The responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain)

We have evaluated the effects of short-term changes in incident irradiance and temperature on oxygenic photosynthesis and oxygen consumption in a hype rsaline cyanobacterial mat from the Ebro Delta, Spain, in which Microcoleus chthonoplastes was the dominant phototrophic organism. The mat was incubat ed in the laboratory at 15, 20, 25 and 30 degrees C at incident irradiances ranging from 0 to 1000 mu mol photons m(-2) s(-1). Oxygen microsensors wer e used to measure steady-state oxygen profiles and the rates of gross photo synthesis, which allowed the calculation of areal gross photosynthesis, are al net oxygen production, and oxygen consumption in the aphotic layer of th e mat. The lowest surface irradiance that resulted in detectable rates of g ross photosynthesis increased with increasing temperature from 50 mu mol ph otons m(-2) s(-1) at 15 degrees C to 500 mu mol photons m(-2) s(-1) at 30 d egrees C, These threshold irradiances were also apparent from the areal rat es of net oxygen production and point to the shift of M, chthonoplastes fro m anoxygenic to oxygenic photosynthesis and stimulation of sulphide product ion and oxidation rates at elevated temperatures. The rate of net oxygen pr oduction per unit area of mat at maximum irradiance, do, did not change wit h temperature, whereas, J(Zphot), the flux of oxygen across the lower bound ary of the euphotic zone increased linearly with temperature. The rate of o xygen consumption per volume of aphotic mat increased with temperature. Thi s increase occurred in darkness, but was strongly enhanced at high irradian ces, probably as a consequence of increased rates of photosynthate exudatio n, stimulating respiratory processes in the mat. The compensation irradianc e (E-c) marking the change of the mat from a heterotrophic to an autotrophi c community, increased exponentially in this range of temperatures.