Diatoms

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Diatoms

Diatoms are microscopic, single-celled algae found in both fresh water and marine ecosystems. Many diatom species are planktonic, although some are found clinging to surfaces or in sediments. Diatoms are surrounded by intricate glass-like outer shells or frustrules partially composed of silicon. Different species of diatom can be identified based upon the structure of frustrules, which are extremely beautiful when examined under a microscope.

Diatoms belong to the taxonomic phylum Bacillariophyta. There are approximately 10, 000 known diatom species. Of all algae phyla, diatom species are the most numerous. Diatoms frustules are used to differentiate species. Diatom frustules are composed of very pure hydrated silica within a layer of organic, carbon containing material. Frustules are comprised of two parts: the larger upper portion of the frustule is called the epitheca and the smaller lower piece is the hypotheca. The epitheca fits over the hypotheca like the lid fits over a shoebox. The singular diatom cell lives protected inside the frustule halves like a pair of shoes snuggled within a shoe box.

Frustules are very ornate, having intricate designs delineated by patterns of holes or pores. The pores that perforate the frustules allow gases, nutrients, and metabolic waste products to be exchanged between the watery environment and the algal cell. The frustules themselves may exhibit bilateral symmetry or radial symmetry. Bilaterally symmetric diatoms are elongated and referred to as pennate. Radially symmetric diatom frustules are called centric. Centric diatoms tend to be round and flattened, like apple pies. Because they are composed of silica, a very inert material, diatom frustules remain well preserved over vast periods of time within sediments.

Diatom frustules found in sedimentary rock are microfossils. Because they are so easily preserved, diatoms have an extensive fossil record. Specimens of diatom algae extend back to the cretaceous period, over 135 million years ago. Some kinds of rock are formed nearly entirely of fossilized diatom frustules. Rock that has rich concentrations of diatom fossils is known as diatomaceous earth, or diatomite. Diatomaceous earth, existing today as large deposits of chalky white material, is mined for commercial use in abrasives and in filters. The fine abrasive quality of diatomite is useful in cleansers, like bathtub scrubbing powder. Also, many toothpaste products contain fossil diatoms. The fine porosity of frustules also makes refined diatomaceous earth useful in fine water filters,

KEY TERMS

Bacillariophyta— The biological phylum of organisms composed of diatoms.

Carbon fixation— The removal of atmospheric carbon in the form of carbon dioxide gas in air to be converted into organic molecules like sugar by primary producers.

Chloroplast— Green organelle in higher plants and algae in which photosynthesis occurs.

Diatom— A microscopic, single-celled alga having complex two-sided cell walls composed of silicon.

Diatomite— Sedimentary rock rich in fossilized diatoms, also called diatomaceous earth.

Frustule— The silicon containing, complex, ornate cell wall structures of diatomous algae.

acting like microscopic sieves that catch very tiny particles suspended in solution.

Sediments containing fossilized diatoms provide important information about past environmental conditions. Diatom deposits indicate environmental conditions. Certain conditions are more favorable to different species of diatoms. Changes in both the rate of deposition and the species composition or the sediments provide clues about prehistoric climates.

Ecologically vital, diatoms account for just under one quarter of photosynthetic production of organic material on Earth. In addition, they may be found in extremely high concentrations, or blooms, where their abundance makes them important food sources in aquatic ecosystems. They are a major food source for many microorganisms, aquatic animal larvae, and grazing animals like mollusks (snails).

Diatoms most often reproduce by simple asexual reproduction. During division, the diatom cell divides, the epitheca and hypotheca separate, one daughter cell remaining in each half. The two cells then produce a new hypotheca or epitheca, whichever is needed. Diatoms do reproduce sexually, but not with the same frequency.