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

• Chlamydomonas mutant strains with reduced level of FNR were generated by RNAi method.
• fnr-RNAi strains were characterized under sulfur-deprived H2 production condition.
• Profiles of H2 photoproduction, O2 evolution and starch degradation were examined.
• H2-photoproducing capability of fnr-RNAi strains was enhanced under sulfur depletion.
• Reduction of FNR stimulated inactivation of photosynthesis and starch degradation.

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.