Under the Microscope/Prof William Reville: Microscopic green plants called phytoplankton in the surface layer of the oceans remove as much carbon-dioxide gas from our atmosphere each year as do all the land-based green plants of the world.
Carbon dioxide has slowly but steadily increased in concentration in the atmosphere over the past 150 years, producing an enhanced greenhouse effect that has significantly warmed our world.
There is a broad scientific consensus that if this continues it will have very negative effects. There is a possibility that we can reduce the concentration of carbon dioxide in the atmosphere by manipulating phytoplankton.
Carbon moves between the atmosphere, the biosphere and the inorganic earth in a grand cycle. Carbon is present in the atmosphere as the inorganic gas carbon dioxide at the relatively low concentration of about 365 parts per million (ppm). Its concentration has slowly increased to this level from a pre-industrial concentration of 270 ppm. The increase in carbon-dioxide concentration has largely been due to emission of the gas when fossil fuels - coal, oil, gas, peat - are burned industrially.
Carbon dioxide is removed from the atmosphere by green plants. These plants have the ability, using the energy of sunlight, to split water into hydrogen and oxygen and to combine the hydrogen with carbon dioxide to form the organic carbon compounds on which life is based, such as nucleic acids, proteins and sugars. Oxygen is released to the atmosphere from photosynthesis as a by-product.
Animals are unable to carry out photosynthesis and so depend on photosynthetic organisms to supply vital organic ingredients. Animals eat plants and incorporate the organic compounds into their own substance. Animals and plants also chemically burn some of these organic compounds (principally sugars) in their cells to generate energy.
Burning is done in the presence of oxygen and reduces the organic compounds to carbon dioxide and water, which are returned to the atmosphere.
Carbon dioxide is also removed from the atmosphere when it dissolves directly in our oceans and lakes. Phytoplankton in the top layer of the oceans photosynthesises this carbon dioxide into organic compounds. The plankton is eaten by small marine animals, which are eaten by small fish, which, in turn, are eaten by bigger fish, and so on.
Phytoplankton that is not eaten by little animals further up the food chain dies and sinks to deeper layers, where it is mostly eaten by bacteria, which burn the organic content to produce carbon dioxide and water.
Because these deeper layers of water are colder and denser than upper layers, they don't mix readily with the upper layers; the released carbon dioxide therefore doesn't return to the surface layer for 100-200 years. It is estimated that about 15 per cent of the carbon dioxide photosynthesised by surface-layer phytoplankton is temporarily buried in this deeper layer sink.
A very small fraction of the dead phytoplankton sinks to the bottom of the sea, where it becomes incorporated into sedimentary rocks or petroleum or gas deposits. Some of this organic material can also slowly return, over millions of years, to the atmosphere through volcanic eruptions.
One way to reduce the carbon dioxide in the atmosphere would be to enhance green plants' photosynthetic activity. This is a large part of the motivation behind the large-scale forest plantation schemes under way in Europe.
It was thought that ocean phytoplankton "fixed" a relatively small fraction of carbon dioxide from the atmosphere compared with land-based green plants. Recent measurements have shown, however, that ocean phytoplankton fixes as much carbon dioxide from the atmosphere as do land-based plants. If we destroyed all marine phytoplankton, the concentration of carbon dioxide in the atmosphere would rise by a third over the next few hundred years.
In order to thrive, phytoplankton needs nutrients other than carbon dioxide, such as nitrogen in the form of ammonium nitrate, phosphates and iron. Studies have indicated that availability of iron is a limiting factor. Apparently, the main source of iron is wind-blown iron-rich dust.
Marine studies over the past 10 years have clearly shown that the addition of low levels of iron to limited areas of ocean surface water greatly increases phytoplankton activity within eight weeks. Many scientists and industrialists are seriously contemplating the hypothesis that artificially stimulating the phytoplankton in our oceans would significantly slow down the enhancing greenhouse effect by removing larger amounts of carbon dioxide.
I wouldn't hold out much hope that more good than harm would result from interfering with the phytoplankton. These organisms form part of the base of a gigantic food chain. If you tamper with it, this will have effects elsewhere along the chain, perhaps catastrophically.
Perhaps it would be worth trying this approach if no other avenue were open to us to slow down the greenhouse effect. One blindingly obvious avenue is open, however: drastically reducing our emission of carbon dioxide.
• William Reville is associate professor of biochemistry and director of microscopy at University College Cork