Investigation of light/dark cycles effects on the photosynthetic growth of Chlamydomonas reinhardtii in conditions representative of photobioreactor cultivation

Abstract

In microalgae cultivation, the combination of light attenuation and culture mixing exposes the microalgae to fluctuating light regimes, which is a factor that can interact with cell metabolism and affect photosynthetic conversion. Here we investigate the effect of light/dark cycles (L/D cycles) on the growth of microalga Chlamydomonas reinhardtii using a photobioreactor equipped with a light-emitting diode (LED) panel to simulate L/D cycles. The L/D cycles applied were engineered to mimic L/D cycles obtained in photobioreactor operating conditions, i.e. L/D cycle frequencies and light attenuation conditions applied during the light period. Long-term experiments were run in continuous mode to investigate whether cells adapt to applied L/D cycles.For cycles of duration higher than 40s, microalgal response was close to the response assuming no L/D cycle effect (no light integration). Biomass growth rate showed responses at durations below 12s, which points to partial light integration resulting in an increase of measured biomass growth rate. This increase is known as the “L/D cycle effect”, where L/D cycle frequencies are found to generate positive effects for photosynthetic conversion. To investigate whether the growth increase was explained by L/D cycle effects, results were compared against the predictions of a kinetic model assuming no light integration but time-solved for light regimes corresponding to L/D cycles applied experimentally. The model was found to very accurately represent any of the light regimes applied, including those leading to an increase in biomass growth rate. It was then concluded that expected primary coupling between L/D cycles and photosynthetic conversion was negligible. The main influence was on pigment adaptation, which when integrated into the kinetic model assuming no L/D cycle effects was found to very accurately predict culture growth under all light regimes.