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

We performed a ¹⁵N-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 ¹⁵N pulse–chase experiment. After injection of ¹⁵NH₄⁺ into the sediment, ¹⁵N was traced into bulk sediment, total hydrolyzable amino acids (THAAs, representing bulk proteinaceous biomass), the bacterial biomarker D-alanine, and fauna over a 30- d period. Additional experiments included short-term (24 h) incubations of sediment cores injected with different ¹⁵N-labeled substrates (NH₄⁺, NO₃^-, urea, and an amino acid mixture) and sediment core incubations for analysis of benthic fluxes of O₂, dissolved inorganic carbon, NH₄⁺, NO_x^-, dissolved organic nitrogen, and N₂. 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 (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.