Cultivation of microalgae: effect of light/dark cycles on biomass yield

Abstract

In this thesis the efficiency of light utilization of microalgae was studied under light/dark cycles encountered in photobioreactors. Phototrophic microorganisms such as microalgae and cyanobacteria could provide valuable compounds. For many of these applications it is essential to use enclosed photobioreactors in which monocultures can be maintained for an extended time. Because light energy is the growth limiting substrate, light supply plays a key role in photobioreactor design.The light regime inside photobioreactors is complicated: algae are exposed to high-intensity sunlight in the so-called photic zone close to the reactor surface and complete darkness in the interior. As a result of mixing of the reactor liquid, algae are exposed to light-gradient/dark cycles and this fluctuating light regime could have a considerable influence on the efficiency of light utilization for biomass growth, also referred to as the photosynthetic efficiency.The present study therefore was limited to medium-frequency light/dark cycles. These have a duration of a few seconds to 100 s. The efficiency of light utilization was determined as the yield of biomass, measured as protein, on light energy, measured as 'photosynthetic active radiation' (PAR, 400 - 700 nm). In addition, the quantum yield of photochemistry was determined using pulse-amplitude-modulation (PAM) fluorometry. Both determinations showed that the efficiency of photosynthesis is lower during the light period of light/dark cycles in comparison to continuous illumination.In addition to this general conclusion, it was found that, in the situation the dark period comprises was less than 33 % of the full cycle duration (= dark fraction), the yield was about equal to the one determined under continuous illumination. The biomass yield was mainly affected by the light fraction (e), whereas cycle duration only had a small influence. Additional experiments showed that together with the biomass yield on light energy also the capacity of the photosynthetic apparatus decreased. Furthermore, evidence is presented photoacclimation occurs after a shift from continuous illumination to medium-frequency light-dark cycles. This could explain the decrease in efficiency in the light period. An efficiency increase was only observed under short light/dark cycles (0.19 s) in comparison to continuous illumination.As a summary, the state-of the-art of enclosed outdoor photobioreactors was analyzed. Characteristic examples were selected and studied with respect to the light regime and the photosynthetic efficiency. It is shown that high photosynthetic efficiencies only can be reached at high biomass concentration in short light-path bioreactors. It is demonstrated, however, that these and also the other types of photobioreactors are poorly scalable and/or not applicable for cultivation of monocultures. This is the reason new photobioreactor designs are proposed in which light capture is physically separated from photoautotrophic cultivation