What's in ocean water? Shards of bacteria

 When a team of oceanographers used state-of-the-art techniques to analyze seawater, they came to a surprising conclusion: Tattered remnants of bacteria constitute much of the dissolved matter in the open ocean. This finding adds to a growing appreciation of bacteria's role in the seas.

"Bacteria are seen as more and more central to every aspect of what's going on in the open ocean," says Matthew D. McCarthy of the University of Washington in Seattle.

 McCarthy and his colleagues collected thousands of liters of water from remote spots in the central Pacific, the Gulf of Mexico, and the North Sea. They pumped the water through a series of filters, the last of which were so fine that they sifted out individual biological molecules-material collectively called dissolved organic matter.

 The oceanographers focused on amino acids, the building blocks of peptides and proteins. They found that four of the amino acids in the dissolved matter appeared in two flavors, left-handed and right-handed forms. This observation indicates that bacteria produced the amino acids, the scientists conclude. All other types of organisms make only the left-handed versions. The amino acid fingerprint, the mix of the various forms, indicates that they came from peptidoglycans, the main structural molecules in bacterial cell walls, says McCarthy, who collaborated with Washington's John I. Hedges and Ronald Benner of the University of Texas Marine Science Institute in Port Aransas. They report their findings in the July 10 Science.

 McCarthy and his colleagues rule out other possible sources of the right-handed amino acids. These uncommon structures can form naturally when left-handed amino acids undergo a transformation known as racemization. This process, however, takes many thousands to millions of years-too long to explain the amino acids' presence in ocean water. Moreover, the ratios of right- and left-handed amino acids in the seawater samples do not match the ratios expected from racemization.

 Jeffrey L. Bada of the Scripps Institution of Oceanography in La Jolla, Calif., agrees that bacteria are the source of the right-handed amino acids. "You really can't explain it any other way," he says. Bada notes that bacteria coat themselves with right-handed amino acids because the unusual structures provide a tough exterior that resists other organisms. This is what helps bacteria evade digestive enzymes in human stomachs, he says.

 From the ratios of right- to left-handed amino acids in seawater, McCarthy and his colleagues conclude that a substantial fraction of the dissolved organic matter comes from bacteria. This challenges the traditional view that algae produce most of the ocean's soluble biological material.

 Until recently, oceanographers thought of algae as the main photosynthesizers in the ocean, occupying a niche similar to plants on the continents. Bacteria were considered instead to be the consumers that break down leftover pieces of algae and other organisms. Researchers, however, are gradually realizing that bacteria play an important role as primary producers in the nutrient-deprived ocean areas far from land, says McCarthy. "This suggests that actually the trees and grasses and bushes of the open ocean are largely bacteria."