Getting Your Diatoms in a Row

Diatoms are one of the world's primary groups of aquatic unicellular algae. Perhaps only the cyanobacteria rival them for ecological significance. They play a crucial role in the production and fixation of nutrients on which other organisms depend.

Colony of Melosira moniliformis attached to some sort of weed, copyright Frank Fox. The last individual seems to have suffered some unfortunate bisection.

Diatoms live protected in a siliceous test or, to put it another way, they really do live in glass houses. The test is composed of a pair of opposed valves; as noted by Round & Crawford (1990), the arrangement of valves is commonly compared to that of a Petri dish. The valves themselves do not overlap directly in the manner of a Petri dish, but a series of girdle bands around the edge of each valve does overlap. Diatoms come in a range of shapes and structures (artistically minded microscopists [or microscopically minded artists, however you wish to phrase it] have been known to create kaleidoscopic patterns through the careful arrangement of diatoms on a slide) and have commonly been divided between two major groups on the basis of the main symmetry of the valves. Centric diatoms have valves that are radial in appearance when viewed from above whereas pennate diatoms have elongated, more bilateral valves.

Melosira is a widespread genus of centric diatoms found in both fresh and salt water. It might be considered the classic centric: the test is circular in dorsal view and rectangular in side view so the overall shape is that of a hat box. Individual cells remain united by pads of mucilage following division, resulting in the formation of long chains. Species of the genus differ in their preferred habitats. One freshwater species, Melosira varians, is commonly found in polluted or poor quality waters. Conversely, a marine species M. arctica is the most abundant algal species known from the Arctic Ocean, responsible for nearly half the Arctic's primary production. Diatoms lack flagella for most of their life cycle (only their gametes are ever flagellate) so they are not active swimmers. In life, they are either found attached to a substrate or, if floating as planktonic, suspended in the water column by turbulence. One species, M. italica, is known to survive in sediment during quiescent periods of the year and resume growth when winter turbulence returns it to the light (Round & Crawford 1990).

Auxospores of Melosira varians, copyright Kristian Peters.

When diatom cells divide, each daughter cell receives one of the parent's original test valves and secretes a new valve to match it. As noted above, the marginal girdles of the valves overlap, and the new valve is always secreted as the inner partner of this overlap. As a result, and because the glass valves cannot change in size once secreted, successive generations of diatom cells become inexorably smaller over time. Obviously, this process cannot continue indefinitely least the cells dwindle to extinction, so sexual reproduction plays a vital role in resetting the process of diatom development. Centric diatoms like Melosira produce distinct gamete types, motile spermatozoids and immobile eggs (in contrast, many pennate diatoms produce only a single gamete type with no such distinction). Zygotes produced from the fusion of these gametes grow into a cell called an auxospore that differs from normal diatom cells in possessing a organic cell covering instead of solid glass valves. This organic covering may be reinforced with individual siliceous scales, but some Melosira auxospores remain contained and protected within the valves of their parent and lack scales of their own (Medlin & Kaczmarska 2004). The auxospore will not produce a uniform glass test until it has reached full mature size; in Melosira this initial test differs from the standard in being globular rather than pillbox-shaped. The auxospore will then begin dividing into daughter cells in the usual well which will themselves produce test valves of the standard shape. But each of the auxospore's daughters, of course, will receive one of it's initial valves, so as the Melosira chain develops it will remain hemispherical at each end.

REFERENCES

Medlin, L. K., & I. Kaczmarska. 2004. Evolution of the diatoms: V. Morphological and cytological support for the major clades and a taxonomic revision. Phycologia 43 (3): 245-270.

Round, F. E., & R. M. Crawford. 1990. Phylum Bacillariophyta. In: Margulis, L., J. O. Corliss, M. Melkonian & D. J. Chapman (eds) Handbook of Protoctista. The structure, cultivation, habitats and life histories of the eukaryotic microorganisms and their descendants exclusive of animals, plants and fungi. A guide to the algae, ciliates, foraminifera, sporozoa, water molds, slime molds and the other protoctists pp. 574–596. Jones & Bartlett Publishers: Boston.