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

Microscopic organisms are crucial within 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. They 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 naked eye can see only the largest phytoplankton and protozoa. Most can only be seen under a microscope.


These organisms may be tiny, but they are present in huge numbers. In weight and numbers, micro-organisms are the greatest biomass on earth.

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


Phytoplankton: pasture of the sea

Phytoplankton are plants. They derive their energy for growth from sunlight through the process of photosynthesis. Phytoplankton provide the food and energy for the Southern Ocean food web.

There are approximately 400 phytoplankton species in the Southern Ocean. They can be distinguished by a number of characteristics:

  • Shapes and sizes
  • Photosynthetic pigments
  • If they are enclosed by a cell wall, and the material of this wall
  • Arrangement of tiny scales and spines on their surface

These species are so small that details can only be seen by using a high magnification electron microscope. Very few phytoplankton are toxic.

In the Antarctic seas, the light available for photosynthesis is limited for much of the year. Ice covers much of the surface, the angle of the sun is low, and days are short. As a result, many species diminish in number during winter and bloom again in summer when light conditions are better.

Both phytoplankton and protozoa live in the upper layers of the Southern Ocean. Protozoa are single-celled animals that feed on bacteria. Phytoplankton and protozoa are both eaten by larger animals, including krill.


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

Remarkably, these tiny animals have all the functions of larger creatures. They take in food, excrete wastes, reproduce and communicate. They feed 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. They convert their microscopic food into their own cell mass. Some protozoa can withdraw from unfavourable conditions by becoming dormant.


Bacteria are 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 germinated from Antarctic ice and snow which fell centuries ago. The most common types of bacteria vary in size from 0.0005 mm to 0.005 mm. It would take a row of 100 of the largest bacteria to 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. The vast majority are ‘decomposer organisms’, and feed on wastes or dead organisms. They are of vital importance to life on Earth. 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 sulphur, iron, hydrogen gas or other chemicals 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. They 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 microbial communities in Antarctic waters.