No abstract

The quantitative contribution of bacteria to total microbial uptake of nitrogenous substrates is an aspect of the aquatic nitrogen cycle that is still largely unclear, mainly because existing methods are generally inadequate. We investigated the feasibility of measuring

Cold-water corals (CWC) are widely distributed around the world forming extensive reefs at par with tropical coral reefs. They are hotspots of biodiversity and organic matter processing in the world’s deep oceans. Living in the dark they lack photosynthetic symbionts and are therefore considered to depend entirely on the limited flux of organic resources from the surface ocean. While symbiotic relations in tropical corals are known to be key to their survival in oligotrophic conditions, the full metabolic capacity of CWC has yet to be revealed. Here we report isotope tracer evidence for efficient nitrogen recycling, including nitrogen assimilation, regeneration, nitrification and denitrification. Moreover, we also discovered chemoautotrophy and nitrogen fixation in CWC and transfer of fixed nitrogen and inorganic carbon into bulk coral tissue and tissue compounds (fatty acids and amino acids). This unrecognized yet versatile metabolic machinery of CWC conserves precious limiting resources and provides access to new nitrogen and organic carbon resources that may be essential for CWC to survive in the resource-depleted dark ocean.

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We performed a 15 N-labeling study to investigate nitrogen incorporation and retention by the benthic microbial community (bacteria and benthic microalgae) and fauna in the intertidal sediment of the subtropical Australian Brunswick Estuary. The main experiment involved an in situ 15 N pulse-chase experiment. After injection of 15 NH 15 N was rapidly incorporated and strongly retained in microbial biomass (THAAs) during the 30-d period in situ, indicating efficient recycling of 15 N by the benthic microbial community. Analysis of 15 N in D-alanine revealed a major bacterial contribution (50-100%) to total microbial 15 N incorporation and retention. 15 N was also incorporated into fauna via grazing on 15 N-labeled microbial biomass, but this was a negligible fraction (,1%) of total 15 N in the sediment. Altogether, results show that efficient recycling of nitrogen by the benthic microbial community can be an important mechanism for nitrogen retention in the sediment and an important pathway supporting benthic microbial production.