Nitrogen incorporation and retention by bacteria, algae, and fauna in a subtropical intertidal sediment: An in situ

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 15NH z4 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 30d period. Additional experiments included short-term (24 h) incubations of sediment cores injected with different 15N-labeled substrates (NH z4 , NO { 3 , urea, and an amino acid mixture) and sediment core incubations for analysis of benthic fluxes of O2, dissolved inorganic carbon, NH z 4 , 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 (,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. Nitrogen is often a limiting element in coastal sediments, which results in strong competition between different groups of microorganisms (bacteria and benthic microalgae [BMA]) that need dissolved nitrogenous substrates to support their growth as well as for catabolic processes such as nitrification and denitrification (e.g., Rysgaard et al. 1995, Risgaard-Petersen 2003, Sundbäck et al. 2004). Moreover, one would expect the benthic microbial community to be very economical with available nitrogen. This is reflected by results from studies on benthic nitrogen fluxes in different subtropical Australian estuaries (Ferguson et al. 2004, Eyre and Ferguson 2005) as well as other coastal systems (e.g., Lomstein et al. 1998, Anderson et al. 2003, Cook et al. 2004) that suggest a relatively strong retention of nitrogen in the sediment. This retention is thought to be due to incorporation of remineralized nitrogen by BMA or bacteria or both (e.g., Lomstein et al. 1998, Anderson et al. 2003, Ferguson et al. 2004) and possibly by transfer of nitrogen to higher trophic levels (Eyre and Ferguson 2005). However, ‘‘traditional’’ measurements of sediment–water nitrogen fluxes only allow analysis of the net result of all nitrogen cycling processes in the sediment (i.e., the sediment remains a ‘‘black box’’), meaning that elucidation of the numerous nitrogen transformation processes and nitrogen flows through (micro-)organisms within the sediment requires an alternative approach. A useful approach is stable isotope labeling with the nitrogen isotope 15N. 15N-labeling has previously been used to measure uptake of nitrogenous substrates by the microbial community in the water column (e.g., Dugdale and Goering 1967, McCarthy and Eppley 1972). However, applications in studies on sediments have so far been limited to studies on nitrogen transformation processes such as nitrification and denitrification (e.g., Rysgaard et al. 1993; Ottosen et al. 2001) or analysis of 15N uptake by Acknowledgments We thank Lexie Walker for doing the fauna extractions, Iain Alexander for the nutrient analysis, alkalinity titrations, and N2/ Ar analysis, Peter van Breugel for analysis of 15N in bulk sediment and fauna, Marco Houtekamer for his help with the GC-c-IRMS analyses, Nicole Dijkman for the PLFA extractions, Cobie Kleppe for the pigment analyses, and Dick van Oevelen for the model simulations of 15N in fauna. We also thank Ron Benner, Gerhard Herndl, and two anonymous reviewers for their useful