Photosynthetic characteristics of Spirulina platensis grown in commercial-scale open outdoor raceway ponds:: what do the organisms tell us?

At least six important factors that determine productivity in mass algal cultures have been identified. These are (1) the culture depth or optical cross section, (2) turbulence, (3) nutrient content and supply, (4) cultivation procedure, (5) biomass concentration and areal density, and (6) photo-acclimation. Since the efficient capturing of light energy relates to high productivities and efficiencies, all potential losses and inefficiencies need to be managed and eliminated. Photoinhibition could reduce areal productivities by up to 30% and more, where photoinhibition is the decline in photosynthetic rates at supra-optimal irradiancies. It is, however, unclear whether this occurs in high density and turbulent mass algal cultures. Using chlorophyll a fluorescence, it was possible to show how the maximum quantum efficiency of dark adapted cells (Phi(max)) decreased at midday conditions of high irradiancies. Neither photochemical (qP) nor, to a lesser extent, non-photochemical quenching (qN), could explain the midday depression. Using chlorophyll a fluorescence transient analyses it was shown that, although light absorption increased towards midday, the captured energy was essentially lost as heat dissipation. This was clearly shown in low-density cultures where the average light per cell was high, compared to denser cultures where the effects of high light exposure were significantly reduced. In low-density cultures, more than 60% of the reaction centres (RCs) became "silent", meaning that they neither reduce Q(A), nor return their excitation energy to the antenna. At higher cell densities, losses due to photoinhibition and the number of "silent RCs" were much reduced. Elucidation of the relationship between active RCs and productivity should be a priority for optimising photobioreactor productivity.