IMIS

Nannochloropsis sp. (Eustigmatophyta) is a unicellular algal species popular for mass algal cultures because it contains high concentrations of the polyunsaturated fatty acid eicosapentaenoic acid (EPA- 20:5É3). In this experiment we followed the photosynthetic behaviour and biomass parameters of this alga during 50h in two outdoor mass cultures: a flat panel photobioreactor (FPP) and a high rate algal pond (HRAP) in order to detect acclimation patterns to different optical conditions, with the FPP having a small gradient in light intensity and the HRAP having large gradient in irradiance. Algae of both cultures showed a depression of photosynthetic activity during the afternoon, which was more pronounced in the FPP. Despite the fact that the maximum irradiance experienced by the algae was the same in both systems, the HRAP algae showed classic shade adaptation whereas the FPP algae showed high light acclimation in photosynthetic parameters. The FPP showed diurnal changes in the maximum rate of photosynthesis and respiration and maximum rates of photosynthesis were lower at night than during the day. High temperatures (41 oC) on day 3 in the FPP caused a large reduction in Fv/Fm and an over-reduction of the PSII acceptors which did not recover during the light period. PAM measurements with and without dark adaptation showed that this decreased photosynthetic activity was due to both chronic and dynamic downregulation. Photodamage was higher in the FPP, but most damaged seem to be repaired during the evening. A comparison between the quantum efficiencies for PSII charge separation and oxygen evolution revealed a close coupling between the two.