Microscopic organisms

Image seen through microscope of tiny phytoplankton shapes
Phytoplankton (Photo: ACE CRC)
White on back scan of a heliozoan; round body with tiny fibres protruding.SYBR Green 1 stained filter showing two highly fluorescent protist cells (yellow), many less fluorescent bacteria (green/yellow) and large numbers of very small viruses (pale green) (Photo: Andrew Davidson)

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

They 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.

In addition, these microscopic organisms 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 such as the greenhouse effect and the depletion of the ozone layer.

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

  • algae, which are single celled plants also known as phytoplankton (from the Greek, meaning drifting plants)
  • protozoa, which are single celled animals also known as zooplankton (also Greek, meaning drifting animals)
  • bacteria, which are the most abundant organisms on earth
  • viruses, which are the most abundant biological agents in seawater where they infect phytoplankton, protozoa and bacteria, and may be important in controlling their abundance and composition


Bacteria are the smallest micro-organisms, ranging from between 0.0001 and 0.001mm in length. Phytoplankton and protozoa cover about the same size range. The smallest are phytoplankton and protozoa which are about 0.001 in diameter and range up to about 0.25mm. The largest phytoplankton and protozoa can be seen with the naked eye, but most can only been seen under a microscope. Even though it's difficult to see these organisms, they are present in huge numbers.


Each litre of sea water contains

  • about a billion bacteria
  • about a 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. What they lack in size they make up for in number.


Phytoplankton: pasture of the sea

Phytoplankton are plants and derive their energy for growth from sunlight during the process of photosynthesis.

There are a large number of phytoplankton species, approximately 400 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 wall, and if so, what this 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 of these algae are toxic, the vast majority are not.

Phytoplankton can be regarded as the pasture of the sea, providing the food and energy for the Southern Ocean food web. For much of the year, the Antarctic seas are covered by ice. This, coupled with the low angle of the sun and short length of days, limits the light available for photosynthesis by the algae. Thus many species diminish in number during the winter months and bloom again in spring and summer when light conditions become more favourable.

As well as phytoplankton that float in the upper layers of the ocean there is algae and single-celled animals called protozoa that feed on bacteria. The algae and protozoa are eaten by larger animals of 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, generally wherever there is moisture, 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. It is remarkable that such a small single unit can carry out all the activities performed by larger animals. They can feed directly on the phytoplankton, bacteria, other protozoa, or on organic matter. Some protozoa can withdraw from unfavourable conditions by using a dormant stage of their life cycle. This can be used for protection or dispersal of the species.


Bacteria have been found everywhere 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 was deposited in the Antarctic centuries ago. The most common types of bacteria vary in size from 0.0005mm to 0.005mm. 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 and 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.

This page was last modified on 20 February 2012.