Interplay between light intensity, chlorophyll concentration and culture mixing on the hydrogen production in sulfur‐deprived Chlamydomonas reinhardtii cultures grown in laboratory photobioreactors

Abstract

Relationships between light intensity and chlorophyll concentration on hydrogen production were investigated in a sulfur-deprived Chlamydomonas reinhardtii culture in a laboratory scale photobioreactor (PBR) equipped with two different stirring devices. In the first case, the culture was mixed using a conventional magnetic stir bar, while in the second it was mixed using an impeller equipped with five turbines. Experiments were carried out at 70 and 140 micromol photons m(-2) s(-1) in combination with chlorophyll concentrations of 12 and 24 mg L(-1). A high light intensity (140 micromol photons m(-2) s(-1), supplied on both sides of the PBR) in combination with a low chlorophyll concentration (12 mg L(-1)) inhibited the production of hydrogen, in particular in the culture mixed with the stir bar. An optimal combination for hydrogen production was found when the cultures were exposed to 140 micromol photons m(-2) s(-1) (on both sides) and 24 mg L(-1) of chlorophyll. Under these conditions, the hydrogen production output rate reached about 120 mL L(-1) in the culture mixed with the stir bar, and rose to about 170 mL L(-1) in the one mixed with the impeller. These outputs corresponded to a mean light conversion efficiency of 0.56% and 0.81%, respectively. However, the efficiency increased to 1.08% and 1.64%, respectively, when maximum hydrogen rates were considered. The better performance of the dense cultures mixed with an impeller was mainly attributed to an intermittent illumination pattern to which the cells were subjected (time cycles within 50-100 ms) which influenced the hydrogen production (1) directly, by providing the PSII with a higher production of electrons for the hydrogenase and (2) indirectly, through a higher synthesis of carbohydrates. The fluid dynamics in the PBR equipped with the impeller was characterized. The better mixing state achieved in the PBR of the new configuration makes it a useful tool for studying the hydrogen production process involving photosynthetic microorganisms, and provides a better insight into the physiology of the process.