Sigman Contributes to New Discoveries about Critical Link in the Earth’s Nitrogen Cycle

Igor Heifetz ・ Princeton Environmental Institute

DAVIS — Two species of bacteria living on the ocean floor have teamed up in a unique symbiotic relationship to form a critical link in the Earth’s nitrogen cycle, reports a research team that includes two University of California, Davis, microbiologists.

The scientists, led by researchers affiliated with the University of Southern California, will publish their findings about the novel bacterial partnership Aug. 8 in the journal Nature.

At the heart of the study are the long, thin, hairlike bacteria called Thioploca (meaning “sulfur braids” in Spanish) and the tiny anammox bacteria, which use ammonium and nitrite released by Thioploca as an energy source. Both species of bacteria dwell in the ocean floor off of the coast of Baja California as well as the Pacific coast of South America.

The Thioploca live as filaments inside sheaths that glide vertically through marine sediment, generating nitrite and “fixed” nitrogen. The anammox bacteria live in close association with, or piggy-back on, Thioploca, allowing them to consume the byproducts produced by Thioploca.

“All life — including microbes — rely upon one another to survive,” said UC Davis microbiologist Scott Dawson, a co-author on the paper. “A key ecological link in the global nitrogen cycle between microbes with these physiologies has been postulated for some time, yet this study provides the first direct evidence of this intimate, symbiotic association,” he said.

Marissa Hirst, a graduate student in Dawson’s laboratory, provided molecular and microscopic evidence for the presence of anammox bacteria in close association with Thioploca. The research involved identifying anammox-specific genes as well as showing that anammox bacteria were found in Thioploca sheaths, using fluorescence microscopy.

Newly discovered link in nitrogen cycle

Nitrogen is a prerequisite for photosynthesis and a crucial building block of life. It is abundant in Earth’s atmosphere but to be useful to most living organisms must be converted from its nonreactive atmospheric form into the biologically available, or “fixed,” form as ammonium.

The specialized organisms that carry out this nitrogen-conversion process are called nitrogen fixers. Other organisms use up the fixed nitrogen and convert it back to the atmospheric form of nitrogen, which is di-nitrogen gas.

Thioploca filaments store high concentrations of nitrate; the concentration of this valuable resource is several thousand-fold higher compared to that of the surrounding sediment. As Thioploca glides down vertically through the sediment, it encounters sulfide, which is used to produce fixed nitrogen or ammonium.

For most other bacteria, sulfide is a roadblock to metabolism, but Thioploca is able to react nitrate with sulfide, producing ammonium and nitrite, which the anammox then consumes and churns out as di-nitrogen gas. With the anammox cells living off of the Thioploca byproducts, both microbes thrive in a unique microbial niche where others perish.

The symbiosis between Thioploca and anammox does not cause widespread changes in the nitrogen cycle throughout the ocean; rather, it creates localized zones where fixed nitrogen is depleted more quickly from sediments than most scientists had expected.

Other collaborators on the study include lead author Maria Prokopenko, formerly at USC and now at Pomona College; William Berelson and Lauren Chong, both of USC; Bo Thamdrup and Loreto De Brabandere, both of the University of Southern Denmark; EJ Crane and DJP Lawrence, both of Pomona College; Julie Granger of the University of Connecticut; Bonnie Chang and Daniel Sigman, both of Princeton University; and Amy Townsend-Small of the University of Cincinnati.

Funding for the study was provided by the National Science Foundation, grant number OCE-0727123.