Nitrogen incorporation and retention by bacteria, algae, and fauna in a subtropical intertidal sediment:: An in situ 15N-labeling study

We performed a N-15-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 N-15 pulse-chase experiment. After injection of (NH4+)-N-15 into the sediment, N-15 was traced into bulk sediment, total hydrolyzable amino acids ( THAAs, representing bulk proteinaceous biomass), the bacterial biomarker D-alanine, and fauna over a 30d period. Additional experiments included short-term ( 24 h) incubations of sediment cores injected with different N-15-labeled substrates ( NH4+, NO3-, urea, and an amino acid mixture) and sediment core incubations for analysis of benthic fluxes of O-2, dissolved inorganic carbon, NH4+, NOx-, dissolved organic nitrogen, and N-2. 15N was rapidly incorporated and strongly retained in microbial biomass ( THAAs) during the 30-d period in situ, indicating efficient recycling of 15N by the benthic microbial community. Analysis of 15N in D-alanine revealed a major bacterial contribution ( 50-100%) to total microbial 15N incorporation and retention. 15N was also incorporated into fauna via grazing on 15N-labeled microbial biomass, but this was a negligible fraction (, 1%) of total 15N 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.