Irradiance and temperature regulation of oxygenic photosynthesis and O-2 consumption in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

Short-term effects of temperature and irradiance on oxygenic photosynthesis and O-2 consumption in a hypersaline cyanobacterial mat were investigated with O-2 microsensors in a laboratory. The effect of temperature on O-2 flu xes across the mat-water interface was studied in the dark and at a saturat ing high surface irradiance (2162 mu mol photons m(-2) s(-1)) in the temper ature range from 15 to 45 degrees C. Areal rates of dark O-2 consumption in creased almost linearly with temperature. The apparent activation energy of 18 kJ mol(-1) and the corresponding Q(10) value (25 to 35 degrees C) of 1. 3 indicated a relative low temperature dependence of dark O-2 consumption d ue to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperat ure up to 40 degrees C and exhibited a Q(10) value (20 to 30 degrees C) of 2.8. Both O-2 dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 degrees C, with the most pronounced in crease of gross photosynthesis at the mat surface between 25 and 35 degrees C (Q(10) of 3.1). In another mat sample, measurements at increasing surfac e irradiances (0 to 2319 mu mol photons m(-2) s(-1)) were performed at 25, 33 (the in situ temperature) and 40 degrees C. At all temperatures, areal r ates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of sa turation (E-k = 148 to 185 mu mol photons m(-2) s(-1)) estimated from P ver sus E-d curves showed no clear trend with temperature, while maximal photos ynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 mu mo l photons m(-2) s(-1), where oxygenic gross photosynthesis and also the thi ckness of the photic zone was significantly reduced at 40 degrees C. The co mpensation irradiance increased with temperature, from 32 mu mol photons m( -2) s(-1) at 25 degrees C to 77 mu mol photons m(-2) s(-1) at 40 degrees C, due to increased rates of O-2 consumption relative to gross photosynthesis . Areal rates of O-2 consumption in the illuminated mat were higher than da rk O-2 consumption at corresponding temperatures, due to an increasing O-2 consumption in the photic zone with increasing irradiance. Both light and t emperature enhanced the internal O-2 cycling within hypersaline cyanobacter ial mats.