High-intensity pulsed-light cultivation of unicellular algae: Photosynthesis continues in the dark.

Experiments have shown that photon exploitation efficiency in unicellular algal biomass production under a pulsed-light regime with a high-photon flux is higher than the efficiency under continuous illumination with the same flux. This observation has been explained theoretically to be a consequence of the improved efficiency of exploitation of photons by Photosystem II (PS II) thanks to the combined effect of photon-absorption statistics, a rate-limiting time scale and the size of the PQ pool. Exploiting the same ideas, it is shown in this paper that, under a pulsed-light regime, there is a pulse-time length, for which the average exploitation efficiency of PS II absorbed photons is maximal. Under ideal conditions, this maximum is close to 100%. The optimal pulse-time length is roughly proportional to the size of the PQ pool, N PQ . This is clearly seen for τ (the average time gap between consecutive photons absorbed by the PS II-Chlorophyll antenna) of the order of 1 ms or less (corresponding to a high photon flux and/or a large photon absorption cross-section area of the antenna) and for small N PQ . The width of the plot of efficiency vs. pulse length around the optimum is then small and the optimal pulse length is well defined. As τ is increased beyond 1 or N PQ becomes large, the width grows, allowing for a broad choice of pulse lengths, for which efficiency is very close to the maximum. These observations open the door to future designs of highly productive bioreactors.