Diatoms are single-celled photosynthetic algae with cell walls made of transparent silica. These walls, called frustules, consist of two interlocking halves that fit together like a Petri dish. Diatoms occur singly or in chains and are major contributors to oxygen production. They are found in oceans, freshwater bodies and soils and belong to the class Bacillariophyceae.
Explanation
The diatom cell wall, or frustule, is composed of hydrated silicon dioxide and is ornamented with intricate pores and grooves that are species-specific. Each frustule has an upper valve (epitheca) and a lower valve (hypotheca) that overlap. Diatoms range in size from about 2 µm to over 2 mm and exhibit two main morphological groups: centric diatoms, which are radially symmetrical and often planktonic, and pennate diatoms, which are bilaterally symmetrical and may attach to surfaces. Most diatoms reproduce asexually by mitosis, with each daughter cell inheriting one of the parental valves and secreting a new, smaller valve. Over successive divisions, average cell size decreases until a sexual reproduction phase restores maximum size. Diatoms perform oxygenic photosynthesis using chlorophylls a and c and the accessory pigment fucoxanthin, giving them a golden-brown colour. They are primary producers in aquatic ecosystems and contribute an estimated 20 %–30 % of the oxygen on Earth. Through photosynthesis and sinking of their silica shells, they play a significant role in global carbon and silicon cycling. Diatom blooms form the base of marine food webs, supporting zooplankton and higher trophic levels.
Ecology and uses
Diatoms inhabit a range of environments from polar oceans to tropical lakes and even moist soils. Species diversity reflects adaptations to light, nutrients and turbulence. Some diatoms form mucilaginous stalks or are motile due to a longitudinal slit called a raphe. Fossilised frustules accumulate on ocean floors to form diatomaceous earth, which is mined for use as a filtration medium, mild abrasive, insulation and insecticide. The regular nanoscale patterns of frustules inspire research into biomimetic materials and photonic devices. Because diatoms are sensitive to changes in water chemistry, assemblages are used as indicators of water quality and past environmental conditions in paleolimnology. Artists and microscopists have long admired their geometric designs and symmetry.
These unique algae are important members of plankton communities and have both ecological and practical significance.
Related Terms: Algae, Phytoplankton, Frustule, Silica, Diatomaceous earth