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. The abundance and types of phytoplankton present are being determined using their photosynthetic pigments, by light and electron microscopy, and using electronic particle counters. Controls on population abundance are being studied in relation to oceanography, nutrients and grazers. This information is vital in developing global carbon budgets.