Aircleaners in the Ocean. CAN A ONE-CELLED SEA PLANT COOL THE `GREENHOUSE'?

WILL a tiny, floating creature - phytoplankton - rescue man from the greenhouse effect? Scientists at the Bedford Institute of Oceanography here are asking this as they conduct research into the abundance and distribution in the world's oceans of these one-celled green plants.

``I am not saying that they are going to wipe it [the greenhouse effect] out,'' notes Trevor Platt, a biological oceanographer. ``But the ocean is interfering in a way that might be beneficial to man.''

That's because phytoplankton, like plants living on solid earth, consume carbon dioxide (CO2) as part of their nutrition.

These ocean plants are so small that a 10-gallon jug of seawater may contain more such cells than there are people on earth. Nonetheless, phytoplankton in the ocean use an estimated 50 billion tons of carbon per year as CO2. This is about 10 times the annual global emission of CO2 to the atmosphere from combustion of fossil fuels - gasoline in cars and trucks, coal in power plants, and so forth.

Humans have been spewing so much CO2 into the air, increasing the concentration of this invisible gas in the earth's atmosphere, that the heat waves and drought of last summer raised fears that the long-predicted greenhouse effect had already begun.

If so, more of the sun's heat would be trapped in the atmosphere, raising its temperature, melting the polar ice caps, boosting ocean levels, and increasing the violence of hurricanes and other storms.

Alternatively, it is theoretically possible that phytoplankton will thrive on the extra atmospheric CO2, multiplying sufficiently to create a new ``steady state'' in the ocean and atmosphere that would not be as catastrophic to man.

Dr. Platt has pioneered the study by satellite of plankton production. Because phytoplankton, floating in the top 30 feet or so of the ocean, make chlorophyll pigment, they color themselves and the water green. When the sea is photographed from a satellite, concentrations of these creatures stand out as green against a blue background.

Data on the ocean's color were gathered by a sensor aboard the NASA satellite Nimbus-7 that orbited the globe 14 times a day from 1978 to 1986. After some years of research learning how to interpret the data and taking advantage of faster computers, Platt and others produced ``ocean color'' maps of the sea surface. (Another satellite-borne sensor is expected to go into orbit in 1991.) These maps provided far more coverage of the ocean than had been previously obtained by ships towing fine-mesh plankton nets at 5 miles per hour.

Says Platt: ``The global satellite coverage has shown that the open ocean, which was once thought to be relatively dead, is a noisy place biologically.''

That means more of the biological ``Pac Men'' are gobbling up CO2 than previously estimated. Most of the CO2 ``fixed'' from the atmosphere by phytoplankton is returned to the ecosystem when the organisms decompose. But some carbon is lost to the system when a portion of the plankton falls to the bottom and is buried in ocean sediments. That carbon could - over eons - turn into coal or oil.

The conventional view among scientists has been that about 5 percent of the plankton sinks to the bottom. Platt believes it is more like 33 percent.

Satellite data have shown great variability in plankton production over time periods as short as a few days. Platt and his colleagues believe that this change depends largely on how much nitrogen in deep water is brought to the surface by storms or ocean currents for use by the phytoplankton. They have revised upward estimates of available nitrogen.

The oceans are doing more atmospheric ``house cleaning'' than previously understood.