Microscopic organisms are tiny life forms, often consisting of a single cell. They are very sensitive to change.

Microscopic organisms are vitally important in the food chain and to the health of our planet. They are the base of the marine food web and, directly or indirectly, are food for everything else in the open sea.

Microscopic organisms also have a role in maintaining the Earth’s atmosphere. They help remove carbon dioxide and release chemicals that help form clouds. Scientists study microscopic organisms in the Antarctic so they can better understand atmospheric changes and the depletion of the ozone layer.

The four main types of micro-organisms in the ocean are:

  • Algae — these are single celled plants also known as phytoplankton (from the Greek, meaning drifting plants).
  • Protozoa — these are single celled animals also known as zooplankton (from the Greek, meaning drifting animals).
  • Bacteria — the most abundant organisms on earth.
  • Viruses — the most abundant biological agents in seawater. They infect phytoplankton, protozoa and bacteria, and may be important in controlling their abundance and composition.

'Protists' is the general term for single celled organisms, including phytoplankton and protozoa.


Bacteria are the smallest micro-organisms, ranging from between 0.0001 mm and 0.001 mm in size. Phytoplankton and protozoa range from about 0.001 mm to about 0.25 mm. The largest phytoplankton and protozoa can be seen with the naked eye, but most can only been seen under a microscope.


These organisms may be tiny, but they are present in huge numbers.

Each litre of sea water contains:

  • between 1 and 4 billion viruses
  • about 1 billion bacteria
  • about 1 million phytoplankton
  • about half a million protozoa

When the weight and number of these organisms are multiplied together, micro-organisms are the greatest biomass on Earth.


Phytoplankton: pasture of the sea

Phytoplankton are plants. They derive their energy for growth from sunlight through the process of photosynthesis.

There are approximately 400 phytoplankton species in the Southern Ocean. The various species are distinguished by their shape and size, or by their photosynthetic pigments. They can also be distinguished by whether or not they are enclosed by a cell wall, and if so, what this cell wall is made from. Many can also be identified by the arrangement of the tiny scales and spines that cover their surface. These scales are so small that details of their shapes and patterning can only be seen by using a high magnification electron microscope. Very few phytoplankton are toxic.

Phytoplankton can be thought of as the pasture of the sea. They provide the food and energy for the Southern Ocean food web. For much of the year the Antarctic seas are covered by ice. This, combined with the low angle of the sun and the short days, limits the light available for photosynthesis. As a result, many species diminish in number during the winter months and bloom again in spring and summer when light conditions are more favourable.

In the upper layers of the ocean, the phytoplankton are accompanied by single-celled animals called protozoa. Protozoa feed on bacteria. Both phytoplankton and protozoa are eaten by larger animals in the Southern Ocean, including krill.


Protozoans occur in a great variety of habitats including the sea, freshwater, soil and the bodies of other organisms. In fact, wherever there is moisture, generally protozoans occur. Some species are limited to cold water and are only found in the Southern Ocean.

These minute animals have all the functions of larger creatures: they take in food, excrete wastes, reproduce and communicate. They feed directly on phytoplankton, bacteria and other protozoa. Their respiration releases much of the carbon dioxide incorporated by phytoplankton. However, they also help remove carbon dioxide from the atmosphere by converting their microscopic food into their own cell mass. It is remarkable that such a small single unit can carry out all the activities performed by larger animals. Some protozoa can withdraw from unfavourable conditions by becoming dormant.


Bacteria have been found everywhere that life can exist. They occur in the depths of the ocean and in the ice sheets at the poles. Some have been recovered and germinated from ice and snow which fell centuries ago in the Antarctic. The most common types of bacteria vary in size from 0.0005 mm to 0.005 mm. This means that a row of 100 of the largest bacteria would reach across the full stop at the end of this sentence.

Most bacteria are consumers. A very small minority can cause disease in other living things, but the vast majority feed on wastes or dead organisms. These are decomposer organisms. They are of vital importance to life on Earth because they circulate the elements that make up living things.

A few bacteria are producers. Some are like plants in that they produce energy by photosynthesis. Other bacteria use chemicals such as sulphur, iron, hydrogen gas or nitrogen compounds in a process called chemosynthesis.


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. Each virus infects a particular species of microbe. Viruses may be important in controlling the abundance and composition of microbial communities in Antarctic waters.