The most abundant forms of life in the seas are organisms less than 1/1000 in. in their longest dimension – the minute protists, which include algae, flagellates, bacteria, yeasts, and fungi. Some of these are plant-like, relying on a photosynthetic source of energy for their life processes; others behave as particle-ingesting animals, while still others, such as the yeasts and bacteria, depend on the absorption of penetrable nutrients from their surroundings. The photosynthetic algae and flagellates live mostly in the illuminated-surface layers of the ocean; other protists are to be found in a greater variety of depths and as inhabitants of bottom sediments.

By taking their nutrients from sea water, microorganisms appear as the producers of organic matter, and hence initiate the marine food chains that support the world's commercial fisheries. These are also primarily responsible for breaking down the organic remains of dead plants and animals to elemental nitrogen, phosphorus, carbon, and sulphur, thereby balancing the economy of these important constituents of living matter.

The specialised techniques of microbiology commonly include the setting up of live, axenic (single species) cultures of marine micro-organisms that can be maintained indefinitely in the laboratory. These may then serve as a basis for experimental studies of life cycles and habits, physiological characteristics, temperature and salinity optima for growth, and so on. This type of information assists the interpretation of how each organism is sustained in its native environment, and in what manner it may influence other species. Investigation of the biochemical activities, communal relationships, and successional development of the differing marine microbial populations are so directed towards a broader understanding of factors controlling the fertility of the seas.

by Terence Martin Skerman, M.SC., Senior Scientific Officer, New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, Wellington.