Carbon metabolism and energy conversion of Synechococcus sp PCC 7942 under mixotrophic conditions: comparison with photoautotrophic condition

To investigate the carbon metabolism and energy conversion efficiency of the cyanobacterium Synechococcus sp. PCC 7942 under mixotrophic conditions, we studied its growth characteristics in mixotrophic cultures with glucose and with acetate, respectively, and further discussed the carbon metabolism and energy utilization based on metabolic flux analysis. Results showed that both glucose and acetate could enhance the growth of Synechococcus sp. PCC 7942. The metabolic flux through the glycolytic pathway, tricarboxylic acid cycle, and mitochondrial oxidative phosphorylation was affected by the two organic substrates. Additionally, the cellular composition was also modulated by glucose and acetate. Under mixotrophic conditions, glucose exerts more significant impact on the diminishment of photochemical efficiency. Although the contribution of light energy was smaller, the cell yields based on total energy in mixotrophic cultures were higher compared with that of photoautotrophic one. On the basis of chlorophyll fluorescence analysis, the actual energy conversion efficiencies based on ATP synthesis in the photoautotrophic, glucose-mixotrophic, and acetate-mixotrophic cultures were evaluated to be 4.59%, 5.86%, and 6.60%, respectively.