Metabolic specialization of denitrifiers in permeable sediments controls N2 O emissions.

Hannah K Marchant,Frank Schreiber,Gaute Lavik,Jon Graf,Marc Mussmann,Moritz Holtappels,Marc Strous,Soeren Ahmerkamp,Halina E Tegetmeyer,Marcel M M Kuypers

doi:10.1111/1462-2920.14385

Abstract

Coastal oceans receive large amounts of anthropogenic fixed nitrogen (N), most of which is denitrified in the sediment before reaching the open ocean. Sandy sediments, which are common in coastal regions, seem to play an important role in catalysing this N-loss. Permeable sediments are characterized by advective porewater transport, which supplies high fluxes of organic matter into the sediment, but also leads to fluctuations in oxygen and nitrate concentrations. Little is known about how the denitrifying communities in these sediments are adapted to such fluctuations. Our combined results indicate that denitrification in eutrophied sandy sediments from the world's largest tidal flat system, the Wadden Sea, is carried out by different groups of microorganisms. This segregation leads to the formation of N2 O which is advectively transported to the overlying waters and thereby emitted to the atmosphere. At the same time, the production of N2 O within the sediment supports a subset of Flavobacteriia which appear to be specialized on N2 O reduction. If the mechanisms shown here are active in other coastal zones, then denitrification in eutrophied sandy sediments may substantially contribute to current marine N2 O emissions.

Highlights

The continental shelves and coastal regions act as a buffer between the land and open ocean, making them vulnerable to anthropogenically induced eutrophication
Sands cover more than 50% of the seafloor (Emery 1968, Hall 2002), little is known about the role of sandy sediments in global biogeochemical cycles of carbon and nitrogen
In the world’s largest intertidal area, the Wadden Sea, which is predominantly comprised of sandy sediments, nitrate inputs have increased eightfold since the 1930’s

Summary

Introduction

The continental shelves and coastal regions act as a buffer between the land and open ocean, making them vulnerable to anthropogenically induced eutrophication In these regions, sands cover more than 50% of the seafloor (Emery 1968, Hall 2002), little is known about the role of sandy sediments in global biogeochemical cycles of carbon and nitrogen. The redox conditions in permeable sands are highly variable over time scales of minutes to hours (Huettel et al, 2014, Jansen et al, 2009) This is a result of changes in the magnitude of porewater flow, which is controlled by bottom water currents, sediment topography and sediment transport (Ahmerkamp et al, 2015, Ahmerkamp et al, 2017). Little is known about how the microbial communities mediating denitrification in permeable sediments cope with the large fluctuations in oxygen and substrate availability that occur on small spatial and temporal scales

Results

Discussion

Conclusion