Image shows a number of diatoms with their characteristic spherical shape

What are they?

Diatoms are single-celled phytoplankton of the class Bacillariophyceae, and constitute a major component of marine phytoplankton communities. Different genera of diatoms vary greatly in size, from 2 to 500 micrometres (0.5 mm) in diameter. They are unique in having a cell wall composed of the inorganic silica, a component of glass, quartz and opal. Indeed, diatoms are sometimes called “living opals” due to the brilliant structural colouration of their siliceous cell wall, or frustule. Most diatoms are autotrophs, using chlorophyll a  and other photosynthetic pigments to photosynthesise their food using sunlight. Diatoms are stunningly diverse and the total number of species in the group remains uncertain, however, there are 12,000 described species and likely 1000s more as yet undescribed.

Ecological importance

Diatoms are vital to the world’s ecosystems. They dominate the base of marine foodwebs, providing a key food resource for countless other species, which in turn feed the larger species higher up the food web. In addition,  they produce an estimated 20 – 50% of the oxygen in the atmosphere. The majority of diatoms are most likely not consumed but rather die and sink, and therefore are a key part of global nutrient cycles as they transfer silica, carbon and other elements to the sea floor.  

 Life cycle

Diatoms have a remarkable and unusual life cycle imposed by the constraint of their silica frustule. This cell wall is divided into two parts, the larger epitheca and smaller hypotheca. When a diatom reproduces through binary fission, each daughter cell inherits one of the thecae. These silica structures cannot expand, so the cell that inherits the smaller hypotheca cannot grow as large as its parent was. Over successive generations, diatoms thus become smaller and smaller. Eventually, they undergo sexual reproduction to produce an auxospore, which grows into a full-sized diatom. Diatoms can reproduce rapidly, doubling their population every 24 hours in the right conditions, although they only live for a maximum of six days.

By Peter Ntsako Lahiff