How do algae regulate photosynthesis in mixotrophic growth?

Mixotrophic growth of microalgae is gaining increasing interest because of its potential application as an efficient biomass production strategy for biofuel production. The Chlorella sorokiniana cultured mixotrophically grew faster, and achieved higher biomass concentration than that of photoautotrophic and heterotrophic cultures. The exact mechanism for the improved performance however, is still unclear.  In our study, the effects of glucose added on primary and secondary photosynthetic reactions are assessed to elucidate mixotrophic capacities of the alga. After 24 hours, the light saturation rate of photosynthetic O₂ evolution in mixotrophic culture increased by approximately 288% and 388%, compared with 1% CO₂ and air supplemented photoautotrophic culture. This enhancement in photosynthetic efficiency of photosystem II was also indicated by increased quantum yield of electron transfer through PSII (ΦII), minor nonphotochemical quenching (NPQ), and 77K fluorescence spectra of mixotrophic cells. The upregulation of primary photosynthetic reactions was accompanied by increased ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) total carboxylase activity. The results were corroborated by expression levels of rbcL (ribulose 1, 5-bisphosphate carboxylase/oxygenase large subunit) and prk (phosphoribulokinase subunit) genes through real-time PCR. Following glucose depletion after 72 hours, photosynthetic O₂ evolution of mixotrophic cells in stationary phase was strongly reduced, accompanied by reduced total Rubisco carboxylase activity. The results suggested that in mixotrophic culture of C. sorokiniana the utilization of glucose enhanced the photosynthesis efficiency, and had pronounced effects on growth rate of the algal biomass.