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Marine Ecology Progress Series

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MEPS 482:57-68 (2013)  -  DOI:

Evidence and a conceptual model for the co‑occurrence of nitrogen fixation and denitrification in heterotrophic marine sediments

R. W. Fulweiler1,*, S. M. Brown2, S. W. Nixon3, B. D. Jenkins2, 3 

1Department of Earth and Environment & Department of Biology, Boston University, Boston, Massachusetts 02215, USA
2Department of Cell and Molecular Biology, University of Rhode Island, Kingston, Rhode Island 02881, USA
3Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA

ABSTRACT: Marine waters are often nitrogen (N) limited. Denitrification, the microbial conversion of nitrate to dinitrogen (N2) gas, is responsible for significant N removal from the coastal ocean. In contrast, nitrogen fixation, the microbial transformation of N2 to ammonium, is typically regarded as an inconsequential N source. The imbalance between these 2 processes is responsible, at least in part, for N limitation in the coastal ocean. Organic matter quality and quantity has been shown to determine rates of these critical N cycling processes. We hypothesized that the timing of organic matter deposition to the benthos might also be important in determining which process dominates. We tested this hypothesis using a coupled biogeochemical-molecular approach. We report directly measured net sediment denitrification rates and corresponding expression of  nirS, a gene in the denitrification pathway, with the simultaneous expression of nifH, a gene associated with nitrogen fixation. The timing of organic matter deposition determined the magnitude of the net sediment N2 fluxes. Highest rates of denitrification occurred soon after deposition, and the lowest rates occurred over 200 d after the last deposition event concomitant with increased nifH expression. Phylogenetic evidence suggests that sulfur and sulfate reducers are responsible for the nitrogen fixation. Globally, warming water temperatures, changes in light, and reduced nutrient loads through management intervention have been linked to decreases and/or altered phenology of water column productivity. Based on a conceptual model developed here, we suggest that in these systems, heterotrophic sediment nitrogen fixation may become an important component of the nitrogen budget.

KEY WORDS: Denitrification · Heterotrophic nitrogen fixation · Marine sediments · nifH expression · Direct sediment N2 fluxes

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Cite this article as: Fulweiler RW, Brown SM, Nixon SW, Jenkins BD (2013) Evidence and a conceptual model for the co‑occurrence of nitrogen fixation and denitrification in heterotrophic marine sediments. Mar Ecol Prog Ser 482:57-68.

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