Modelling time-series of photosynthesis and comparisons with the fluorescence yield in Chlorella vulgaris: a study of adaptation, inhibition and recovery
Time-series of 14C uptake and fluorescence yield (i.e., the fluorescence enhancement after addition of the photosynthetic inhibitor DCMU) were measured in Chorella vulgaris at various light intensities. Adaptation and recovery processes after alteration of the light intensity were also studied. At a constant light intensity, both the rate of 14C uptake and the fluorescence yield decreased with time. Comparison of time-series data of 14C uptake at different light intensities showed that this phenomenon consisted of several processes (i.e., at low light intensities small changes in uptake rates were mainly due to photoadaptation, while at higher light intensities relatively larger changes occurred, as result of photoinhibition). Transfer of an algal sample to low light intensities after a period of exposure to inhibiting light intensities resulted in an exponential recovery of the 14C uptake rate with time, coupled with an exponential recovery of the fluorescence yield. A mechanistic model is presented, which describes the algal 14C uptake rate as a function of time and light intensity. The model includes adaptation, inhibition and recovery. Six parameters, characterising the algal suspension, have to be estimated from the results of one P versus I curve and one time-series 14C uptake, which includes a period of recovery. Using these parameters the model can predict the time-course of 14C uptake at every constant light intensity, as well as when the light intensity is changed during the experiment. When applied to a culture of C. vulgaris, the theoretical values closely approach the actual measurements. The resemblance between the measured time-series of fluorescence yield and the rate of 14C uptake indicates, that the changes in the rate of 14C uptake are due to changes in the photosynthetic apparatus, rather than to changes of diffusion of 14C into the cell.