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Coccolithophorids, a type of phytoplankton

Microscopic organisms

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

Phytoplankton Photo: ACE CRC

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.

Size

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.

Abundance

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

Types

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.