Elusive microbe fertilizes oceans

By Larry O'Hanlon,

updated 12:03 p.m. ET Nov. 14, 2008

After a long search researchers think they have found a cryptic microbe that helps fertilize ocean waters worldwide. Or at least they have found the single-celled critter's very telling and surprising genome.

The actual microbe — a type of bacteria known as cyanobacteria — has so far eluded direct observation, although perhaps not for much longer.

"This is a microscopic organism that I've been chasing for 10 years now," said researcher Jonathan Zehr of the University of California at Santa Cruz. "We couldn't culture it (in a laboratory) and couldn't see it." He is the lead author of a paper on the discovery featured in the Nov. 14 issue of the journal Science.

Hints of the mysterious organism have been popping up all over the world in DNA analysis of sea water, said Zehr. Those hints indicated that there was some small organism which was rigged to grab nitrogen from the air and feed the microscopic plants — called phytoplankton — that form the base of the ocean food chain. This makes it a rather important player in the oceans.

"In order to pull down carbon dioxide from the atmosphere, we need phytoplankton," explained Woods Hole marine scientist Anton Post. And in order for sun-loving phytoplankton to grow, they need a host of nutrients —  just like land plants. "More often than not, nitrogen is the limiting nutrient."

Tracking down a widespread organism that gets nitrogen into the ocean food web has implications for global warming, which is driven by excessive carbon dioxide in the atmosphere.

By applying a powerful suite of new technologies to the task, Zehr and his colleagues were finally able to nail down the size and color of the organism and map out its genome. What they found was a shock.

"The first thing that surprised us is that this turned out to be a small cell," Zehr said. It also lacked photosynthetic plant pigments or any of the genes for being a photosynthetic plant. That means it had to make a living from other living things.

The discovery is remarkable because the only other nitrogen-fixing organisms found in the ocean also perform photosynthesis — which creates the oxygen we breathe. The two processes are tricky to accomplish side-by-side because the molecular equipment used to grab nitrogen is destroyed by oxygen.

"Some cyanobacteria get around this by just fixing nitrogen at night, when there is less oxygen" because photosynthesis is not happening in the dark, Zehr said.

The newfound organism can fix nitrogen during the day, however, and it lacks genes for living like a plant, he said. So it's either a weird relict organism from the early days of life on Earth, before photosynthesis evolved — or it lost the genes for living like a plant in favor of some other arrangement.

"One of our hypotheses is that it's living in symbiosis with some other organism," Zehr said. But that other organism, too, has proven elusive.

Fortunately, now that they have clues to the nitrogen-fixing cyanobacteria's lifestyle, it might be finally possible to grow it in a lab and then really study it in detail, Zehr explained.

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