Sulfate reduction rates and low molecular weight fatty acid concentrations in the water column and surficial sediments of the Black Sea

Abstract

Sulfate reduction rates and concentrations of low molecular weight organic acids were measured in the water column and surficial sediments at two sites in the central Black Sea. Water column sulfate reduction rates were much lower than previously reported. The highest rate measured was 3.5 nM day −1 and on a depth integrated basis values of 1.2 and 0.22 mmol m −2 day −1 were obtained for the two sites. Sediment sulfate reduction rates were within the ranges previously reported but were higher than some for comparable abyssal sites. Rates were about 21 μM day −1 in the flocculent layer at the sediment-water interface, decreasing to 2–3μM day −1 at 20 cm depth. On an areal, depth integrated basis, rates at the two sites were 1.45 and 1.29 mmol m −2 day −1. Thus, the water column and sediments have comparable areal rates, but on a volume basis the sediment rates are several thousand times higher than the water column rates. Organic acid concentrations in the anoxic Black Sea water column were surprisingly high, reaching several μM in some cases. One deep sample contained 60μM acetate. Lactate, acetate and formate were the only acids detected in the water column. Some propionate was seen in sediment porewaters. Apparent turnover times of the organic acids in the water column, calculated for utilization solely by sulfate reducing bacteria, are tens to hundred of years. This suggests that sulfate reduction rates in the water column were not limited by organic substrate supply. In the sediments, apparent acid turnover times calculated in this way are generally less than one day, suggesting that sulfate reduction may be limited to by the supply of these substrates through fermentation reactions.