MSU study finds new microbial source of nitrogen fixation

June 28, 2001

A team of scientists from Michigan State University has discovered a new and unusual source of nitrogen fixation, the process that converts the nitrogen in the atmosphere into a form that is used by all life on Earth.

In a paper published in the June 29 issue of the journal Science, MSU microbiologist John Breznak and colleagues report that spirochetes - corkscrew-shaped bacteria that are found practically everywhere but are particularly abundant in termite guts - are "fixers" of nitrogen. Once fixed by microbes such as spirochetes, the nitrogen ultimately becomes available to plants and animals.

Nitrogen makes up about 80 percent of the air humans breathe. However, only certain microbes are able to convert that nitrogen gas - that is, fix it - into a form that can be used to make proteins and other cell materials.

"Every living thing on Earth needs nitrogen," said Breznak, MSU distinguished professor of microbiology and molecular genetics. "The question is, where do we get it from?"

Nitrogen fixation occurs in certain plants, Breznak said, particularly in the root nodules of certain legumes. However, this only occurs because of the presence of specific microbes in those nodules.

It had been known for many years that nitrogen fixation took place within termites and could provide up to 60 percent of the nitrogen needs of the insects. But the particular microbes performing the fixation were in question.

Recently, termite gut spirochetes were isolated by Breznak's group and grown in the laboratory, where they were shown to contain the genes for nitrogen fixation and also carry out the process.

"Spirochetes are a large group of bacteria in which nitrogen fixation had never been recognized before," Breznak said.

This is not only good news for termites, whose food is very low in nitrogen, but for the rest of the world as well. Despite their reputation as destructive pests, termites occupy an important ecological niche as decomposers of dead plant material such as wood, releasing the degradation products back for use by other organisms.

"Given the ubiquity of spirochetes in freshwater and marine habitats, our work reveals that spirochetes are newly discovered participants in this globally important process," Breznak said. "Our work also illustrates the ways in which microbes can contribute to animal nutrition and health, not just cause disease."

Other members of Breznak's research team were Timothy Lilburn, research associate in MSU's Center for Microbial Ecology; Kwi Kim, a research technician in Breznak's laboratory; Nathaniel Ostrom, an MSU associate professor of geological sciences; Catherine Byzek, a former MSU graduate student; and Jared Leadbetter, who received his doctorate under Breznak and is now on the faculty at the California Institute of Technology.
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Michigan State University

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