Permeable sands cover more than half of the coastal and riverine environments, where they serve as major contributors to nitrogen (N) loss. The extent of this N-loss is enhanced by the occurrence of denitrification in apparently oxic sediments. This has been attributed to microbial adaptations to the frequent oscillations in oxygen (O₂) availability that characterize sands. Yet, the microenvironments around individual sand grains that are important factors in controlling the microbial metabolism are only little understood. Here, we used a new non-invasive microfluidic technique that allowed us for the first time to observe O₂ concentrations and distributions of microorganisms at the single sand grain level. This revealed a high heterogeneity in O₂ production and consumption rates, which varied by two orders of magnitude, matching the patchy microbial colonization. We used our observations to develop a transport-reaction model to further investigate how this microbial activity shapes anoxic microenvironments in sands. This showed that anoxic microenvironments form within the boundary layer of sand grains even at O₂ concentrations as high as 80 µM. Based on a new scaling relationship, we predict that more than 70% of denitrification that occurs in apparently oxic sands can be attributed to the presence of anoxic microenvironments. This study therefore indicates a previously underestimated contribution of anoxic microenvironments to N-loss by denitrification in oxic sediments.

Primary Presenter: Farooq Moin Jalaluddin, Max Planck Institute for Marine Microbiology, Bremen (fjalalud@mpi-bremen.de)

Authors:

Farooq Moin Jalaluddin, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany   (fjalalud@mpi-bremen.de)

Soeren Ahmerkamp, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany  (sahmerka@mpi-bremen.de)

Hannah Marchant, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany  (hmarchan@mpi-bremen.de)

Volker Meyer, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany  (vmeyer@mpi-bremen.de)

Klaus Koren, Aarhus University Centre for Water Technology, Department of Biology, Aarhus University, 8000 Aarhus, Denmark  (klaus.koren@bio.au.dk)

Marcel Kuypers, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany  (mkuypers@mpi-bremen.de)