Scientists estimate the amount of nitrogen in the ocean by comparing samples from deep water and surface water, and then considering how they mix together, Zehr said. Recent calculations indicate that there is more nitrogen fixation in the surface water than scientists previously thought.
"There seems to be a lot more nitrogen than we can account for," Zehr said. "It's like making a big budget, but the budget doesn't balance."
The unexpectedly large amount of nitrogen in the open ocean set Zehr looking for its sources. Over the past 12 years, Zehr has uncovered evidence of dozens of nitrogen-fixing bacteria by looking not for the organisms themselves but for their DNA fingerprints--specifically, for a gene encoding the protein responsible for nitrogen fixation.
"We initially went out thinking we were going to prove that there are no other nitrogen-fixing bacteria besides the ones we already know," Zehr said. "When we found these genes, we were very surprised."
The nitrogen-fixing marine Synechocystis is the first of these organisms Zehr's team has succeeded in cultivating in the laboratory. It was isolated from samples collected at a long-term monitoring site near Hawaii where Zehr works with David Karl of the University of Hawaii in Honolulu. To cultivate the bacteria, Zehrs collaborator John Waterbury of the Woods Hole Oceanographic Institution subjected a soup of organisms to an environment containing all the factors needed for growth except nitrogen compounds. Deprived of usable nitrogen, they reasoned, all the organisms would die out, except those that could fix nitrogen from the atmosphere.
"We gave the organisms we were interested in an ecological edge," Zehr said.