The synergistic energy and carbon metabolism under mixotrophic cultivation reveals the coordination between photosynthesis and aerobic respiration in Chlorella zofingiensis

Microalgal biofuels have attracted much attention in recent years. Nevertheless, the relatively low growth rate and low cell density limits its industrial application. Mixotrophic cultivation has been considered as a good strategy to increase the specific growth rate and biomass yield in some microalgal species. However, the metabolic mechanism has not been well elucidated. To address it, Chlorella zofingiensis was employed in the present study. The biomass productivity and maximum dry weight achieved under mixotrophic condition were greater than the sum of those under photoautotrophic and heterotrophic conditions. Interestingly, compared with photoautotrophic cultures, the RuBisCO activity was reduced for mixotrophic cultures, indicative of the down-regulation of the Calvin cycle. When compared with heterotrophic cultures, the citrate synthase activity was attenuated for mixotrophic cultures, indicative of the down-regulation of Tricarboxylic Acid cycle. Biochemical analysis showed that mixotrophic cells contained more starch but less total fatty acid content than heterotrophic cells. Besides, the biomass yield on glucose was 0.789 g g(-1) for mixotrophic cultures, 63.7% higher than that for heterotrophic cultures, and the enhancement effect of light under mixotrophic cultivation was 7.35-fold. Based on these results, a novel mechanism for the synergistic effects in mixotrophic cultivation was proposed, in which glucose was more efficiently utilized for biomass production by the possible coordination of energy and carbon metabolism between photosynthesis and aerobic respiration.