In addition to the aforementioned efforts to ensure deep penetration of light into the culture, it must be constituted that the presence of dark volume elements in a pho­tobioreactor does not necessarily decrease volumetric productivity of the system. On the contrary, the circulation of algal cells between sufficiently illuminated and
dark volume elements can increase the overall volumetric productivity in reactors at saturating light intensities. This phenomenon is referred to as intermittent (or “flashing”) light effect [22]. As long as microalgae are located in illuminated areas, photons are captured and the photosynthetic apparatus generates ATP and NADPH (light reactions). Light reactions stop when cells are located in a dark volume ele­ment. Nevertheless, dark reactions, that are driven by ribulose bisphosphate car­boxylase (Rubisco) and that are kinetically limiting for the overall CO2 fixation process, can proceed. The overall productivity can eventually be increased in a reac­tor if light intensities exceed saturation levels and frequencies of circulation between dark and illuminated volume elements are beneficial for the algae cells.

Experiments with Dunaliella have shown that photosynthetic efficiency can be increased in comparison to continuous illumination for light/dark cycles of 5.32 Hz but efficiency is lower under slow cycles of 0.17 Hz [21]. Reasons for reduced efficiency when slow light/dark cycles are prevalent are not clear, yet. One possible reason might be interference with intracellular control loops on the epigenetic level [30]. Cycle frequencies of >1 Hz are recommended for P. tricornutum [24]. Optimal cycle frequencies are certainly strain-dependent and also strongly influenced by photon flux densities and spatial distribution of light in the liquid volume.

Usually, favorable light/dark cycle frequencies can be attained when prevalent flow regimes in reactors are turbulent so that sufficient radial mixing along the light path is guaranteed. However, this demand imposes two restrictions. Firstly, some algae species are sensitive to shear stress. Microeddies with dimensions comparable to cell size should be avoided. This restriction can be crucial for scale-up when specific light/dark cycles should be obtained but the high energy input required gener­ates cell damaging flow regimes [25]. Secondly, high levels of auxiliary energy input are usually required to attain light/dark frequencies in a desirable order of magnitude. These energy inputs significantly exceed values necessary for sufficient mass transfer. Especially for energetic utilization of biomass the energy balance requires that energy input is minimal. Energy content of algae can range from 20 to 30 MJ/kg for oil rich algae [30]. Unfortunately, little information is available about the correlation between energy input and frequencies of light/dark cycles in different reactor geometries.

Computational flow dynamics (CFD) simulations can be implemented to estimate radial mixing velocities and therewith residence times in dark or illuminated volume elements. Biomass concentration, pigment composition, and light intensities at the surface of a reactor strongly influence the spatial light distribution and the associated ratio of dark and illuminated volume elements. Exemplary simulations for tubular reactors show that static helical mixers could increase light/dark cycle frequencies in a tubular reactor with a factor higher than 20 compared to plain tubes [27].