Effect of light energy variations and vertical mixing on photosynthetic budget for intensive outdoor algal cultures
Experiments were carried out to define the effect of a particular mixing system (circular round a horizontal axis) upon the primary productivity of high-rate alga cultures. We know that mixing prevent algal cell sedimentation, gaseous and nutritional gradients, and variations in algae light reception. So, we wanted to quantity the effect of mixing on algal productivity with regard to flashing light (positive effect in comparison with continuous light). We used water from waste stabilization ponds (secondary pond output), with the native algae population dominated by Euglena and Chlorella (in Aveyron, South of France). The method used compares algal production of a water column with 14C-labelled samples i) in ten flasks placed along a classical vertical profile, ii) in a rotating system with a flask moving between the top and the bottom of the pond (0.5 m). The production is here considered as the mean of a mixed vertical profile. At the same time, physico-chemical and biological parameters were recorded (high load for N and P). Experiments were made periodically in the mid-afternoon over one year, and were completed by 8 incubations made between sunrise and sunset for on day (in August 1989). The aim was to evaluate the mixing affect with various environmental conditions. Physico-chemical parameters vary greatly over a day, and higher production values are measured in the mid-afternoon. Production observed for the stationary profile fitted the Steele model, and showed active photosynthetic from sunrise until sunset, with inhibition at high light intensities; maximal photosynthesis rates were measured between 4.00 and 5.00 PM. Results are compared to moving flask values : in all the summer experiments photosynthesis was higher in the moving flask. Light variation effects (through water movements) on the water column photosynthetic budget are particularly notable at sunrise and sunset bouts : this budget may be over 2,5 times that of homogeneous nonturbulent water. During winter however, photosynthesis is higher for the stationary profile. We compared production recorded with the two systems, and calculated the mathematical relationship between the two systems over one day (exponential) and over the year (linear relationship). By extrapolation of the instant photosynthesis profile to a 24-hour period, we can evaluate the production gain with culture mixing. We calculated the net production for various days over the year; around the end of April, the mixed system became more productive than the stationary one. Considering our results in environmental conditions, it is obvious that they are not representative of all mixing systems, particularly the techniques in which algae cell position shift continually with respect to the photic zone : the positive effect of mixing in our ponds may be linked to the flashing light effect. In the literature, we can see that photosynthetic activity increases with flash time duration. Our production figures for various hours along the day (or various flash durations) are in agreement with these results. During low photosynthetic activity periods, the flashing light effect can not act since light is available at all depths of pond. The most important factor acting, in our experiments, seems to be light, since the two systems are in similar conditions. These results have clear implications for monitoring intensive outdoor algal cultures.