Diatoms are the most common of the eukaryotic algae. They belong among the heterokonts along with the golden and brown algae, as shown by the pigmentation of the chloroplasts and the structure of the gametes, although these have lost the mastigonemes found in other such groups. Most diatoms are single cells and live between two silicate shells collectively called a frustule or a test, one overlapping the other like the two halves of a petri dish. When the creature divides, each half keeps one side and grows a new shell within it, meaning that smaller and smaller diatoms are produced. Once a certain minimal size is reached, the cell undergoes meiosis to produce heterokont gametes, which fuse to form a zygote that divides and releases new large diatoms.

The diatoms are divided into two main groups: the pennate diatoms, which are bilaterally symmetrical, and the centrate diatoms, which are radially symmetric. The latter appear to be paraphyletic to the former. Both groups show a wide diversity of forms, some quite beautiful and ornate. A few diatoms form radial or filamentous colonies. Most diatoms are non-motile but some are capable of an oozing motion. Diatoms are found in both freshwater and marine environments, and are especially important in the latter, where they are estimated to contribute up to 45% of total oceanic primary (Mann, 1999). They also play a key role in the regulation of the silicon cycle in the modern ocean (Treguer et al., 1995; Yool & Tyrrell, 2003), having displaced the radiolarians and siliceous sponges from that role in earlier geological epochs (Racki & Cordey, 2000; Kidder & Erwin, 2001).

Evolutionary history

The earliest known fossil diatoms date from the early Jurassic (~185 Mya; Kooistra & Medlin, 1996) although recent genetic (Kooistra & Medlin, 1996) and sedimentary (Schieber, Krinsley & Riciputi, 2000) evidence suggests an earlier origin. Medlin et al. (1997) suggest that their origin may be related to the end-Permian mass extinction, after which many marine niches were opened. Major deposits of fossil diatoms are found as far back as the early Cretaceous, and some rocks (called diatomaceous earth) are composed almost entirely of them.

Observational notes

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. Diatoms 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. They can be made to emerge from the mud by putting black paper around the jar and letting direct sunlight fall upon the surface of the water. The diatoms, within a day or less, will come to the top in a scum which can be easily isolated and secured.

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. Marine diatoms can be collected by direct water sampling, though benthic forms can be secured by scraping barnacles, oyster shells, and other shells.

Fresh-water and marine diatoms appear in greatest abundance early in the year as part of the phenomenon known as the "spring bloom", which occurs as a result of the availablity of both light and (winter-regenerated) nutrients. They are comparatively scarce in summer when nutrient concentrations are reduced, but often reappear in autumn (though not as abundantly as in spring) when water column mixing re-introduces nutrients. Some diatom species survive the poor growing conditions of the winter as "hibernating" resting spores.

The silicious shells of diatoms are among the most beautiful objects which can be examined with the microscope. To obtain perfectly clean mounts requires considerable time and patience, but once the material is cleaned, preparations may be made at any time with very little trouble.

The terms diatomaceous earth, diatomite and kieselguhr all refer to mineral deposits of fossiled diatoms.

Note : Much of the text above (from Living diatoms on) is from Methods in Plant Histology from the 1900s. Handle with care!

References

• Kidder, D. L. and Erwin, D. H. (2001). Secular distribution of biogenic silica through the Phanerozoic : Comparison of silica-replaced fossils and bedded cherts at the series level. J. Geol. 109, 509-522.

• Kooistra, W. H. C. F. and Medlin, L. K. (1996). Evolution of the diatoms (Bacillariophyta) : IV. A reconstruction of their age from small subunit rRNA coding regions and the fossil record. Mol. Phylogenet. Evol. 6, 391-407.

• Mann, D. G. (1999). The species concept in diatoms. Phycologia 38, 437-495.

• Medlin, L. K., Kooistra, W. H. C. F., Gersonde, R., Sims, P. A. and Wellbrock, U. (1997). Is the origin of the diatoms related to the end-Permian mass extinction? Nova Hedwegia 65, 1-11.

• Racki, G. and Cordey, F. (2000). Radiolarian palaeoecology and radiolarites : is the present the key to the past? Earth-Science Reviews 52, 83-120.

• Schieber, J., Krinsley, D. and Riciputi, L. (2000). Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature 406, 981-985.

• Treguer, P., Nelson, D. M., Van Bennekom, A. J., DeMaster, D. J., Leynaert, A. and Queguiner, B. (1995). The silica balance in the world ocean : A reestimate. Science 268, 375-379.

• Yool, A. and Tyrrell, T. (2003). Role of diatoms in regulating the ocean's silicon cycle. Global Biogeochemical Cycles 17, 1103, doi:10.1029/2002GB002018.