Climate Key May Lie Many Leagues Under the Sea

Next time you draw a breath, you might give a nod to sea floor bacteria. Philippe Van Cappellen at the Georgia Institute of Technology in Atlanta thinks they help keep our air breathable.

Dr. Van Cappellen and Ellery Ingall at the University of Texas at Austin are studying computer models of a feedback system by which organisms in the sea help keep atmospheric oxygen concentration within limits safe for organic life. Van Cappellen says their work ''shows that life has developed ways of compensating for any drastic changes in its environment.''

Scientists, it seems, have to be alert for unsuspected biological processes if they want to understand global climate change. It isn't enough just to look at the physics and chemistry involved.

The level of oxygen in the air is a key factor. Too much oxygen would feed rampant wild fires and poison many organisms. Too little oxygen would asphyxiate higher life forms. The geological record shows little sign of such catastrophes even though processes have been at work that could have driven the atmosphere toward either extreme.

On land, weathering - with oxidation of minerals and organic matter - takes oxygen from the air. Photosynthesis by microscopic marine plants - the ''grass'' of the sea - restores it. Oxygen removal is rapid when there's a lot of new land, say from mountain building, to be weathered. This could deplete atmospheric oxygen within a few million years. That hasn't happened. Conversely, when weathering is relatively inactive, marine photosynthesis could pump up the atmosphere's oxygen concentration to toxic levels. This doesn't appear to have happened either.

As Van Cappellen and Dr. Ingall explained recently in Science, they think the process is controlled by bacteria in sea floor sediments. It controls the availability of phosphorus - an essential plant nutrient - and hence controls marine photosynthesis.

Since oxygen dissolves readily in sea water, oxygen levels in the air and sea tend to be in balance. When those levels are high, phosphorus is taken out of sea water and sequestered in sediments. When oxygen is low, the reverse happens.

Van Cappellen says the sea floor bacteria can live by means of more than one food and energy cycle. When oxygen is plentiful, they use a cycle that sequesters phosphorus in their cells. This damps down the marine plants' oxygen-producing activity. When sea water oxygen is low, the bacteria switch to an oxygen-free energy cycle that releases the phosphorus. Marine plants then burgeon. This is a quick response feedback system. It can switch modes within a million years or so. That's a mere wink of an eye in geological time.

As Lee Kump and Fred Mackenzie of the University of Hawaii in Honolulu note in commenting on this research in Science, these computer studies are crude. They need to be expanded and refined to take account of what's happening on land as well as in the sea. Van Cappellen agrees. He adds, however, that, since the same organisms that produce oxygen also remove heat-trapping carbon dioxide from the air, even these crude computer models can help scientists studying global climate change.

As Van Cappellen and Dr. Mackenzie also noted in telephone interviews, the key to better understanding probably lies in the geological record encoded in sediments on continental shelves. And political restrictions are making those data hard to get. The Law of the Sea Treaty gives coastal nations jurisdiction over these shelves as exclusive economic zones, and permission to sample sediments there is hard to get. Mackenzie calls it a major barrier to research.

Van Cappellen and Ingall's studies have their uncertainties. But one point seems clear. If humanity is to understand the mechanisms of the global environment, nations must replace nationalistic mistrust with scientific cooperation to carry out the necessary research within exclusive economic zones.