The earth is full of locales seemingly inhospitable to life. In areas like that deep beneath the ocean's mud floor, oxygen cannot penetrate. In such anoxic environments, the simple cellular precursors of all life--bacteria and archea--thrive, but the single-celled ancestors of more complex life-forms, known as eukaryotes, were thought to suffocate. Now new research has shown that at least one eukaryotic species--a shelled, amoebalike creature called a foraminifer--can prosper without oxygen by respiring nitrogen instead.
Marine ecologist Nils Risgaard-Petersen of the National Environmental Research Institute in Silkeborg, Denmark, and his team investigated the flora in a sediment core from Gullmar Fjord, Sweden. Deep into the mud, the researchers found a pool of nitrate that seemed to correlate with foraminifera abundance. This foraminifer--Globobulimina pseudospinescens--also contained nitrate concentrations at least 500-fold higher than the surrounding sludge. A second sediment core collected in Chile's Concepcion Bay confirmed that foraminifera were hoarding nitrate.
Accumulating nitrate requires energy and therefore is unlikely to be undertaken unless it benefits the organism in some way. Archaea and bacteria benefit by breathing the stuff in the surrounding mud, turning nitrate into dinitrogen gas and getting rid of their organic waste in the process, a chemical reaction known as denitrification. In the lab, G. pseudospinescens consistently respired the nitrate, and close examination of the eukaryote's makeup showed that it did not contain any bacteria that could assist in this process. Rather, the organism used its own cellular machinery to control the process, storing as much as a month's worth of nitrate.
The finding marks the first time that more complex animals have been proved to play a role in denitrification, a crucial part of the nitrogen cycle, and have shown an ability to proliferate in anoxic environments. G. pseudospinescens likely acquired the trick because nitrogen is a nontoxic ion and can be safely stored as the organism plunges into deeper strata. (In contrast, stored oxygen, because it is so reactive, can wreak internal havoc in such microbes.) And it is probably not alone; two other species of foraminifera found in the Chilean sediment showed nitrate concentrations "up to 15,000-fold higher than the maximum porewater concentration," the researchers write in the paper presenting the finding in today's Nature.