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Scientists find key to ocean bacterium that helps control greenhouse gas

August 27, 2003

Scientists are a step closer to understanding how the world's oceans influence global warming - as well supply us with the oxygen we breathe.

A study led by Imperial College London has revealed how the most abundant ocean bound photosynthetic bacterium helps control levels of the greenhouse gas, carbon dioxide.

Reporting in Nature the researchers provide new detail on how Prochlorococcus cyanobacteria traps atmospheric carbon dioxide and stores it in the deep sea.

Working with colleagues from the Observatoire Oceanologique de Roscoff, France, they reveal that iron plays a crucial role in the ability of this marine organism to use energy from light to convert carbon dioxide into organic molecules by the process of photosynthesis.

Professor Jim Barber of Imperial College London and senior author of the study, said:

"Until recently the contribution of marine photosynthesis to the global carbon cycle was grossly underestimated. We now know that over 50 per cent of global photosynthetic activity takes place in the oceans.

"A 10 per cent decrease in carbon dioxide uptake by the oceans would leave about the same amount of carbon dioxide in the atmosphere as is produced by fossil fuel burning each year. Therefore, understanding what factors influence this bacterium's ability to regulate carbon dioxide is crucial for humans' continued survival."

Since the beginning of the industrial revolution atmospheric concentrations of carbon dioxide have increased by 30 per cent. Evidence strongly suggests this increase is due to increased combustion of fossil fuels and other human activity.

Until now it was not known how Prochlorococcus, which lives at a large range of ocean depths from the surface to 200 metres, has adapted to maximise the available light to fuel photosynthesis.

Previous work by the Imperial group has shown how Prochlorococcus regulates the interaction of the two powerhouse protein complexes that drive photosynthesis by recruiting additional, or 'antenna' proteins.

Using genetic analyses, the researchers compared three strains of the bacteria that have adapted to varying light intensities at different ocean depths. They found that evolution of the two powerhouse protein complexes is directed by environmental constraints.

Prochlorococcus that survives in extreme low light conditions has more of the antenna proteins compared with strains that live close to the surface and iron is the limiting nutrient that regulates the organism's ability to harvest light energy.

"Iron is critical for the health of organisms and the low levels in the ocean significantly decreases the ability of Prochlorococcus to grow and reproduce," explained Professor Barber.

"Iron is the fourth most abundant element in the Earth's crust. Yet its levels in the aquatic ecosystem, particularly in open oceans where most cyanobacteria are found is low. Indeed, in experiments where regions of the ocean have been artificially 'seeded' with extra iron there is a dramatic increase in biomass production due to an increased amount of Prochlorococcus and other photosynthetic organisms. Our research opens up the possibility of artificially increasing ocean levels of iron to combat global warming."

Imperial College, University of London