Ocean Science

Reduced Nitrogen

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Science  06 May 2005:
Vol. 308, Issue 5723, pp. 759
DOI: 10.1126/science.308.5723.759a

The growth of phytoplankton is limited by the loss of fixed nitrogen from the world's oceans. This loss occurs predominantly in zones of low oxygen (< 25 μM), such as the Black Sea, Chilean waters, and the Benguela upwelling off the Namibian coast. Classically, N2 was thought to be produced by denitrification—the reduction of nitrate to N2 by heterotrophic bacteria—but Kuypers et al. show that a large contribution may come via the anammox process: the anaerobic oxidation, carried out by bacteria known as Planctomycetes, of ammonium by nitrite. They present five corroborating strands of evidence. First, concentrations of nitrate drop at the bottom of the oxic zone; second, ammonium concentrations in the suboxic zone are low; third, water samples doped with [15N]nitrate and [14N]ammonium produced significant amounts of 14N15N; fourth, ladderane lipids, characteristic of the anammoxosome membrane, were present; fifth, fluorescence in situ hybridization and ribosomal RNA sequence analysis revealed an abundance of Planctomycetes in the suboxic zone. One unknown is why there are anammox bacteria in the Benguela upwelling at depths where there is free oxygen (9 μM). Either these cells are quiescent, or there may be a suboxic microenvironment available, such as marine snow. — CA

Proc. Natl. Acad. Sci. U.S.A. 102, 6478 (2005).

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