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 experienced by micro-algae in externally-illuminated (sunlight) air-lift loop bioreactors with internal draft tube. Biomass yield in relation to light energy was determined as g protein per mol of photons (400-700 nm). Between 600 and 1200 ?mol m^-2 s^-1 the yield at a 10/5 s light/dark cycle was equal to the yield at continuous illumination. Consequently, provided that the liquid circulation time is 15 s, a considerable dark zone seems to be allowed in the interior of air-lift loop photobioreactors (33␟/v) without loss of light utilization efficiency. However, at a 5/10 s light/dark cycle, corresponding to a 67␟/v dark zone, biomass yield decreased. Furthermore, both algae, C. reinhardtii and D. tertiolecta, responded similarly to these 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 (average) photon flux densities (57-370 ?mol m^-2 s^-1) than at high PFDs (> 600 ?mol m^-2 s^-1) and it was shown that D. tertiolecta was cultivated at a sub-optimal temperature (20 ^°C).