The Nitrogen Cycle Intensified

In a discovery that further demonstrates just how unexpected and unusual nature can be, scientists have found two strains of bacteria whose symbiotic relationship is unlike anything seen before.

Working with William Berelson, chair of the Department of Earth Sciences at USC Dornsife, Maria Prokopenko was lead author of a paper on the research that appeared in Nature earlier this month.

Long, thin, hairlike Thioploca (meaning “sulfur braids” in Spanish) trichomes form chains down into marine sediment, which tiny anammox cells ride down like an elevator. At the bottom, the anammox cells consume nitrite and ammonium, or “fixed” nitrogen, the waste products of the Thioploca.

Nitrogen is a crucial building block of life, a prerequisite for photosynthesis. While nitrogen is present in abundance in the Earth’s atmosphere, to be useful for most living organisms, the nonreactive atmospheric nitrogen that diffuses into the ocean from the air must be converted into the biologically available “fixed” forms ammonium, nitrate and nitrite by specialized organisms called nitrogen fixers. Other organisms use up this fixed nitrogen and convert it back to di-nitrogen gas.

Living together in the mud beneath areas of high plant productivity, Thioploca and anammox intensify this part of the nitrogen cycle.

Gliding down through the mud, Thioploca chains bring down nitrate — a highly desirable resource in the harsh environment of oxygen-free sediments. As Thioploca encounters sulfide (which is a roadblock for most other bacteria) formed from the reaction of organic matter from above and sea water sulfate, it helps react nitrate with sulfide, producing nitrite and ammonium, which the anammox consumes and churns out di-nitrogen gas.

The anammox cells ride on Thioploca, living off its waste, and so both microbes thrive where others perish. Overall, however, they lock up an important resource for life in the ocean, making it unusable by the organisms at the base of the food chain that rely on photosynthesis to survive.

“The symbiotic relationship we discovered is an incredibly elegant chemical tandem between two chemolithotrophs — organisms which derive their metabolic energy purely from inorganic chemistry. We first predicted the symbiosis based on realization that Thioploca’s waste [nitrite and ammonium] are ‘bread and butter’ for anammox,” Prokopenko said. “The prediction was confirmed by our team, proving that the symbiotic pair builds a very efficient natural ‘waste-treatment plant’ — destroying substantial quantities of fixed nitrogen while linking sulfur and nitrogen cycles in oxygen-free sediments.”

Prokopenko is currently a visiting scholar at Pomona College, but completed the research while a research assistant professor working with Berelson.

Prokopenko and Berelson collaborated with researchers from the University of California, Davis; the University of Southern Denmark; Pomona College; the University of Connecticut; Princeton University and the University of Cincinnati.

The symbiosis between Thioploca and anammox is not one creating widespread change throughout the ocean, but rather one that creates localized zones where fixed nitrogen is depleted faster than most expected.

Most of the samples collected were found off the coast of Baja California.

“As important as nitrogen is to life on this planet, it is amazing that we can discover new pathways and chemical reactions and biological partnerships involving this compound,” Berelson said.

Prokopenko, Berelson and others are presently studying nitrogen cycling in waters off Chile and Peru and are also investigating the history of nitrate preserved in ancient rocks.

The research was funded by the National Science Foundation (grant number OCE-0727123).