MEPS 276:37-52 (2004)  -  doi:10.3354/meps276037

Anoxic nitrification in marine sediments

Robert J. G. Mortimer1,*, Sansha J. Harris1, Michael D. Krom1, Thomas E. Freitag2, James I. Prosser2, Jonathan Barnes3, Pierre Anschutz4, Peter J. Hayes5, Ian M. Davies5

1Earth and Biosphere Institute, School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
2Department of Molecular & Cell Biology, University of Aberdeen, Institute of Medical Sciences, Foresterhill, Aberdeen AB25 2ZD, UK
3Department of Marine Sciences and Coastal Management, University of Newcastle-upon-Tyne, Newcastle, UK
4Université Bordeaux 1, Departement de Geologie et Oceanographie (D.G.O.), CNRS UMR 5805, 33405 Talence Cedex, France
5Fisheries Research Services, Marine Laboratory, Aberdeen AB11 9DB, UK

ABSTRACT: Nitrate peaks are found in pore-water profiles in marine sediments at depths considerably below the conventional zone of oxic nitrification. These have been interpreted to represent non-steady-state effects produced by the activity of nitrifying bacteria, and suggest that nitrification occurs throughout the anoxic sediment region. In this study, ΣNO3 peaks and molecular analysis of DNA and RNA extracted from anoxic sediments of Loch Duich, an organic-rich marine fjord, are consistent with nitrification occurring in the anoxic zone. Analysis of ammonia oxidiser 16S rRNA gene fragments amplified from sediment DNA indicated the abundance of autotrophic ammonia-oxidising bacteria throughout the sediment depth sampled (40 cm), while RT-PCR analysis indicated their potential activity throughout this region. A large non-steady-state pore-water ΣNO3 peak at ~21 cm correlated with discontinuities in this ammonia-oxidiser community. In addition, a subsurface nitrate peak at ~8 cm below the oxygen penetration depth, correlated with the depth of a peak in nitrification rate, assessed by transformation of 15N-labelled ammonia. The source of the oxidant required to support nitrification within the anoxic region is uncertain. It is suggested that rapid recycling of N is occurring, based on a coupled reaction involving Mn oxides (or possibly highly labile Fe oxides) buried during small-scale slumping events. However, to fully investigate this coupling, advances in the capability of high-resolution pore-water techniques are required.

KEY WORDS: Anoxic nitrification · Diffusive Equilibrium in Thin films (DET) · 16sRNA

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