Geoff Thomas: Plankton earth's building blocks

Dolphins are at the top of the food chain which starts with plankton. Photo / Paul Thomas

Life began in the oceans billions of years ago. Today the seas are vital to all life, not just for the fish that we hunt but also for the tiniest organisms producing the oxygen we breathe and in past millennia even contributed to the oil that we use as fuel.

To understand the processes more clearly, we need to look at how the different elements are inter-woven in a complex symbiosis where each is affected by the success or failure of others.

The oceans and seas cover more than 70 per cent of the surface of the planet, and most life in the sea relies on the energy from the sun. The closer to the surface one looks, the richer the life is.

Microscopic plants and bacteria live among the plankton, converting the energy from the sun through the process of photosynthesis, just like plants and trees on the land.

This process was started by microbes more than three billion years ago.

The key to the food chain which starts here is phytoplankton. These are myriad forms of plant plankton, as distinct from the animal plankton which make up the zooplankton and which could be regarded as the next step up the ladder of life as they eat phytoplankton.

The phytoplankton are so vast they are like a forest in the sea, and the numbers are incomprehensible.

For example one group, called diatoms, are preyed on by other plankton predators. Diatoms are about as small as you can get. They are single-celled organisms less than half a millimetre in size that come in every shape you can imagine, from cylinders to round and triangular.

Their bodies contain carbon which is accumulated during photosynthesis and when they die they become intricate skeleltons which fall to the depths. It is believed that over the millennia vast deposits of diatom remains have been transformed by heat and pressure into the oil we seek so avidly.

Diatoms are found in every type of aquatic environment, from the inside of a toilet cistern or aquarium to alpine tarns to the algae blooms that appear in lakes and at sea.

One of the smallest members of this group may be responsible for the largest contribution to marine photosynthesis - the blue-green bacterial algae called cyanobacteria. They create the slimy filaments found in geothermal pools, and are thought to be responsible for half of all photosynthesis at sea.

Diatoms are giants compared to cyanobacteria, which are among the smallest microbes on Earth.

Try to imagine 0.5-0.7 microns, when one micron is one-thousandth of a millimetre. One drop of sea water can contain up to 20,000 of these cells.

About half of the varieties of plant plankton use the sun's energy to convert water and carbon dioxide into sugars, which provides their food. The other half are hunters, feeding on fellow life forms.

Scientists believe phytoplankton are critical to the survival of the planet, as they are responsible for half of the carbon dioxide which is removed from the atmosphere by photosynthesising organisms - which includes all plants and trees - and replacing it with oxygen.

Like all populations in nature, plankton follow boom and bust cycles. The blooms occur when conditions combine to create an environment favourable for growth - a combination of nutrients, perhaps following upwellings from deep water as a result of wind conditions, warm temperatures and plenty of sunlight (read summer).

The most common is the "red tide" seen around our coast. This is a natural phenomenon, but still can raise concerns; particularly when phytoplankton produce toxins that become concentrated in some species of fish and in filter feeders such as scallops, mussels and pipis. The culprits are diatoms and another type of plant plankton- dinoflagellates.

Extreme cases can result in shellfish poisoning of people, and mass deaths of fish such as occurred in the Firth of Thames one summer when many thousands of pilchards were washed up on beaches, and at Orewa in October, 2002, when thousands of dead fish washed up on the beach.

In the sea everything gets eaten by something and the plankton are the basis of the food chain. They also remove half of the carbon from the atmosphere and free up oxygen. Useful little critters, aren't they? We probably wouldn't be here if it wasn't for their existence.