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Abstract

We used microsensors to study short-term temperature effects on gross photosynthesis, oxygen consumption and sulfate reduction in an extremely hypersaline (salinity 200‰) cyanobacterial mat. In situ, the mat surface can reach high temperatures (ca. 50°C in summer), and experiences daily temperature fluctuations due to tidal inundation. A mat sample was incubated in the laboratory at 25, 35, 45, 50 and 60°C. Areal rates of gross and net photosynthesis increased with increasing temperature, and maximum rates were detected at 45°C. The photosynthetic zone (i.e. the depth zone in which oxygenic photosynthesis was detectable by the light–dark shift method) decreased from 3 to 1.75 mm with increasing temperature. Above 50°C, photosynthesis was completely inhibited, probably due to high sulfide concentrations. This inhibition was confirmed by the pH profiles, where the peak in the top 2 to 4 mm at 25 to 45°C disappeared at 50 to 60°C. Between 25 and 45°C, areal oxygen consumption rates in the light did not change significantly but showed a decreasing trend in the photosynthetic zone, suggesting a decoupling between photosynthesis and oxygen consumption in the light. In both the light and dark, sulfide production was strongly enhanced by increasing temperature to a maximum flux at 50°C, above which it became inhibited. At 50 to 60°C, the sulfide produced could not be completely removed by sulfide oxidation, resulting in sulfide leakage to the overlying water. Our data demonstrate that these mat ecosystems are well adapted to high ambient temperatures. Photosynthesis, oxygen consumption in the light and sulfide production are differentially controlled by temperature. However, the individual processes in this complex community are not only directly controlled by temperature, but also by the temperature-dependent response of other processes.