Effect of photoacclimation on microalgae mass culture productivity

Microalgae are capable of adapting their pigmentation to the light regime to which they are exposed. In high density microalgae cultures exposed to sunlight, the high pigment content leads to oversaturation of the photosystems resulting in increased light energy dissipation at the reactor surface, reducing the light use efficiency. In theory, therefore, pigment reduction would maximize biomass productivity. In this study, we have measured the long-term biomass productivity and short-term oxygen production rate of low-pigmented cells of Chlorella sorokiniana under mass culture conditions. Reduced pigmentation was obtained through the natural process of photoacclimation under high irradiance. During the time that the pigmentation was reduced, mass culture productivity, light absorption, and light use efficiency were investigated. Photoacclimation kinetics were investigated in a light shift experiment in which the increase in absorption cross section was followed in time upon a shift from high to low light intensity. Improved productivity of low-pigmented cells under mass culture conditions was not observed in any of the experiments. There is no solid explanation based on the experimental data. The most likely explanations are that thermal dissipation mechanisms were still activated and that the photoacclimation process itself consumed a substantial amount of energy at cost of growth processes. It is suggested that photoacclimation can only be exploited in the situation that microalgal cells are grown at a fixed position (e.g. in a biofilm, or multicompartment reactor) without being exposed to rapid light fluctuations.