Enhancing overall carbon fixation and energy conversion with formate in green microalga Chlamydomonas reinhardtii

Abstract

The natural carbon-concentration mechanism (CCM) is the solution to achieve high photosynthetic efficiency. Mimicking the natural CCM, exogenous formate which can be produced by renewable energy and CO2 artificially, was introduced into a model photosynthetic organism, Chlamydomonas reinhardtii, to improve the overall photosynthesis. To release the adverse effects of formate, the combination of different strategies, including co-substrate, evolution, and conditions optimization, the formate tolerance to over 40 mM, which was higher than previous irreversible inhibition limitation of formate on C. reinhardtii, was achieved, and formate-mediated enhancement of growth and photosynthesis activity was obtained with a 27 % biomass improvement and a 60 % increase in photosynthetic oxygen evolution. Interestingly, 82 % of formate was converted with the illumination, indicating the existence of energy interaction between photosynthesis and formate assimilation. Meanwhile, formate suppressed respiration and reduced a 29 % biomass decline in the night. Moreover, supported by the enzyme inhibitor, formate consumption, and biochemical compositions, it was postulated that formate was assimilated via the pyruvate formate-lyase (PFL) enzyme to stimulate the increase of photosynthetic components to maintain photosynthesis activity. With the overexpressed PFL and conditions optimization, formate could be further used as the sole organic carbon source for higher biomass in C. reinhardtii. This work presents the potentiality of introducing formate as artificial carbon intermediates to natural metabolic pathways for higher-than-nature photosynthetic efficiency.