Marine plankton put nitrogen in a fix

New genetic analyses of tropical marine microorganisms have revealed that some species of single-celled plankton are converting significant amounts of nitrogen from the air into nutrients, helping to fortify the base of the ocean’s food pyramid.

“This [finding] may give scientists a hint of where to look to solve one of the ocean’s biggest biogeochemical puzzles,” says Douglas G. Capone, an oceanographer at the University of Southern California in Los Angeles. There’s much more nitrogen capture going on in the oceans than known processes can account for.

By detecting the molecular machinery used by the plankton to create one of the enzymes needed to split nitrogen molecules apart, scientists appear to have discovered a new microbial source of the ocean’s nitrogen-bearing nutrients.

Nitrogen makes up almost 80 percent of the atmosphere, where the gas occurs as two-atom molecules, a form that plants and animals can’t use directly. On land, bacteria in the roots of some plants split nitrogen molecules apart and incorporate the single nitrogen atoms that result from this process into compounds that the plants can use. Coastal waters contain other bacteria that have the genes to perform this vital trick–an energy-consuming set of chemical reactions known as nitrogen fixing.

But only a couple such organisms have been found in the open ocean, and they’re present in numbers far too small to account for the amount of nutrients in the water, says Jonathan P. Zehr, a microbiologist at the University of California, Santa Cruz. So, he and his colleagues turned to genetic analysis to search for the missing microbial alchemists.

The researchers decided to look for a marker for the gene that produces nitrogenase, the enzyme catalyzing the final step in the nitrogen-fixing reactions. The group screened a vast number of microbial species that are between 0.2 and 10 micrometers in diameter.

However, only several types of photosynthesizing plankton collected near Hawaii at depths of as much as 150 meters had this genetic marker, and they produced the active enzyme, says Zehr. He and his coauthors report their findings in the Aug. 9 Nature.

The use of genetic evidence to find bacteria that could convert nitrogen to nutrients is “really cool,” says Tracy A. Villareal, a biological oceanographer at the University of Texas’ Marine Science Institute in Port Aransas.

Because nitrogen fixing requires about 20 genes and much energy, the plankton probably wouldn’t make nitrogenase unless they were producing nutrients and thereby benefiting from it, Villareal says. “If you find these genes, you can be pretty sure the cells are using them,” he says.

Zehr says the next challenges are to determine the global distribution, growth rate, and life cycle of these nitrogen-fixing microorganisms. The microbes may be common enough to match the nutrient production of Trichodesmium, the cyanobacterium previously thought to be the nitrogen-fixing champ among ocean plankton.

Even so, the newly recognized nitrogen fixers may not be prevalent enough to balance the ocean’s nutrient budget. Zehr notes that oceanographers recently have discovered other plankton that might make up the difference. They suspect, but haven’t verified, that their finds include two new types of nitrogenase-producing organisms: bacteria that don’t photosynthesize and ocean microbes that have formed symbiotic relationships with larger organisms.

“There’s about a million bacteria in each milliliter of seawater, and we don’t have any idea what most of these guys are doing,” notes Capone.

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