Response of fermentation and sulfate reduction to experimental temperature changes in temperate and Arctic marine sediments

Anaerobic degradation of organic material generally proceeds through a sequence of steps, including polymer hydrolysis, fermentation and respiration or methanogenesis. The intermediates, such as volatile fatty acids (VFA) or H(2), are generally maintained at low concentration, showing a close coupling of the terminal oxidation to fermentation. We exposed marine sediments to extreme temperature perturbations to study the nature and robustness of this coupling. Bacterial sulfate reduction and its dependence on fermentation were studied experimentally over a broad temperature range of -0.3 to 40 degrees C in sediments from temperate and permanently cold environments. In an Arctic sediment from Svalbard, the apparent optimum temperature for sulfate reduction decreased with prolonged incubation, whereas sulfate reduction rates increased. In a temperate sediment from the North Sea, the apparent optimum temperature was higher and did not change with incubation time. Up to a critical temperature, the concentrations of VFA remained low, <3 microM for acetate and <1 microM for the other VFA, the H(2) concentration showed thermodynamic control by sulfate-reducing bacteria, revealing a close coupling of fermentation and sulfate reduction. Above the critical temperature, the concentrations of VFA and H(2) increased transiently by 100-1000-fold. According to the different in situ temperatures of the samples, the critical temperature was lower for sediments from the Arctic than from the North Sea. The H(2) concentrations decreased again upon prolonged incubation to values typical for sulfate-depleted methanogenic sediments. This suggests that fermentative bacteria and methanogenic archaea in both sediments tolerated higher temperatures than the sulfate-reducing community.