Diatoms (class Bacillariophyta) are a type of mainly aquatic, photosynthetic algae. Similar to many other algae, they can live as unicellular organisms, colonial, or filamentous. Unlike most algae, though, they have a solid shells made of silica. Because of such shells, diatoms have major economic importance in industry while also having a major role in biological and chemical processes.
Almost all of the extant and extinct species of Bacillariophyta are aquatic. They are found in marine and freshwater ecosystems as well as brackish water (Bold, 1978). Diatoms can also be found in terrestrial environments in the soil where moisture is at least periodic (Bold, 1978). They can be found all around the world; from the tropics to the arctic zones (Tiffany, 1968). In water, diatoms live attached to rocks, plants, or be free floating but they are best known for being part of the drifting planktonic mass (Garrison, 1997).
Of the 200 genera and 5000 species known, all are eukaryotic and photosynthetic (Alexopoulos, 1967). They contain chlorplasts that have been found to have numerous photosynthetic pigments giving the chloroplasts a typically golden brown color (Garrison, 1997) Photosynthetic pigments include chlorophylls a and c (green), as well as B-carotene (yellow), fucoxanthin (brown), and small amounts of diatoxanthin, diadinoxanthin, and other carotenoids (Bold, 1978). Because of the photosynthetic nature of diatoms, they have traditionally been placed in the plant kingdom, but many scientists today place them in the kingdom Protista (Garrison, 1997)
The external morphology of diatoms is based on the solid silica shell or frustule that they all have in common. It is the shell that is used in species identification and comparison. All diatom skeletons are made of silica and consist of two parts or frustules that fit inside each other like a petri dish: the epitheca and the hypotheca (Alexopoulos, 1967). The hypotheca is smaller and fits inside the larger epitheca. The shape of the frustule is the defining feature that is used to break the diatoms into two distinct classes: the centric or Centrobacillariophyceae and the pennate or Pennatibacillariophyceae. The pennate diatoms are usually radially symmetrical while the centric diatoms are generally bilaterally symmetrical (Alexopoulos, 1967). These two classes can be found in both marine and freshwater habitats, but centric diatoms are more likely found in the oceans while the pennate diatoms are predominately found in freshwater (Round, 1990).
Reproduction of diatoms can be either sexual or asexual (cellular division). Cellular division is the ordinary method of reproduction in diatoms (Bold 1978). In this method, during the processes of mitosis and cytokinesis, the two valves (hypotheca and epitheca) separate slightly and the division of the protoplast occurs in a plane parallel to the valves. Both parts of the parent frustule become the epitheca of the new cells resulting in one of the two cells being smaller than the parent (Bold, 1978). The progressive reduction in individuals' size is overcome because of the flexibility of the new cell walls (Alexopoulos, 1967) or by sexual reproduction.
The predominant method of reproduction in diatoms is sexual reproduction. Bold (1978) writes that sexuality in diatoms has been associated with diatom size: only individuals less than a certain size can reproduce sexually. Sexual reproduction in the centric diatoms is oogamous, meaning that this process has a motile sperm or nonmotile spermatium that reaches a nonmotile egg. The pennate diatoms are a bit different. This order has isogamous sexual reproduction meaning that the gametes (egg and sperm) are indistinguishable. The "offspring" of diatoms are called auxospores. These new diatoms will increase in volume while forming vegetative cells and solid silica shells (Alexopoulos, 1967).
The importance of diatoms must not be overlooked. These tiny organisms have been around for billions of years and play major roles in chemical and biological processes. Diatoms are estimated to be responsible for 20% to 25% of all the organic carbon fixation, are major sources of atmospheric oxygen, and are a major food source for aquatic microorganisms and insect larva (anonymous, 1999). Tiffany (1968) writes that marine diatoms are considered so important that they have been called "grass of the sea". This is because diatoms are major contributors to primary productivity in the oceans and create a beginning to the food chain. Another important use of diatoms in the biological realm is in water quality testing. Research by Dixit et al (1999) show that diatoms can be used for present water quality but also used to determine former water quality and trends over the years. The sediments of lakes and rivers hold chemical and biological clues to the environment and water quality of the past and present. Diatoms are one of these clues. Because diatoms are ecologically diverse in almost every freshwater habitat, the dead and living diatoms can be found in the substrate. Diatoms in the first centimeter represent the current condition of the water, while the diatoms found in deeper sediment are representative of past water quality. The high reproductive rates of diatoms makes them respond quickly to environmental changes and many diatom species, as well, have specific tolerances for water quality. An important result of this research is that diatoms can be used to determine former water quality. This means that pre-colonial water quality can be estimated and used as a baseline to work from in determining anthropogenic effects on water quality. Diatoms help biologists see trends from past to present based on the sheer number, diversity and tolerance of diatoms in the sediment (Dixit et al, 1999)
Economically and industrially, diatoms are of huge importance. Billions of years of diatom frustules being naturally fossilized has created huge deposits of these shells or diatomaceous earth particularly in western United States. One deposit in Lompoc, California is as deep as 1400 feet and covers and area of twelve square miles (Alexopoulos, 1967). These deposits are mined to be used as filtering aids, abrasives, cleansers, and paints. Other deposits hold pockets of oil. It is estimated that a significant portion of the world's oil supply comes from diatom fossil beds (Prescott, 1968). For the average person, all this means that the wine we drink may have been filtered with the aid of diatom fossils, or the toothpaste we use may clean our teeth with the help of diatom fossils as an abrasive, and the gas we use to drive our cars may all come from a diatomaceous origin.
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