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

Abstract

We investigated short-term temperature effects on oxygen and sulfide cycling with O2, pH, and H2S microsensors in a hypersaline cyanobacterial mat, incubated in darkness and at a down-welling irradiance, E(d) (PAR), of 425 μmol photons m-2 s-1 in a laboratory. The incubation temperature was increased from 25 to 40°C in 5°C intervals. Areal rates of gross and net photosynthesis, of O2 consumption in the aphotic zone and of dark O2 consumption were maximal at 30°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 O2 consumption was diffusion limited at all temperatures. Sulfide production increased strongly with temperature in both the dark and light incubated mat (Q10 = 1.8 to 3.2), and this led to saturation of sulfide oxidation and an increased sulfide efflux out of the dark incubated mat, which was maximal at 35°C. In the uppermost layer of the dark incubated mat, pH decreased due to aerobic respiration, sulfide oxidation and fermentation, and this decrease was enhanced 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 sulfide boundary and thus the zone of sulfide oxidation rose closer to the mat surface in the light incubated mat. Areal rates of sulfide oxidation increased more than 2-fold from 25 to 40°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 incubated mat. Both the photosynthetically induced pH maximum and the overall pH of the mat decreased with increasing temperature due to enhanced heterotrophic activity, sulfide oxidation, and a changed depth distribution of these processes. Our data demonstrate a close coupling of oxygen and sulfur cycling in hypersaline microbial mats, that is strongly regulated by temperature.