CYCLING OF SOME LOW-MOLECULAR-WEIGHT VOLATILE FATTY-ACIDS IN A PERMANENTLY ANOXIC ESTUARINE BASIN

Low molecular weight volatile fatty acids (VFAs) are important product s of the anaerobic fermentation of organic matter. However, little is known of the cycling of VFAs in the water column. In this study, the c oncentrations of acetate, propionate, isobutyrate and butyrate were me asured in the water column of a permanently anoxic basin by a static d iffusion method. Acetate concentrations varied from 0.8 to 6.1 mu M, w ith the maximum value measured near a peak of anoxyphototrophs. The ac etate concentration was relatively uniform except in the peak. The con centration of propionate varied from 44.3 nM to 191.0 nM, with a maxim um value found in a layer dominated by the flagellated protozoan, Eugl ena proxima, which was located just below the depth at which O-2 went to zero. The concentration profiles of butyrate and isobutyrate were s imilar to the concentration profile of propionate. Acetate turnover ra te constants varied from 0.1 to 5.1 h(-1). A maximum in the acetate up take rate constant was found during the day in the layer rich in E. pr oxima, but not at night, and possibly was associated with O-2 producti on from photosynthesis at this depth. Acetate uptake rate constants al so were higher during the day as compared with the night in a deeper l ayer with abundant anoxyphototrophic bacteria, suggesting photoassimil ation of acetate by these organisms may be important. In the E. proxim a layer, and at the base of the oxycline, most acetate uptake was inhi bited by BES in a short term incubation, suggesting that methanogens m ight be a sink for acetate. Integrated acetate oxidation rates in this system were almost twice as high as the integrated rates of primary p roduction.