Marine Microbial Ecology

Microbial Components

Phytoplankton are single-celled algae. Like all plants, they use carbon dioxide and light to produce food in the process of photosynthesis. Most range in size from 1 - 100 micrometres (1 micrometer = 1/1000 millimetre, abbreviation = µm), although some needle-shaped species reach 4 mm in length. Phytoplankton are commonly subdivided into three size classes: picoplankton (<2 µm), nanoplankton (2 - 20 µm), microplankton (> 20 µm). For most of the year, nanoplankton dominate the Southern Ocean with superimposed blooms of microplankton in summer. Picoplankton are much less important in the Southern Ocean than in temperate waters. There are at least 360 different phytoplankton species identified in Antarctic waters, many of which can swim.

Protozoa are single celled animals that consume phytoplankton, bacteria and organic matter [see video images in Agents of death]. Their respiration releases much of the carbon dioxide incorporated by phytoplankton. However they also help remove CO₂ from the atmosphere by converting their microscopic food into their own cell mass, making it available for higher levels of the food web whose bodies and faecal pellets sink into the deep ocean.

Protists is the general term for single celled organisms, including phytoplankton and protozoa. Each litre of surface water from the Southern Ocean can contain from 0.5 million to 60 million protists.

Bacteria are abundant in the Southern Ocean. Typically there are about 600,000 cells per ml of seawater. They are vital components of the microbial community, breaking down particulate matter (cells and detritus), 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. Concentrations in Antarctic waters range from 1 to 4 million particles per ml. They infect phytoplankton, protozoa and bacteria and may be responsible for up to 50% of deaths of marine bacteria. 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.

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