Scientists Fit Another Piece of Global-Warming Puzzle ; Bacteria Smaller Than a Speck Are Identified as Food for Plankton Plants, Which Break Down Carbon Dioxide

Article excerpt

Tiny, previously unknown bacteria are turning out to be key sustainers of ocean life and its ability to sequester global- warming carbon dioxide.

Like the bacteria that live on the roots of peas and beans, these marine microbes take nitrogen from the air and "fix" it in chemical forms that fertilize plants.

Microscopic marine plants feed microscopic animals. This mix of floating organisms, known collectively as plankton, underlies all marine food chains. That's why scientists call the tiny plants the grass of the sea.

Now you can extend that metaphor to call nitrogen-fixing bacteria the sea's legumes.

Biologists have known the ocean has a massive nitrogen-fixing system, but they haven't been able to identify all the sources. Jonathan Zehr at the University of California in Santa Cruz explains that "in the open ocean, there are only one or two organisms known to fix nitrogen."

Now, however, he and colleagues at several other institutions have identified a third: a type of photosynthetic bacteria. In reporting it today in the journal Nature, they say there likely are many other unknown nitrogen-fixing microbes in the sea. This photosynthetic one is the first they have been able to cultivate in the laboratory and put through its nitrogen-fixing paces.

The discovery is as important for climate scientists as it is for marine biologists. The ocean's ability to remove atmospheric carbon dioxide has been poorly known, because the biological cycle involved hasn't been fully explained.

What is known is that the plankton's photosynthetic plants break down carbon dioxide and lock up the carbon in their shells. …