Cycling of some low molecular weight volatile fatty acids in a permanently anoxic estuarine basin

Low molecular weight volatile fatty acids (VFAs) are important products of the anaerobic fermentation of organic matter. However, little is known of the cycling of VFAs in the water column. In this study, the concentrations of acetate, propionate, isobutyrate and butyrate were measured in the water column of a permanently anoxic basin by a static diffusion method. Acetate concentrations varied from 0.8 to 6.1 μM, with the maximum value measured near a peak of anoxyphototrophs. The acetate concentration was relatively uniform except in the peak. The concentration of propionate varied from 44.3 nM to 191.0 nM, with a maximum value found in a layer dominated by the flagellated protozoan, Euglena proxima, which was located just below the depth at which O2 went to zero. The concentration profiles of butyrate and isobutyrate were similar to the concentration profile of propionate. Acetate turnover rate constants varied from 0.1 to 5.1 h−1. A maximum in the acetate uptake rate constant was found during the day in the layer rich in E. proxima, but not at night, and possibly was associated with O2 production from photosynthesis at this depth. Acetate uptake rate constants also were higher during the day as compared with the night in a deeper layer with abundant anoxyphototrophic bacteria, suggesting photoassimilation of acetate by these organisms may be important. In the E. proxima layer, and at the base of the oxycline, most acetate uptake was inhibited by BES in a short term incubation, suggesting that methanogens might be a sink for acetate. Integrated acetate oxidation rates in this system were almost twice as high as the integrated rates of primary production.