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Short-term effects of temperature and irradiance on oxygenic photosynthesis and O2 consumption in a hypersaline cyanobacterial mat were investigated with O2 microsensors in a laboratory. The effect of temperature on O2 fluxes across the mat–water interface was studied in the dark and at a saturating high surface irradiance (2162 μmol photons m−2 s−1) in the temperature range from 15 to 45 °C. Areal rates of dark O2 consumption increased almost linearly with temperature. The apparent activation energy of 18 kJ mol−1 and the corresponding Q 10 value (25 to 35 °C) of 1.3 indicated a relative low temperature dependence of dark O2 consumption due to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperature up to 40 °C and exhibited a Q 10 value (20 to 30 °C) of 2.8. Both O2 dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 °C, with the most pronounced increase of gross photosynthesis at the mat surface between 25 and 35 °C (Q 10 of 3.1). In another mat sample, measurements at increasing surface irradiances (0 to 2319 μmol photons m−2 s−1) were performed at 25, 33 (the in situ temperature) and 40 °C. At all temperatures, areal rates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of saturation (E k = 148 to 185 μmol photons m−2 s−1) estimated from P versus E d curves showed no clear trend with temperature, while maximal photosynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 μmol photons m−2 s−1, where oxygenic gross photosynthesis and also the thickness of the photic zone was significantly reduced at 40 °C. The compensation irradiance increased with temperature, from 32 μmol photons m−2 s−1 at 25 °C to 77 μmol photons m−2 s−1 at 40 °C, due to increased rates of O2 consumption relative to gross photosynthesis. Areal rates of O2 consumption in the illuminated mat were higher than dark O2 consumption at corresponding temperatures, due to an increasing O2 consumption in the photic zone with increasing irradiance. Both light and temperature enhanced the internal O2 cycling within hypersaline cyanobacterial mats.
