MICROBIAL SULFATE REDUCTION IN LITTORAL SEDIMENT OF LAKE CONSTANCE

Porewater concentrations of SO4(2-) 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-mu-M at the surface to 10-20-mu-M 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 S(o) 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-degrees-C). Concentrations of sulfate greater 60-mu-M 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 formed parallel to the decrease of thiosulfate, most likely due to bacterial thiosulfate disproportionation.