Acetate bioavailability and turnover in an estuarine sediment
An enzymatic method for the determination of acetate in marine porewaters was cross-calibrated against a bioassay technique to investigate whether the enzymatic method directly measured the bioavailable acetate pool. Cells of the acetate-oxidising sulfate-reducing bacterium Desulfobacter sp. (DSM 2035) were added to sterile porewater from an intertidal estuarine sediment (Southdown, Tamar Estuary, UK), at three different depths. The porewater was inoculated with [1(2)-14C]acetate and incubated. Removal of [14C]acetate, production of 14CO2 and changes in acetate pool concentrations, measured by both chemical derivatisation and the enzyme method, were determined. [14C]acetate, total and bioavailable acetate were initially rapidly removed, but after depletion of bioavailable acetate a significant amount (20–50%) of the original acetate pool remained unmetabolised. After 18 h the samples were respiked with [14C]acetate, which was mineralised rapidly (at the same rate as initial depletion) although the total recalcitrant acetate pool remained constant. Decay rates at each depth for the initial removal of [14C]acetate and after respiking were the same. Degradation rates of acetate in sediments decreased with depth, corresponding with lower biologically available acetate concentrations deeper in the sediment. Although both bioavailable and total acetate methods result in an overestimate when compared to an independent measure of carbon flow within sediments (sulfate reduction), the overestimate is significantly smaller when data from the enzyme method are used, although still not wholly satisfactory (208%). The enzymatic assay offers advantages over chemical determinations of acetate in marine porewaters, although when used to directly predict the proportion of bioavailable acetate, results do not concur with those provided by the bioassay technique, and below 9 cm in the sediment a substantial sulfate reduction rate was measured but no bioavailable acetate was present. The bioassay technique indicates that bioavailable acetate decreases with increasing sediment depth, and that the pools are turned over at different rates: whether this has an effect on carbon flow within sediments requires further investigation.