A new strategy of carbon-energy coupling transfer enhancement by formate on phototrophic green microalga Chlamydomonas reinhardtii

Low efficient CO2 and solar energy transfer in aquatic microalgae cultivation are always challenges and have consumed huge efforts during the last two decades. With the development of solar fuels technologies, formate can come from the reduction of CO2 with green H2. As a water-soluble solar fuel, formate is a good carbon-energy coupling enhancement carrier for biosynthesis. However, formate is also an inhibitor to the electron transfer chain (ETC) in the cell. Based on the chloroplast engineering, a series of Chlamydomonas reinhardtii mutants with chloroplast transit peptide chimeric NADPH-dependent formate dehydrogenase (FDH) were constructed, and these mutants were named as PsFDHs. Results showed that exogenous FDH alleviated the inhibition of formate on the photosynthetic system and promoted the growth, and a win–win result obtained between the natural and artificial photosynthesis: 40 % more carbon from formate was fixed than heterotrophic only, and 62 % increase in dry weight (DW), while no obvious formate consumption was detected in the wide type (WT). The maximum overall biomass productivity was 15.4 mg·L−1·h−1, which was 36 % higher than that of WT. It's postulated that formate enhanced the carbon and energy transportation close to carbon fixation reaction center, and photosynthesis made up for energy shortages in the conversion of formate to biomass. The results showed the potential of formate in achieving a higher-than-nature artificial-natural hybrid photosynthesis by microalgae, to meet requirements of both the carbon neutralization and protein supply for human nutrients.