Scale-up aspects of photobioreactors: effects of mixing-induced light/darkcycles

The green micro-algae Chlamydomonas reinhardtii and Dunaliella tertiolecta were cultivated under medium-duration square-wave light/dark cycles with a cycle time of 15 s. These cycles were used to simulate the light regime exp erienced by micro-algae in externally-illuminated (sunlight) air-lift loop bioreactors with internal draft tube. Biomass yield in relation to light en ergy was determined as g protein per mol of photons (400-700 nm). Between 6 00 and 1200 mu mol m(-2) s(-1) the yield at a 10/5 s light/dark cycle was e qual to the yield at continuous illumination. Consequently, provided that t he liquid circulation time is 15 s, a considerable dark zone seems to be al lowed in the interior of air-lift loop photobioreactors (33% v/v) without l oss of light utilization efficiency. However, at a 5/10 s light/dark cycle, corresponding to a 67% v/v dark zone, biomass yield decreased. Furthermore , both algae, C. reinhardtii and D. tertiolecta, responded similarly to the se cycles with respect to biomass yield. This was interesting because they were reported to exhibit a different photoacclimation strategy. Finally, it was demonstrated that D. tertiolecta was much more efficient at low (avera ge) photon flux densities (57-370 mu mol m(-2) s(-1)) than at high PFDs (> 600 mu mol m(-2) s(-1)) and it was shown that D. tertiolecta was cultivated at a sub-optimal temperature (20 degreesC).