freie Universitat Berlin, Berlin, Germany
Abstract
Diatoms are an exceptionally successful group of unicellular microalgae with a large contribution of global primary production in aquatic environments and contributing a significant amount of oxygen to both hydro- and atmospheres. They are fascinating throughout their life and even after death, thanks to their unique cell walls made from ornamented silica. The diatoms include centric species, which may have radial or polar symmetry, and pennates, which include araphid, monoraphid, and biraphid species. Several applications have utilized diatomite, i.e., the fossil form of diatom frustules. To date, many diatoms’ secrets have been understood; however, there are still more hidden. Thus, there is a need for more research on diatom basic biology and applications. Seeking this goal, more people should be encouraged to work on diatoms. Often novice researchers are overwhelmed by the terminology associated with the diverse morphology, the discrepancy between expected features for published descriptions, and the actual observation of those complex 3D organisms, which can be a barrier for more progress. Here, we provide a brief introduction to the beginners with a guide to approach the complex diatom morphology focusing on the tools that can be used for its study.
Keywords: Diatom morphology, tutorial, LM and SEM, frustule morphology
1.1 Diatoms in Brief
Diatoms are unicellular, eukaryotic, microscopic algae (range from 1.5 μm to 5 mm in length, or diameter [1.9]), which maintain large population numbers and contribute considerably to the carbon and oxygen cycle on a global scale [1.8]. This ecologically successful group of algae is present in all aquatic habitats e.g. [1.1, 1.2] and even extends to humid terrestrial places. In aquatic habitats, diatoms are present in the photic zone, i.e., the region of water that light strongly penetrates, as well as in the benthic zone, i.e., the lowest level of water adjacent to the bottom with dim light conditions, depending on water column height and water’s turbidity. Diatoms can exist as planktonic (i.e., suspended in the water column), benthic (i.e., living near the bottom), epiphytic (i.e., adhered to aquatic plants [1.19], Figures 1.2c–d), or epizoic (i.e., adhered to a wide range of marine organisms such as crustaceans, mollusks, and vertebrates [1.19, 1.38]), or epilithic (i.e., attached completely or partially to submerged rocks). The adhesion ability of some diatoms is related to their mucilage secretion from specialized areas within their rigid cell walls (such as examples shown in Figures 1.1d and 1.2c–d). Some diatoms can form colonies in different arrangements such as chains and ribbons (examples shown in Figures 1.1 and 1.2).
Diatoms are a unique group of microalgae for several reasons, but one of the most notable and unique differences is the glass cell walls they possess [1.45]. This cell wall is called the “frustule” and is composed of amorphous hydrated silica that gives it unique properties. In general, the frustule is composed of two pieces that fit together like a petri-dish, meaning that the lower part of the frustule, called the hypotheca, sits inside of the upper part of the frustule, called the epitheca. The frustule volume extends by adding strips of silica called girdle bands (cingulum) to the mantle, i.e., the curved edge of the valve. It should be noted that there are plenty of frustule morphologies that vary between taxa.
Diatoms reproduce both asexually (visible in Figure 1.6) and sexually. Most of the time, they reproduce asexually via binary fission through adding new hypovalves to the parent valves. Those new hypovalves are synthesized inside the silica deposition vesicle (SDV). Only after the new hypovalves have completely synthesized and the protoplast cleavage, as well as the exocytosis of siliceous parts, has occurred, the final splitting apart will occur, leaving two daughter diatoms in place. Because the SDV forms inside of each new cell before splitting into two, each new cell creates a new interior of the petri-dish structure. What this means is that the cell that originally contained the upper part of the petri dish (the epitheca) remains the same size, whereas the cell that originally contained the lower part of the petri-dish (the hypotheca) becomes smaller, since it has now built a smaller hypovalve to fit into it. Repeated cell division, therefore, leads to some part of the resulting population becoming smaller and smaller. Were asexual reproduction the only method by which diatoms reproduces, this could lead the population eventually to become vulnerable to dying out, but diatoms are ingenious and have gotten around this problem. At some point, sexual reproduction is initiated by a number of steps, including meiotic divisions to produce male and female gametes. These cells can find each other, fuse to form a zygote and create a structure known as an auxospore, out of which a new large cell of the diatom species will form, restoring its optimal size, which also depends on the environmental circumstances surrounding the auxospores. Some new research proposes chemical communication with pheromones between the male and female gametes [1.20].
Figure 1.1 Living diatoms as observed under LM, brightfield. (a) Two living cells of Actinoptychus senarius (Ehrenberg) Ehrenberg at the valve view. (b) The valve view of a single living cell of Coscinodiscus wailesii Gran and Angst. (c) The girdle view of a single living cell of Coscinodiscus granii L.F. Gough. (d) Two living cells of Achnanthes brevipes C. Agardh at the girdle view attached to each other with a prolonged stalk for the attachment to the substrate. (e) A living colony of Stephanopyxis turris (Greville) Ralfs with visible linking spines. (f) A living colony of Odontella longicruris (Greville) M.A. Hoban with discoid chloroplasts. Copyright reserved Mary Ann Tiffany, used with her permission. The identification was carried out by Mary Ann Tiffany. All the scale bars are 50 μm.
Figure 1.2 Live centric (a, b) and pennate (c–h) diatoms. (a, b) Pleurosira laevis (Ehrenberg) Compère shown from girdle view, frustules with numerous girdle bands in straight filaments with discoid chloroplasts, chains connected with mucilage pads released from ocelli; in (b), visible diameter size restoration within the chain; (c, d) Epiphytic diatoms on Cladophora glomerata (Linnaeus) Kützing, in (c) focus on Cocconeis spp. With visible one flat C-shaped plastid; in (d) focus on Rhoicosphenia spp.; (e) Cymbella sp. partial valve and girdle views, visible chloroplast bridge connecting the chloroplast plates; (f) Eunotia cf. camelus Ehrenberg in girdle view with visible discoid chloroplasts; (g) Amphora ovalis (Kützing) Kützing with H shaped chloroplast; (h) Rhoicosphenia sp. girdle view with visible lobes of the plastid. Scale bars, 10 μm. These micrographs were obtained and identified by KMM.
Figure 1.3 Specific diatom morphology gleaned from images with whole and partial valves views of Navicula oblonga (Kützing) Kützing; (a) live linear-lanceolate cell with visible two plates like brown chloroplasts, visible linear striae, and proximal raphe ends deflected slightly toward the secondary side. (b) Valve view after cleaning, axial area is linear, widening toward the central area and about twice the width of the raphe. The central area orbicular. The raphe is lateral, becoming filiform near the proximal ends, which are simple. Central striae do not reach valve edge. These micrographs were obtained and identified by KMM.
Details shown:
1 Central area is more or less orbicular and two to three times wider than the axial area. Proximal raphe ends are simple and barely wider than the raphe. Striae are finely lineate and the individual areolae are difficult to distinguish.
2 Round,