No abstract

To be able to study the effect of mixing as well as any other parameter on productivity of algal cultures, we designed a lab-scale photobioreactor in which a short light path (SLP) of (12 mm) is combined with controlled mixing and aeration. Mixing is provided by rotating an inner tube in the cylindrical cultivation vessel creating Taylor vortex flow and as such mixing can be uncoupled from aeration. Gas exchange is monitored on-line to gain insight in growth and productivity. The maximal productivity, hence photosynthetic efficiency, of Chlorella sorokiniana cultures at high light intensities (1,500 micromol m(-1) s(-1)) was investigated in this Taylor vortex flow SLP photobioreactor. We performed duplicate batch experiments at three different mixing rates: 70, 110, and 140 rpm, all in the turbulent Taylor vortex flow regime. For the mixing rate of 140 rpm, we calculated a quantum requirement for oxygen evolution of 21.2 mol PAR photons per mol O(2) and a yield of biomass on light energy of 0.8 g biomass per mol PAR photons. The maximal photosynthetic efficiency was found at relatively low biomass densities (2.3 g L(-1)) at which light was just attenuated before reaching the rear of the culture. When increasing the mixing rate twofold, we only found a small increase in productivity. On the basis of these results, we conclude that the maximal productivity and photosynthetic efficiency for C. sorokiniana can be found at that biomass concentration where no significant dark zone can develop and that the influence of mixing-induced light/dark fluctuations is marginal.

No abstract

No abstract

BackgroundDay/night cycles regulate the circadian clock of organisms to program daily activities. Many species of microalgae have a synchronized cell division when grown under a day/night cycle, and synchronization might influence biomass yield and composition. Therefore, the aim of this study was to study the influence of day/night cycle on biomass yield and composition of the green microalgae Neochloris oleoabundans. Hence, we compared continuous turbidostat cultures grown under continuous light with cultures grown under simulated day/night cycles.ResultsUnder day/night cycles, cultures were synchronized as cell division was scheduled in the night, whereas under continuous light cell division occurred randomly synchronized cultures were able to use the light 10–15% more efficiently than non-synchronized cultures. Our results indicate that the efficiency of light use varies over the cell cycle and that synchronized cell division provides a fitness benefit to microalgae. Biomass composition under day/night cycles was similar to continuous light, with the exception of starch content. The starch content was higher in cultures under continuous light, most likely because the cells never had to respire starch to cover for maintenance during dark periods. Day/night cycles were provided in a ‘block’ (continuous light intensity during the light period) and in a ‘sine’ (using a sine function to simulate light intensities from sunrise to sunset). There were no differences in biomass yield or composition between these two ways of providing light (in a ‘block’ or in a ‘sine’).ConclusionsThe biomass yield and composition of N. oleoabundans were influenced by day/night cycles. These results are important to better understand the relations between research done under continuous light conditions and with day/night cycle conditions. Our findings also imply that more research should be done under day/night cycles.