Marlon Lewis: The vast majority of action in the sea is carried out by these microscopic organisms

Those who have travelled by ship, or enjoyed the idle pleasures of peering at the sea from an aircraft window, appreciate that the ocean is a constantly changing mosaic of different colours. The deep purple of tropical waters give way to a stony grey-green as one approaches the shore; these are but two end-members in a spectrum of ocean colour.

For most of the ocean, removed from the direct influence of land, these changing colours – and dramatic changes in ocean transparency which accompany them – are the result of changes in the concentration of microscopic algae and bacteria, the phytoplankton. As well as changing the colour and transparency of the ocean, these freely drifting organisms form the base of the vast majority of the marine food chain.

Ocean phytoplankton are effectively plants; like the more familiar trees, crops and grass, they have pigments and metabolic machinery which make it possible for them to carry the process of photosynthesis. They capture the sun's energy, transform it into organic matter from carbon dioxide and release oxygen as a by-product. In this way, they are able to provide the organic matter which sustains other organisms, ranging from microscopic animals to the largest of ocean whales. Because of the central importance of photosynthesis to biological processes in the sea, the rate at which it proceeds is called primary production.

In addition to being the foodstock for ocean life, the tiny phytoplankton are mighty geochemical factories. Our present atmosphere, and in particular the oxygen that sustains most animals, is almost entirely derived from the activities of photosynthetic bacteria living some 2.5 billion years ago. The oil and gas deposits that we exploit to fly our planes, drive our cars and heat our houses were produced by the sinking and burial of phytoplankton in the geological past. The absorption of solar energy warms and stabilises the upper ocean and as a result affects circulation in both atmosphere and ocean.

The phytoplankton exhibit a vast diversity of species, and range in size from less than a micron to half a millimeter – a factor of over 1,000. Many are really bacteria, while others have all of the cellular machinery commonly found in higher plants. They have diversity in metabolism as well, with some species able to not only photosynthesise, but also to ingest other organisms. Some species produce dimethyl sulphide, which has been implicated as a cloud-forming compound in the atmosphere. Still others are toxic, and contribute to illness and many human deaths each year.

The diversity of the phytoplankton means that they support very different food chains or, more properly, food webs. The smallest of them, including the Prochlorococcus species which appears to be the most abundant photosynthetic organism on the planet, participate in what is called the microbial loop. They are grazed by microscopic organisms, which produce faeces and dead bodies, and then consumed by bacteria, which then return the nutrients to the photosynthetic organisms. The recycling process starts again – it is very efficient, but results in little matter for larger organisms or to sink to the deep sea. Larger phytoplankton provide food for small, insect-like creatures, which then can be used to feed larger organisms up the food chain. Most of the fish we as humans consume comes via this pathway, as does the matter that sinks to the ocean floor to form the precursors for future oil and gas supplies.

The next time you get the chance to visit the ocean, have a look and appreciate the colour and clarity of the water. We like clear blue waters for our holidays, but we must have the darker green waters to provide the organic matter that supports our seafood diets. We tend to focus on the large, charismatic organisms in the sea, but we should appreciate as well that the vast majority of the action is carried out by organisms that can only be seen individually through the microscope.