Omics integration for in-depth understanding of the low-carbon co-culture platform system of Chlorella vulgaris-Escherichia coli

Many fuels and chemicals can be commercially produced by microbial processes to substitute the petrochemical processes. However, the microbial processes suffer from the instability, low efficiency or discontinuous cultures. The continuous culture can greatly increase the productivity and reduce costs for the microbial production. The challenges for the application of continuous culture in industry are high contamination risk and strain degeneration. The co-culture system of photoautotrophs and heterotrophs is probably able to solve bottlenecks of microbial culture. The micro-ecological balance can be achieved for the continuous microbial culture by the co-culture of photoautotrophs and heterotrophs. Here, a system for the co-cultivation of Chlorella vulgaris and Escherichia coli was established for the biosynthesis of isoprene. Compared with axenic culture, the isoprene production in the co-culture process was improved 10-fold to 0.6 g/L and the fermentation was prolonged from 100 h to 350 h. C. vulgaris promoted the isoprene synthesis and E. coli growth, while E. coli restricted C. vulgaris growth. The interactions between E. coli and C. vulgaris were closely associated with oxidative pressure from photomixotrophic metabolism. The consumption of exogenous glucose resulted in excess photomixotrophic electrons and subsequently resultant toxic reactive oxygen species (ROS). The oxidative pressure was reflected by the high activity of intracellular antioxidative (CysK, CysE, SerA, AhpC, AhpF) and repair (YtfE, NfuA, YebG) systems. C. vulgaris might protect E. coli against the oxidative pressure and improve the growth of E. coli through the inter-species cross-feeding. The biosynthesis of cysteine was greatly up-regulated in C. vulgaris to reduce ROS, and the cysteine necessary for antioxidation in E. coli might be provided by C. vulgaris. This study on the co-culture of C. vulgaris and E. coli is significant for revealing the common interactions between photoautotrophs and heterotrophs for the continuous culture.