Effects of drift algae accumulation and nitrate loading on nitrogen cycling in a eutrophic coastal sediment

The permeable (sandy) sediments that dominate the world's coastlines and continental shelves are highly exposed to nitrogen pollution, predominantly due to increased urbanisation and inefficient agricultural practices. This leads to eutrophication, accumulation of drift algae and changes in the reactions of nitrogen, including the potential to produce the greenhouse gas nitrous oxide (N₂O). Nitrogen pollution in coastal systems has been identified as a global environmental issue, but it remains unclear how this nitrogen is stored and processed by permeable sediments. We investigated the interaction of drift algae biomass and nitrate (NO₃^-) exposure on nitrogen cycling in permeable sediments that were impacted by high nitrogen loading. We treated permeable sediments with increasing quantities of added macroalgal material and NO₃^- and measured denitrification, dissimilatory NO₃^- reduction to ammonium (DNRA), anammox, and nitrous oxide (N₂O) production, alongside abundance of marker genes for nitrogen cycling and microbial community composition by metagenomics. We found that the presence of macroalgae dramatically increased DNRA and N₂O production in sediments without NO₃^- treatment, concomitant with increased abundance of nitrate-ammonifying bacteria (e.g. Shewanella and Arcobacter). Following NO₃^- treatment, DNRA and N₂O production dropped substantially while denitrification increased. This is explained by a shift in the relative abundance of nitrogen-cycling microorganisms under different NO₃^- exposure scenarios. Decreases in both DNRA and N₂O production coincided with increases in the marker genes for each step of the denitrification pathway (narG, nirS, norB, nosZ) and a decrease in the DNRA marker gene nrfA. These shifts were accompanied by an increased abundance of facultative denitrifying lineages (e.g. Pseudomonas and Marinobacter) with NO₃^- treatment. These findings identify new feedbacks between eutrophication and greenhouse gas emissions, and in turn have potential to inform biogeochemical models and mitigation strategies for marine eutrophication.