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AME prepress abstract   -  DOI:

Nitrogen cycling in coastal sediment microbial communities with seasonally variable benthic nutrient fluxes

Alexis J. Marshall, Andrew Longmore, Lori Phillips, Caixian Tang, Helen L. Hayden, Karla B. Heidelberg, Pauline Mele

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ABSTRACT: Benthic microbial communities contribute to nitrogen (N) cycling in coastal ecosystems through taxon specific processes such as anammox, nitrification and N-fixation and community attributed pathways such as denitrification. By measuring the total (DNA-based) and active (RNA-based) surface sediment microbial community composition and the abundance and activity profiles of key N-cycling genes in a semi-enclosed embayment—Port Phillip Bay (PPB), Australia—we show that although the total relative abundance of N-cycling taxa is comparatively lower close to estuary inputs (Hobsons Bay; HB), the capacity for this community to perform diverse N-cycling processes is comparatively higher than in sediments isolated from inputs (Central PPB ; CPPB). In HB, seasonal structuring of the sediment microbial community occurred between spring and summer, co-occurring with decreases in the activity profiles of anammox bacteria and organic carbon content. No changes were detected in the activity profiles of nitrifiers or the community-based pathway denitrification. Although no seasonal structuring of the sediment microbial community occurred in CPPB, the activity profiles of key N-cycling genes displayed comparatively higher within-site variability. These results show that despite N-cycling taxa representing a smaller fraction of the total community composition in estuary impacted sediments (HB), these microbial communities consistently engage in N-cycling processes and that seasonal instability in the composition of this community is not reflective of changes in its capacity to cycle N through coupled nitrification-denitrification but potentially via changes within the anammox community.