Efficiency of light utilization of Chlamydomonas reinhardtii under medium-duration light/dark cycles

The light regime inside a photobioreactor is characterized by a light gradient with full (sun)light at the light-exposed surface and darkness in the interior of the bioreactor. Consequently, depending on the mixing characteristics, algae will be exposed to certain light/dark cycles. In this study the green alga Chlamydomonas reinhardtii was cultivated under five different light regimes: (1) continuous illumination; (2) a square-wave light/dark cycle with a light fraction (epsilon) of 0.5 and a duration (t(c)) of 6.1 s; (3) epsilon = 0.5, t(c) = 14.5 s; (4) epsilon = 0.5, t(c) = 24.3 s and (5) epsilon = 0.8, t(c) = 15.2 s. The biomass yield on light energy, protein per photons, decreased under light/dark cycles (epsilon = 0.5) in comparison to continuous light (CL), from 0.207 (CL) to 0.117-0.153 g mol(-1) (epsilon = 0.5). Concomitantly, the maximal specific photosynthetic activity, oxygen production per protein, decreased from 0.94 (CL) to 0.64-0.66 g g(-1) h (-1) (epsilon = 0.5). Also the quantum yield of photochemistry, yield of the conversion of light energy into chemical energy, decreased from 0.47 (CL) to 0.23 (epsilon = 0.5, t(c) = 24.3 s). Apparently, C. reinhardtii is not able to maintain a high photosynthetic capacity under medium-duration light/dark cycles and since specific light absorption did not change, light utilization efficiency decreased in comparison to continuous illumination. (C) 2000 Elsevier Science B.V. All rights reserved.