Hot spots drive uptake and short-term processing of organic and inorganic carbon and nitrogen in intertidal sediments

This study uses dual-labelled (13C and 15N) stable isotope applications to examine uptake and short-term processing of carbon (C) and nitrogen (N) by microbial communities in intertidal sediment from three subtropical estuarine sites. We examine differences in microbial uptake and retention that arise due to domination of microbial processing by either microphytobenthos or heterotrophic bacteria. We compare amino acids and algal dissolved organic matter (Algal DOM) and glucose and NH4+ versus newly fixed microphytobenthos C (MPB-C) and NH4+ using in situ applications across 24 h to identify uptake into the microbial community and sediment OM. Algal DOM had preferential C uptake and more retention across 24 h indicating precursors incorporated into biosynthetic pathways for biomass. Conversely, amino acid C was not incorporated or rapidly respired to DIC but displayed clear preferential uptake and retention of 15N. Short-term (24 h) retention of glucose was higher than MPB-C, while uptake of 15N from NH4+ was similar between treatments, potentially indicating glucose-stimulated export of 15N via coupled nitrification-dentrification. Despite careful selection of similar sites and sediment types, we found substantial variability between replicates and sites in the uptake and processing of labeled substrate that challenged traditional statistical analysis due to non-homogenous variance. Uptake variability across orders of magnitude is likely due to disproportionate processing of substrates occurring in hotspots of microbial processing within sediment. Development of analytical techniques to provide robust strategies to handle variability caused by abiotic and biotic factors will allow greater clarity surrounding in situ biogeochemical processing in intertidal environments.