ACETATE BIOAVAILABILITY AND TURNOVER IN AN ESTUARINE SEDIMENT

An enzymatic method for the determination of acetate in marine porewat ers was cross-calibrated against a bioassay technique to investigate w hether the enzymatic method directly measured the bioavailable acetate pool. Cells of the acetate-oxidising sulfate-reducing bacterium Desul fobacter sp. (DSM 2035) were added to sterile porewater from an intert idal estuarine sediment (Southdown, Tamar Estuary, UK), at three diffe rent depths. The porewater was inoculated with [1(2)-C-14]acetate and incubated. Removal of [C-14]acetate, production of 14 CO2 and changes in acetate pool concentrations, measured by both chemical derivatisati on and the enzyme method, were determined. [C-14]acetate, total and bi oavailable 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 resp iked with [C-14]acetate, which was mineralised rapidly (at the same ra te as initial depletion) although the total recalcitrant acetate pool remained constant. Decay rates at each depth for the initial removal o f [C-14]acetate and after respiking were the same. Degradation rates o f 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 over estimate when compared to an independent measure of carbon flow within sediments (sulfate reduction), the overestimate is significantly smal ler when data from the enzyme method are used, although still not whol ly satisfactory (208%). The enzymatic assay offers advantages over che mical determinations of acetate in marine porewaters, although when us ed to directly predict the proportion of bioavailabie acetate, results do not concur with those provided by the bioassay technique, and belo w 9 cm in the sediment a substantial sulfate reduction rate was measur ed but no bioavailable acetate was present. The bioassay technique ind icates that bioavailable acetate decreases with increasing sediment de pth, and that the pools are turned over at different rates: whether th is has an effect on carbon now within sediments requires further inves tigation.