Newly discovered bacterial partnership changes ocean chemistry

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 the waste products of the Thioploca: nitrite and ammonium, or "fixed" nitrogen.

Nitrogen is a crucial building block of life, a prerequisite for photosynthesis. While nitrogen is present in abundance in Earth atmosphere, to be useful for most of 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 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 foodchain 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. 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," said Maria Prokopenko, lead author of a paper on the research that will appear in Nature on August 8.

Prokopenko is currently a visiting scholar at Pomona College, but completed the research while she was a research assistant professor at USC, working closely with William Berelson, chair of the Earth Sciences Department at the USC Dornsife College of Letters, Arts and Sciences.

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

The symbiosis between Thioploca and Anammox is not one creating widespread change throughout the ocean, but rather 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.