Insights into carbon utilization under mixotrophic conditions in Chlamydomonas

[Display omitted] •Bicarbonate was assimilated preferentially rather than acetate in mixotrophy.•Bicarbonate addition accelerated cell growth by improving acetate utilization.•Energy dissipation was reduced, and energy generation was induced by bicarbonate.•Transcriptional profiles of genes related to carbon fixation deciphered the phenotype. Mixotrophic microalgae cultivation with various carbon resources is considered as a strategy that could increase biomass. However, the mechanism of carbon utilization between inorganic carbon (IC) and organic carbon (OC) remains unknown. In this study, IC and OC consumption, chlorophyll fluorescence parameters, intracellular Nicotinamide adenine dinucleotide phosphate content and transcriptional changes in related genes were characterized. The results showed that IC was utilized preferentially, whereas 76% IC was consumed at 8 h. Subsequently, OC was the dominant carbon resource for fermentation. The cell density in the IC group was 100% higher than that in the group without IC at 24 h. Bicarbonate addition enhanced photosynthesis by dissipating less energy and generating more electrons and energy, which benefited OC assimilation. This finding was verified by qRT-PCR analysis. These results elucidate the carbon utilization mechanism under mixotrophic conditions, which provide clues for promoting microalgae growth by regulating carbon utilization.