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Impact of glacial/interglacial sea level change on the ocean nitrogen cycle

Abstract.

"The continental shelves are the most biologically dynamic regions of the ocean, and they are extensive worldwide, especially in the western North Pacific. Their area has varied dramatically over the glacial/interglacial cycles of the last million years, but the effects of this variation on ocean biological and chemical processes remain poorly understood. Conversion of nitrate to N2 by denitrification in sediments accounts for half or more of the removal of biologically available nitrogen (“fixed N”) from the ocean. The emergence of continental shelves during ice ages and their flooding during interglacials have been hypothesized to drive changes in sedimentary denitrification. [...]"

Source: Proceedings of the National Academy of Sciences of the United States of America (PNAS)
Authors: Haojia Ren et al.
DOI: 10.1073/pnas.1701315114

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Niche construction by non-diazotrophs for N2 fixers in the eastern tropical North Atlantic Ocean

Abstract.

"Diazotrophic dinitrogen (N2) fixation contributes ~76% to “new” nitrogen inputs to the sunlit open ocean, but environmental factors determining N2 fixation rates are not well constrained. Excess phosphate (phosphate–nitrate/16 > 0) and iron availability control N2 fixation rates in the eastern tropical North Atlantic (ETNA), but it remains an open question how excess phosphate is generated within or supplied to the phosphate-depleted sunlit layer. [...]"

Source: Geophysical Research Letters
Authors: Arvind Singh et al.
DOI: 10.1002/2017GL074218

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Biological nitrogen fixation in the oxygen-minimum region of the eastern tropical North Pacific ocean

Abstract.

"Biological nitrogen fixation (BNF) was investigated above and within the oxygen-depleted waters of the oxygen-minimum zone of the Eastern Tropical North Pacific Ocean. BNF rates were estimated using an isotope tracer method that overcame the uncertainty of the conventional bubble method by directly measuring the tracer enrichment during the incubations. Highest rates of BNF (~4 nM day−1) occurred in coastal surface waters and lowest detectable rates (~0.2 nM day−1) were found in the anoxic region of offshore stations. [...]"

Source: The ISME Journal
Authors: Amal Jayakuma et al.
DOI: 10.1038/ismej.2017.97

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Microbial oceanography of anoxic oxygen minimum zones

Abstract.

"Vast expanses of oxygen-deficient and nitrite-rich water define the major oxygen minimum zones (OMZs) of the global ocean. They support diverse microbial communities that influence the nitrogen economy of the oceans, contributing to major losses of fixed nitrogen as dinitrogen (N2) and nitrous oxide (N2O) gases. Anaerobic microbial processes, including the two pathways of N2 production, denitrification and anaerobic ammonium oxidation, are oxygen-sensitive, with some occurring only under strictly anoxic conditions. The detection limit of the usual method (Winkler titrations) for measuring dissolved oxygen in seawater, however, is much too high to distinguish low oxygen conditions from true anoxia.  [...]"

Source: Proceedings of the National Academy of Science of the United States of America (PNAS)
Authors: Osvaldo Ulloa et al.
DOI: 10.1073/pnas.1205009109

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