Microscopic algae in glass shells make a huge share of Earth's oxygen
Diatoms are single cells encased in intricately patterned walls of silica, glassy enough to earn the nickname living opals. They produce an estimated 20 to 50 percent of the planet's yearly oxygen, and dust from their ancient shells blows across the Atlantic from the Sahara to fertilise the Amazon.
These microalgae live in oceans, rivers and soils worldwide, alone or in colonies shaped like ribbons, fans, zigzags or stars. Cells range from 2 to 2,000 micrometres, and in good light and nutrients a population can double in about a day, though an individual cell lives at most around six days. About 12,000 species have been described, and estimates of the true number run to roughly 200,000, split between radially symmetric centric forms and elongated pennate ones.
Their defining feature is the frustule, a two-part silica case whose halves fit together like a box and lid, one slightly smaller than the other. Pores, ribs and spines on the glass create structural colour of unknown purpose; ideas include camouflage and communication. The proteins that ferry silica into the wall are unique to diatoms, and diatoms draw more than 6.7 billion tonnes of silicon from the water every year. Most drift passively on currents, though pennate species can glide and male cells of centric species swim with flagella to find eggs.
Their biochemistry holds oddities. Unusually for photosynthesisers, they run a urea cycle like animals do. Some carry former cyanobacteria that have stopped photosynthesising but still fix nitrogen. Going the other way, certain dinoflagellates have swallowed diatoms so recently in evolutionary terms that the captives keep intact genomes, having lost only their glass, the only shell-less diatoms known. The oldest fossil is a Hemiaulus in Late Jurassic amber from Thailand.
Ecologically they are giants. Seasonal blooms account for about half of ocean photosynthesis, feed marine food webs and carry carbon into the deep sea, and diatoms make up nearly half the organic matter in the oceans. Each year around 27 million tons of their shell dust, much of it from the dry lakebeds of Chad's Bodélé Depression, crosses to South America. Deposits of old shells form diatomaceous earth, crumbled into powder for water filters, mild abrasives, cat litter and even to stabilise dynamite. Scientists also use living diatoms to track water quality.
Source: Diatom
