Enhanced H2 photoproduction by down-regulation of ferredoxin-NADP reductase (FNR) in the green alga Chlamydomonas reinhardtii

Renewable H₂ photoproduction by green algae such as Chlamydomonas reinhardtii is a promising system for solar fuels. However, large-scale application of the system has lagged virtually due to lack of high H₂-producing strains. We previously identified ferredoxin-NADP⁺ reductase (FNR) among the 105 proteins differentially expressed in Chlamydomonas during sulfur-deprived H₂ photoproduction. In this work, we used an RNA interference (RNAi) approach to generate Chlamydomonas mutant strains with reduced levels of FNR. We found that fnr-RNAi strains exhibited higher rates of H₂ photoproduction (2.5-fold) than wild type under sulfur-deprived condition. To elucidate the basis for this increase, we analyzed the physiological characteristics of the fnr-RNAi strains under such condition. Major changes, due to the down-regulation of FNR, included the lower rates of photosynthetic O₂ evolution (44%), greater reduction of Rubisco amounts (60%) and higher rates of starch degradation (140%). These may result in an earlier onset of anaerobiosis and increased electron supply to the hydrogenases in the mutant strains. The results provide new information of FNR in regulating H₂ metabolism as well as potential strains for further improvement of the organism toward application in solar-powered systems.