Scientists solve mystery of world's nitrogen-cycling budget

Washington, October 18 : Researchers at the California Institute of Technology (Caltech) have identified an unexpected metabolic ability within a symbiotic community of microorganisms that may help solve a lingering mystery about the world's nitrogen-cycling budget.

The element nitrogen is a critical part of amino acids, the building blocks of proteins, and therefore essential to all life.

When researchers add up all of the known sources of fixed nitrogen (biological and otherwise) in the global nitrogen cycle and compare it to the sinks-where nitrogen is taken up for growth and energy-they come up short.

It appears that more nitrogen is being used than is being made. The apparent nitrogen budget, in effect, does not balance.

This discrepancy had led scientists to question whether the nitrogen cycle is truly out of balance, or whether the known inventories of sources and sinks are misleadingly incomplete.

Victoria J. Orphan, an assistant professor of geobiology at Caltech, along with graduate student Anne E. Dekas and postdoctoral research scholar Rachel S. Poretsky, suggest the answer is, at least in part, an incomplete catalog of the sources of fixed nitrogen.

The team studied ocean sediment samples obtained in methane cold seeps located at a depth of about 1,800 feet.

The area, known as the Eel River Basin, is located approximately 20 miles off the coast of the northern California town of Eureka, on a continental margin in a region supporting high levels of natural methane seepage at the seabed.

In the laboratory, the researchers examined the methane-rich sediment and the tiny microbial conglomerations that live within.

These spherical cell conglomerates, averaging about 500 cells each, consist of two types of anaerobic microorganisms living in a unique symbiotic relationship fueled by methane.

To determine that the archaea were indeed fixing nitrogen, the researchers first incubated the archaeal-bacterial assemblages with a dinitrogen gas, N2, that was composed of two atoms of nitrogen-15.

The researchers then used a technique called fluorescent in situ hybridization (FISH) to stain the two types of organisms in the sediment, and analyzed these cells for their nitrogen-15 content using a state-of-the-art instrument called a nanometer secondary ion mass spectrometer, or nanoSIMS.

The nanoSIMS, which is housed at the Caltech Center for Microanalysis, is capable of collecting chemical and isotopic data at a spatial scale of 50 to 100 nanometers, or around five to 10 times smaller than the size of a single microbial cell.

Both the archaea and, to a lesser extent, their bacterial neighbors had incorporated the nitrogen-15, which could have happened only if the N2 had been fixed by the archaea-and then shared.

--ANI