Role of the mitochondrial alternative oxidase pathway in hydrogen photoproduction in Chlorella protothecoides.

Abstract

The AOX pathway in C. protothecoides plays an important role in the photoprotection of PSII by alleviating the inhibition of the repair of the photodamaged PSII during H2 photoproduction. We had demonstrated that nitrogen limitation (LN) substantially enhanced H2 photoproduction in Chlorella protothecoides. In the present study, the mitochondrial alternative oxidase (AOX) pathway capacity was found to increase significantly during H2 photoproduction under LN or under LN simultaneously with sulfur deprivation (LNS) conditions. The purpose of this study was to clarify the role of the AOX pathway during H2 photoproduction in C. protothecoides. The AOX pathway can affect H2 photoproduction in the following ways: (1) consuming O2, which is favorable for the establishment of anaerobiosis; (2) consuming NADPH and competing with hydrogenase for photosynthetic electrons, which would decrease the H2 photoproduction; (3) protecting photosystem (PS) II, which is a direct electron source for H2 photoproduction, from photoinhibition. In LN and LNS cultures, the inhibition of the AOX pathway reduced the H2 photoproduction significantly, and did not increase the amount of O2. But, the inhibition of the AOX pathway decreased the maximal photochemical efficiency of PSII (F v/F m) and the actual photochemical efficiency of PSII (Φ PSII) significantly, leading to photoinhibition, which would decrease the photosynthetic electrons transferred to hydrogenase. And, the inhibition of the AOX pathway did not change the level of photoinhibition in the presence of D1 protein synthesis inhibitor chloramphenicol, indicating that the inhibition of the AOX pathway did not accelerate the photodamage to PSII directly but inhibited the repair of the photodamaged PSII. Therefore, the mitochondrial AOX pathway in C. protothecoides plays an important role in the photoprotection of PSII by alleviating the inhibition of the repair of the photodamaged PSII during H2 photoproduction, which is thus able to supply more electrons to hydrogenase under LN and LNS conditions.