Promoting cell growth and characterizing partial symbiotic relationships in the co‐cultivation of green alga Chlamydomonas reinhardtii and Escherichia coli

Background By co‐culturing selected microalgae and heterotrophic microorganisms, the growth rate of microalgae can be improved even under atmospheric conditions with a low CO2 concentration. However, the detailed mechanism of improvement of proliferative capacity by co‐culture has not been elucidated. In this study, we investigated changes in the proliferative capacity of the green alga Chlamydomonas reinhardtii by co‐culturing with Escherichia coli. Main Methods and Major Results In the co‐culture, the number of C. reinhardtii cells reached 2.22 × 1010 cell/L on day 14 of culture. This was about 1.9 times the number of cells (1.16 × 1010 cell/L) on day 14 compared to C. reinhardtii cells in monoculture. The starch content per cell in the co‐culture of C. reinhardtii and E. coli on the 14th day (2.09 × 10−11 g/cell) was 1.3 times higher than that in the C. reinhardtii monoculture (1.59 × 10−11 g/cell), and the starch content per culture medium improved 2.5 times with co‐cultivation. By analyzing the gene transcription profiles and key media components, we clarified that E. coli produced CO2 from the organic carbon in the medium and the organic carbon produced by photosynthesis of C. reinhardtii, and this CO2 likely enhanced the growth of C. reinhardtii. Conclusions Consequently, E. coli plays a key role in promoting the growth of C. reinhardtii as well as the accumulation of starch which is a valuable intermediate for the production of a range of useful chemicals from CO2. Graphical and Lay Summary In this study, we investigated changes in the proliferative capacity of the green alga Chlamydomonas reinhardtii by co‐culturing with Escherichia coli. It was clarified for the first time that co‐culture with C. reinhardtii and E. coli promotes the growth of C. reinhardtii and the starch production. In the future, it is expected that the co‐culture system consisting of C. reinhardtii and E. coli will be developed to produce a range of useful chemicals from CO2.