Cold Deep-Sea N₂ Fixers

Scientists have verified that a collective of uncultured deep-sea microorganisms are capable of fixing nitrogen-a discovery that could add missing information to the still-unbalanced global ocean N₂ budget. Such information is crucial for understanding how oceans respond to global changes in their temperature and carbon dioxide levels.

Researchers have discovered that certain archaea and bacteria that eke out a life together deep in the dark, cold ocean are able to fix dissolved N₂ gas. These organisms turn the N₂ into various biologically useful nitrogen-based compounds, including NO₃, NH₄ ⁺, and CN^-, according to geobiology professor Victoria J. Orphan, graduate student Anne E. Dekas, and postdoc Rachel S. Poretsky of California Institute of Technology (Science 2009, 326, 422).

Scientists have posited for years that the seemingly mysteriously low amounts of N₂ in the oceans could be accounted for by way of microbial action. Then, a few years ago, researchers discovered nitrogen fixation in organisms that live around blisteringly hot undersea hydrothermal vents. What's more, previous genetic and isotopic data had suggested that organisms living in "cold seeps" deep in the ocean could possibly fix N₂.

The Caltech group took samples from cold seeps into the lab and used fluorescence in situ hybridization (FISH) to map the positions of the archaea and the bacteria in each sample. By using nanometer-scale secondary ion mass spectrometry (SIMS) and isotopically labeled N₂, they were then able to map the varying levels of N₂ fixation as it occurred in each sample. They showed that the archaea are largely responsible for fixing N₂ into bioavailable nitrogen atoms, which they then share with their bacterial neighbors.

"This is quite exciting," says Maren Voss, a professor at the Leibniz Institute for Baltic Sea Research, in Germany, who studies oceanic nitrogen fixation. "It's an important contribution to our understanding of cycling of nitrogen in the ocean."

The Caltech group has several future projects planned, including determining how rapidly the archaea fix N₂ in their natural environment and understanding how this N₂ fixation source affects the marine nitrogen cycle, Orphan says.