In this study, a metabolic network describing the primary metabolism of Chlamydomonas reinhardtii was constructed. By performing chemostat experiments at different growth rates, energy parameters for maintenance and biomass formation were determined. The chemostats were run at low irradiances resulting in a high biomass yield on light of 1.25 g  mol−1. The ATP requirement for biomass formation from biopolymers (Kx) was determined to be 109 mmol g−1 (18.9 mol mol−1) and the maintenance requirement (mATP) was determined to be 2.85 mmol g−1 h−1. With these energy requirements included in the metabolic network, the network accurately describes the primary metabolism of C. reinhardtii and can be used for modeling of C. reinhardtii growth and metabolism. Simulations confirmed that cultivating microalgae at low growth rates is unfavorable because of the high maintenance requirements which result in low biomass yields. At high light supply rates, biomass yields will decrease due to light saturation effects. Thus, to optimize biomass yield on light energy in photobioreactors, an optimum between low and high light supply rates should be found. These simulations show that metabolic flux analysis can be used as a tool to gain insight into the metabolism of algae and ultimately can be used for the maximization of algal biomass and product yield.Electronic supplementary materialThe online version of this article (doi:10.1007/s10811-011-9674-3) contains supplementary material, which is available to authorized users.

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.

Respiration and photosynthesis are two important processes in microalgal growth that occur simultaneously in the light. To know the rates of both processes, at least one of them has to be measured. To be able to measure the rate of light respiration of Chlorella sorokiniana, the measurement of oxygen uptake must be fast, preferably in the order of minutes. We measured the immediate post-illumination respiratory O2 uptake rate (OUR) in situ, using fiber-optic oxygen microsensors, and a small and simple extension of the cultivation system. This method enables rapid and frequent measurements without disturbing the cultivation and growth of the microalgae. Two batch experiments were performed with C. sorokiniana in a short light-path photobioreactor, and the OUR was measured at different time points. The net oxygen production rate (net OPR) was measured online. Adding the OUR and net OPR gives the gross oxygen production rate (gross OPR), which is a measure for the oxygen evolution by photosynthesis. The gross OPR was 35–40% higher than the net OPR for both experiments. The respiration rate is known to be related to the growth rate, and it is suggested that faster algal growth leads to a higher energy (ATP) requirement, and as such, respiratory activity increases. This hypothesis is supported by our results, as the specific OUR is highest in the beginning of the batch culture when the specific growth rate is highest. In addition, the specific OUR decreases toward the end of the experiments until it reaches a stable value of around 0.3 mmol O2 h−1 g−1. This value for the specific OUR is equal to the maintenance requirement of C. sorokiniana as determined in an independent study of (Zijffers et al. 2010 (in press)). This suggests that respiration could fulfill the maintenance requirements of the microalgal cells.

High oxygen:carbon dioxide ratios may have a negative effect on growth and productivity of microalgae. To investigate the effect of O₂ and CO₂ concentrations and the ratio between these on the metabolism of Chlamydomonas reinhardtii we performed turbidostat experiments at different O₂:CO₂ ratios. These experiments showed that elevated O₂ concentrations and the corresponding increase in the ratio of O₂:CO₂ common in photobioreactors led to a reduction of growth and biomass yield on light with 20-30%. This is most probably related to the oxygenase activity of Rubisco and the resulting process of photorespiration. Using metabolic flux modeling with measured rates for each experiment we were able to quantify the ratio of the oxygenase reaction to the carboxylase reaction of Rubisco and could demonstrate that photorespiration indeed can cause the reduction in biomass yield on light. The calculated ratio of the oxygenase reaction to the carboxylase reaction was 16.6% and 20.5% for air with 2% CO₂ and 1% CO₂, respectively. Thus photorespiration has a significant impact on the biomass yield on light already at conditions common in photobioreactors (air with 2% CO₂).