Diatoms
If you were directed to this page by the visual guide and you want to return to the previous step in the guide, click the button to the left.
Cool Facts About Diatoms!
The silica cell walls left behind by dead diatoms can be preserved in the fossil record for very long periods of time. By studying fossil diatoms (preserved in sediments over time), researchers can gain information about past environmental conditions in lakes, rivers, and other inland waters (Bishop et al., 2017).
Deposits of fossil diatoms—called diatomaceous earth—are mined for use in a variety of products. It is high in silicon dioxide (silica), and is usually sold in a powdery form. Some diatomaceous earth products have been evaluated and registered by the EPA for use as pesticides. When insect pests (such as fleas and cockroaches) come into contact with this dusty substance, they become dried out and die (Bunch et al., 2013).
Diatom Morphology
As a group, diatoms are fairly easy to tell apart from other types of algae—yay! This is due to their unusual cell walls, which are composed of silicon dioxide. These silica structures are glassy and transparent, and often intricately embellished: patterns of lines, pores, spines, and ridges give diatoms the appearance of tiny works of art. In fact, microscope-savvy artists have been arranging diatoms into minute images and complex patterns since the 1800’s (Gambino, 2014)! In its entirety, the siliceous cell wall is called a frustule. When identification guides describe the morphology of different diatom taxa, “frustule” is sometimes used the same way that the word “cell” is used in reference to other kinds of algae. For example, “frustules are joined together into ribbon-like colonies.”
Diatom chloroplasts typically have a golden-brown color. Because phycologists like to make things complicated, the chloroplasts of diatoms may be referred to as plastids, or as chromatophores (phycologists don’t actually like to make things complicated—algae just are complicated. It isn’t our fault!). The number and shape of chloroplasts varies among taxa (more on this in a bit). As a result of photosynthetic activity, diatoms also produce and store droplets of oil (Kociolek et al., 2015). These oil droplets are often visible as colorless, roughly spherical bodies within the cell, and may be rather large.
Although many diatoms are motile, they do not possess flagella, and instead rely on a raphe system to enable movement. Those that can move do so in a gliding fashion, sometimes traversing a microscope’s field of view at an impressive pace!
As is the case for other types of algae, diatom identification guides often group taxa together based on morphological similarities—not because those similarities represent a shared evolutionary origin, but because it makes the ID process easier to tackle. At the most basic level, taxa are often partitioned into the following two categories: centric diatoms and pennate diatoms.
Centric Diatoms
The valves of centric diatoms are radially symmetric, meaning symmetric about a point. For most centric diatoms, the frustule is circular in outline when it is seen in valve view (looking straight at the valve face), and the point of symmetry is in the center of the valve face. In girdle view (looking at the "side" of the frustule), frustules are typically square or rectangular in outline. Frustules may occur singly, or form chain-like colonies. Centric diatoms do not possess a raphe system, and are not significantly motile. In most taxa, plastids are numerous and shaped like small disks (Kociolek et al., 2015b).
Centric diatoms.
Pennate Diatoms
The valves of pennate diatoms, on the other hand, are more elongated. There is variation amongst taxa in terms of valve symmetry: many are symmetric to both the midline running along the long axis of the valve as well as to the short axis; some are symmetric to only one axis; some are not symmetric to either axis. Shapes varies as well, but many are roughly ellipse-shaped and are either tapered to a point at both ends, or have rounded, knob-like poles. In girdle view, frustules may be wedge-shaped, rectangular, sigmoid, or have undulating margins…among other possibilities! Pennate diatoms occur singly or in various kinds of colonies.
Not all pennate diatoms possess a raphe system, and consequently not all are motile. Taxa without any raphe system are called “araphid.” If a raphe is present on only one of the two valves, the taxon is “monoraphid.” If a raphe is present on both valves, the taxon is “biraphid.” Amongst those that do have a raphe system, there is variation in other features, including where the raphe is positioned on the valve, the number of raphe branches (individual slits), and whether the raphe is supported by silica struts (Kociolek et al., 2015a). Typically, pennate diatoms have one or few large plastids, but some araphid types have many small plastids (Kociolek et al., 2015b).
Pennate diatoms.
Ecology of Diatoms
Diatoms can be found in virtually any freshwater habitat (and in marine waters, too). We’ve seen them in lakes, creeks, vernal pools, and even puddles of rainwater, just to name a few examples! Raphid diatoms account for the largest proportion of all diatom genera recorded in North American freshwater habitats (Wehr & Sheath, 2015a). This is in line with our own observations: the majority of diatom taxa we’ve documented are raphid diatoms.
In lakes, centric and araphid pennate diatoms are more common in the plankton, while many biraphid types can be found in benthic assemblages, either upon surfaces or attached to them (Kociolek et al., 2015b; Wehr & Sheath, 2015b). In streams, diatoms often dominate benthic algal assemblages (Wehr & Sheath, 2015b). Excessive growth of Didymosphenia geminata has become a nuisance in rivers and streams: massive amounts of the cells’ mucilaginous stalks cover substrates and surfaces, interfering with water intake pipes and impacting ecosystem health (Kociolek et al., 2015a; Wehr & Sheath, 2015a; Wehr & Sheath, 2015b).
Above: a variety of epiphytic diatoms attached to filamentous green algae and moss.
Diatoms Commonly Observed by NJCWST
Aulacoseira (centric diatom)
Ulnaria, Fragilaria, Tabellaria, Asterionella (araphid diatoms)
Cocconeis, Cymbella, Gomphonema, Pinnularia, Nitzschia (raphid diatoms)
Image Gallery
Click the arrows next to each genus listed below to view photos.
Acanthoceras
Amphora
Asterionella
Aulacoseira
Caloneis
Cocconeis
Craticula
Cyclotella
Cymbella
Epithemia
Eunotia
Fragilaria
Gomphonema
Gyrosigma
Hantzschia
Lindavia
Melosira
Meridion
Nitzschia
Pinnularia
Rhoicosphenia
Stauroneis
Surirella
Tabellaria
Ulnaria
Urosolenia
References
Bunch, T. R., Bond, C., Buhl, K. & Stone, D. 2013. Diatomaceous Earth General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/degen.html.
Gambino, M. (2014, September 18). Secretive Victorian artists made these intricate patterns out of algae. Smithsonian Magazine. https://www.smithsonianmag.com/arts-culture/secretive-victorian-artists-made-these-intricate-patterns-out-of-algae-180952720/
Kociolek, J.P., Spaulding, S.A., & Lowe, R.L. (2015a). Bacillariophyceae: The Raphid Diatoms. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.
Kociolek, J.P., Theriot, E.C., Williams, D.M., Julius, M., Stoermer, E.F., & Kingston, J.C. (2015b). Centric and Araphid Diatoms. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.
Spaulding, S.A., Bishop, I.W., Edlund, M.B., Lee, S., Furey, P., Jovanovska, E. and Potapova, M. 2017. Diatoms of North America. Retrieved January 8, 2022, from https://diatoms.org
Wehr, J. D., & Sheath, R. G. (2015a). Introduction to the Freshwater Algae. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.
Wehr, J. D., & Sheath, R. G. (2015b). Habitats of Freshwater Algae. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.