Microbial sulfate reduction in littoral sediment of Lake Constance

Porewater concentrations of SO42− and NO3−, the depth distribution of FeS, FeS2 and organically bound sulfur, and the spatial distribution and intensity of dissimilatory microbial sulfate reduction were studied at a littoral site in Lake Constance. Porewater sulfate concentrations dropped steeply from about 300 µM at the surface to 10–20 µM at 5–6 cm depth. Free H2S could not be detected in porewater samples. Of the total sediment sulfur 53% was present in an organically bound form, 41% as pyrite and So and only 6% as acid volatile sulfur (FeS). The concentrations of dissolved anions and the rates of sulfate reduction showed intensive short- and long-term variations consistent with the strong seasonal changes of temperature and water level. Sulfate reduction rates were lowest just after the spring thaw (ca. 300–400 nmol cm−2 day−1), but increased strongly toward summer and reached a maximum of more than 2000 nmol cm−2 day−1 in September. The zone of most intense sulfate reduction was restricted to the upper 3 cm of the sediment with a distinct maximum at a depth of 1–2 cm. In deeper zones sulfate reduction rates declined markedly. The apparent activation energy of sulfate reduction, determined by slurry experiments, was 54.1 kJ mol−1; the corresponding Q10-value was 2.25 (between 5 and 15°C). Concentrations of sulfate greater 60 µM did not increase rates in sediment slurries. The relation between sulfate reduction rates and sulfate concentration was not in accordance with Michaelis-Menten saturation kinetics. Thiosulfate or nitrate added to sediment slurries was rapidly consumed with rates of 2620 nmol cm−3 day−1 and 59800 nmol cm−3 day−1, respectively. Sulfate was formend parallel to the decrease of thiosulfate, most likely due to bacterial thiosulfate disproportionation.