Short-term temperature effects on oxygen and sulfide cycling in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

We investigated short-term temperature effects on oxygen and sulfide cyclin g with O-2, pH, and H2S microsensors in a hypersaline cyanobacterial mat, i ncubated in darkness and at a downwelling irradiance, E-d (PAR), of 425 mu mol photons m(-2) s(-1) in a laboratory. The incubation temperature was inc reased from 25 to 40 degrees C in 5 degrees C intervals. Areal rates of gro ss and net photosynthesis, of O-2 consumption in the aphotic zone and of da rk O-2 consumption were maximal at 30 degrees C, i.e. close to the in situ temperature of the natural habitat. Areal rates of dark oxygen consumption showed only a minor temperature dependence as O-2 consumption was diffusion limited at all temperatures. Sulfide production increased strongly with te mperature in both the dark and light incubated mat (Q(10) = 1.8 to 3.2), an d this led to saturation of sulfide oxidation and an increased sulfide affl ux out of the dark incubated mat, which was maximal at 35 degrees C. In the uppermost layer of the dark incubated mat, pH decreased due to aerobic res piration, sulfide oxidation and fermentation, and this decrease was enhance d with temperature. In the light incubated mat, the thickness of the photic zone decreased with temperature from 0.9 to 0.5 mm. Oxygen penetration and peak oxygen concentration decreased with temperature, whereas the upper su lfide boundary and thus the zone of sulfide oxidation rose closer to the ma t surface in the light incubated mat. Areal rates of sulfide oxidation incr eased more than 2-fold from 25 to 40 degrees C in the light incubated mat. The relative contribution of sulfide oxidation to oxygen consumption in the aphotic zone increased significantly with temperature, indicating that at elevated temperatures incomplete sulfide oxidation occurred in the light in cubated mat. Both the photosynthetically induced pH maximum and the overall pH of the mat decreased with increasing temperature due to enhanced hetero trophic activity, sulfide oxidation, and a changed depth distribution of th ese processes. Our data demonstrate a close coupling of oxygen and sulfur c ycling in hypersaline microbial mats, that is strongly regulated by tempera ture.