Nano fixers balance books

Single-celled microbes fertilize the oceans.
9 August 2001


New-found bacteria fertilize the oceans.
J.P. Zehr

A marine gene-hunt has found the oceans awash with nitrogen-harnessing nanoplankton. The newly found fertilizing bacteria could be crucial for the global cycling of the greenhouse gas carbon dioxide (CO2).

Sifting seawater for pieces of DNA, Jonathan Zehr of the University of California, Santa Cruz, and colleagues found at least two, possibly more, novel nitrogen-fixing bacteria in the ocean.

Nitrogen, being a basic ingredient of proteins, is fundamental to life. Ocean plants and animals get theirs from bacteria that process - 'fix' - dissolved nitrogen, just as land organisms rely on microbes in the roots of legumes (peas and beans) that harvest the gas from the air.

The oceans' main nitrogen-fixer is the bacterium Trichodesmium, found in patches along coastlines. But there is not enough Trichodesmium to account for the amount of nitrogen that appears to be getting fixed.

The new bacteria, like Trichodesmium, belong to the cyanobacteria family - also called blue-green algae because they photosynthesise to make energy. Trichodesmium cling together in clumps or chains; the new microorganisms live singly, earning them the name 'nanoplankton'.

The numbers of nanoplankton the group found at different times and depths suggest "they could easily double the nitrogen fixation rate of the oceans", says Zehr. This would balance the ocean's nitrogen books. It "might fill in the gap", agrees Jed Fuhrman, who studies marine microbes at the University of California, Los Angeles.

This discovery could be crucial to climate scientists modelling the abundance of CO2in the atmosphere. The amount of nitrogen available to algae in the oceans dictates how much they grow and how much CO2 they absorb. So nitrogen fixation is often used as a proxy measure of how much CO2 ends up in the ocean.

Fix up

Previously, Zehr's team found "little glimmers of evidence" that other nitrogen fixers existed. So the researchers looked for genes coding for the nitrogen-fixing enzyme nitrogenase in water samples taken throughout the year from varying depths in the North Pacific Ocean near Hawaii.

The "very clever part of this research", says Fuhrman, is the way in which the researchers confirmed that the genes they came across belonged to bacteria actually working at fixing nitrogen. They looked for messenger RNA made from the genes - a sure sign that the bugs' nitrogen-fixing machinery is switched on.

The team now plans to look for nanoplankton elsewhere in the ocean, estimate their global significance and identify the other nitrogen fixers hinted at in the genetic data. "This could end up being a much larger story than it first appears," says Zehr.

Zehr, J. P. et al. Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean. Nature, 412, 635 - 638, (2001).

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Nature News Service / Macmillan Magazines Ltd 2001