Protists are arguably the most important organisms in the world’s oceans. These microscopic, single-celled plants (phytoplankton or algae) and animals (protozoa) comprise the base of the food web on which, essentially, all other marine life depends. Along with bacteria, they make up more than 95% of the biomass of living organisms in the sea. Marine protists also play a major role in the global carbon cycle, absorbing about 50% of the carbon dioxide produced by living things and producing about 50% of the oxygen we breathe. Some species even produce chemicals which, when released to the atmosphere, promote the formation of clouds.
At the level of single-celled organisms the distinction between plants and animals blurs, because of the strategies used to gain nutrients for growth. Plants derive their energy from sunlight, via photosynthesis, while animals gain theirs by consuming other organisms or organic detritus. While protists share these characteristics with either plants or animals, they also differ in many ways. For example, many phytoplankton can feed on bacteria and other algae, while several protozoa that usually graze on other single-celled organisms, generate their own food via photosynthesis. They do this by eating algae and digesting everything except the photosynthetic machinery (chloroplasts), which they sequester in special vesicles.
Protists range in size from less than a micrometre (one thousandth of a millimetre) to over a millimetre, and can only be identified with a light or electron microscope. However, what they lack in size they make up for in species diversity and abundance — with concentrations reaching millions of cells per millilitre of seawater.
The recently published book, Antarctic Marine Protists, edited by Fiona Scott and Harvey Marchant of the AAD, describes over 550 species of protist from south of the Antarctic Polar Front. The book was published jointly by the Australian Biological Resources Study and the AAD. It draws together information from the widely dispersed literature and provides descriptions, illustrations and a comprehensive bibliography of these unique organisms.
Harvey Marchant, Adaptations to Climate Change Programme, AAD