Acetate cycling in the water column of the Cariaco Basin: Seasonal and vertical variability and implication for carbon cycling

Acetate oxidation frequently has been used as proxy of organic carbon decomposition in marine anoxic sediments. However, the importance of acetate uptake in carbon cycling in marine anoxic water columns is less well studied. Acetate concentrations and uptake rate constants, together with total bacterial numbers, primary and chemoautotro-phic production rates, and particulate organic carbon (POC) fluxes, were measured in the water column of the Cariaco Basin during upwelling and nonupwelling seasons between November 1995 and May 1999 as part of the international CARIACO (Carbon Retention In A Colored Ocean) program. Acetate uptake was found to vary strongly with depth and season. Zones of elevated acetate uptake were found in the surface waters and near the suboxic/anoxic interface. High acetate uptake in the surface oxic layer suggests that acetate cycling may be an important component of organic carbon oxidation in oxic environments as well as under anoxic conditions. Depth-integrated acetate uptake rates were correlated with the rates of organic carbon supply in the two zones (r2 5 0.37, P 5 0.017). Comparisons of acetate oxidation rates with rates of primary production, chemoautotrophic production, and POC flux show that, on average, acetate oxidation can account for respiration of between 16 and 46 % of the organic carbon fixed in the water column. During microbial degradation of organic matter, polymer