EFFECT OF BACK REACTIONS IN THE S-CYCLE ON PHOTOSYNTHESIS IN VERY WEAK LIGHT

Photosynthesis in light which is so weak that excitation rates are com parable to the decay rates of S-states is modelled. It is found that o xygen evolution rate varies in a non-linear way with Photosystem II ex citation rate in this region, but asymptotically approaches a linear r elationship (with a slope of one oxygen molecule per four excitations) as excitation rate increases. The asymptote gives an intercept (zero oxygen evolution rate) on the excitation rate axis. The question is ra ised, whether this model is compatible with photoautotrophy in a red a lga found at 268 m depth and able to carry out photosynthesis in extre mely weak light. It is found that no unorthodox mechanisms for the S-c ycle in this alga have to be assumed to explain its performance.