The environmental controls that govern the end product of bacterial nitrate respiration

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Science  08 Aug 2014:
Vol. 345, Issue 6197, pp. 676-679
DOI: 10.1126/science.1254070

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How microbes compete for nitrate

Much of the ammonia fertilizer we use ends up in surface or coastal waters as nitrate. This runoff lowers water quality and damages ecosystems. Although microbial nitrogen respiration influences the fate of nitrate, it is unclear which environmental factors exert the most control. Kraft et al. performed multiple long-term incubation studies of microbial communities from marine sediments. The relative supply of nitrate and nitrite, as well as total carbon and nitrogen, provided selective pressures that drove communities toward denitrification or ammonification. The average generation time of the community also strongly influenced which respiration pathway dominated.

Science, this issue p. 676


In the biogeochemical nitrogen cycle, microbial respiration processes compete for nitrate as an electron acceptor. Denitrification converts nitrate into nitrogenous gas and thus removes fixed nitrogen from the biosphere, whereas ammonification converts nitrate into ammonium, which is directly reusable by primary producers. We combined multiple parallel long-term incubations of marine microbial nitrate-respiring communities with isotope labeling and metagenomics to unravel how specific environmental conditions select for either process. Microbial generation time, supply of nitrite relative to nitrate, and the carbon/nitrogen ratio were identified as key environmental controls that determine whether nitrite will be reduced to nitrogenous gas or ammonium. Our results define the microbial ecophysiology of a biogeochemical feedback loop that is key to global change, eutrophication, and wastewater treatment.

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