We have evaluated the effects of short-term changes

in incident irradiance and temperature on oxygenic

photosynthesis and oxygen consumption in a hypersaline

cyanobacterial mat from the Ebro Delta, Spain,

in which Microcoleus chthonoplastes was the dominant

phototrophic organism. The mat was incubated

in the laboratory at 15, 20, 25 and 308C at incident

irradiances ranging from 0 to 1000 mmol photons

m22 s21. Oxygen microsensors were used to measure

steady-state oxygen profiles and the rates of gross

photosynthesis, which allowed the calculation of areal

gross photosynthesis, areal net oxygen production,

and oxygen consumption in the aphotic layer of the

mat. The lowest surface irradiance that resulted in

detectable rates of gross photosynthesis increased

with increasing temperature from 50 mmol photons

m22 s21 at 158C to 500 mmol photons m22 s21 at 308C.

These threshold irradiances were also apparent from

the areal rates of net oxygen production and point to

the shift of M. chthonoplastes from anoxygenic to

oxygenic photosynthesis and stimulation of sulphide

production and oxidation rates at elevated temperatures.

The rate of net oxygen production per

unit area of mat at maximum irradiance, J0, did not

change with temperature, whereas, JZphot, the flux of

oxygen across the lower boundary of the euphotic

zone increased linearly with temperature. The rate of

oxygen consumption per volume of aphotic mat

increased with temperature. This increase occurred in

darkness, but was strongly enhanced at high irradiances,

probably as a consequence of increased rates

of photosynthate exudation, stimulating respiratory

processes in the mat. The compensation irradiance

(Ec) marking the change of the mat from a heterotrophic

to an autotrophic community, increased

exponentially