A light meal at sea.

A light meal at sea

In a molecular bucket brigade of life, carbon atoms pass from soil to plant to animal to air. Scientists studying the cycle have long puzzled over the ocean--which seems to hog the buckets rather than pass them on. In the ocean, organic molecules from decaying plants and animals -- known as humic material -- combine into larger molecules that plankton or bacteria can't ingest. These inedible molecules can last thousands of years without decaying.

Now researchers at the University of Miami report sunlight can boost these molecules back into circulation. They exposed samples of seawater to sunlight and found that the light breaks these larger molecules into smaller fragments. Tiny zooplankton and bacteria rapidly gobble them up, returning them to the carbon cycle. The Miami team, led by marine chemist Kenneth Mopper, observed the reaction in coastal waters and open ocean, in the tropical Sargasso Sea and along the Maine coast.

The finding, reported in the Oct. 19 NATURE, both explains what happens to the enormous amount of carbon in the ocean, Mopper says, and accounts for regions where microorganisms thrive without an obvious source of edible carbon.

The marine chemists found a daily rhythm to the reaction, with sunlight breaking down the large molecules slightly faster than the microorganisms eat the results. At night, the creatures finish off the day's production. "Here is a mechanism that makes sense," says oceanographer John I. Hedges of the University of Washington in Seattle. "It's not [the result of] some little organism that lives only in Antarctica. It's global in scope -- it occurs wherever sunlight hits the ocean."