Protozoa are single celled animals that are consumers of phytoplankton, bacteria and organic matter. Their respiration releases much of the carbon dioxide that was incorporated by the phytoplankton. However they also help remove CO2 from the atmosphere by repackaging food into faecal pellets that sink into the deep ocean. Phytoplankton and protozoa together are called Protists. Each litre of surface seawater contains about a million protists.
Bacteria are abundant in the Southern Ocean.
They break down particulate matter, releasing nutrients for use by other organisms and releasing CO2. They also take up dissolved organic matter, converting it to cell mass, and making it available to grazers. Viruses are the most abundant biological agents in seawater. Bursting cells release their contents into the water, where they fuel bacterial growth. As each virus infects a particular species of microbe, viruses may be important in controlling the abundance and composition of microbial communities in Antarctic waters.