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.

Changes of Photosynthetic Behaviors and Photoprotection during Cell Transformation and Astaxanthin Accumulation in Haematococcus pluvialis Grown Outdoors in Tubular Photobioreactors

“…Maximal photochemical efficiency of PSII ( F v / F m ) is an indicator of photoinhibition in plants [ 25 , 26 ]. To examine the change in the capacity of photoprotection during the cell transformation and astaxanthin accumulation, we measured the change in F v / F m in H. pluvialis at predawn (6:00) and at midday with photo flux density (PFD) approximately 2300 μmol·m −2 ·s −1 (12:00) during the incubation ( Figure 10 ).…”

“…After five days of incubation, during astaxanthin accumulation ( Figure 1 ), the fact that the maximal amplitude in the I-P phase of the OJIP transient decreased obviously over time ( Figure 3 E) indicates a decrease in the pool size of PSI end electron acceptors, leading to an over-reduction of the PSI acceptor side [ 17 , 22 , 27 ]. Moreover, the relative variable fluorescence kinetics at the J-step increased with increased time after five days of incubation ( Figure 3 B), which indicates that the ratio of Q A − /Q A increased [ 19 , 26 ], resulting in an over-reduction of the PSII acceptor side during astaxanthin accumulation.…”

“…The L -bands increased significantly over time after five days of incubation ( Figure 3 C), indicating that the PSII units were less grouped, resulting in a poorer utilization of the excitation energy and a lower stability of the system [ 17 , 26 ] during astaxanthin accumulation in H. pluvialis . The K -bands ( Figure 3 D) increased significantly with increased time, which suggests that the donor side of PSII might be impaired [ 15 , 17 ] during astaxanthin accumulation.…”

“…Maximal photochemical efficiency of PSII ( F v / F m ) of cultures during incubation was measured using a Handy PEA fluorometer (Hansatech, Norfolk, UK) [ 26 ]. F v / F m measurement was performed using cultures that had been dark-adapted for 10 min prior to doing the measurement.…”

The effect of salinity shock on the growth and rapid light curve of dunaliella salina

“…An FMS‐2 pulse‐modulated fluorometer (Hansatech Instruments, Norfolk, UK) used to measure the modulated parameters of chlorophyll fluorescence such as F v / F m (the maximum light energy conversion efficiency), F v ’ / F m ’ (the maximum antenna conversion efficiency), Φ PSII (the actual photochemical efficiency of PSII) and qP (photochemical quenching coefficient) of D . salina according to the method listed in our previous publication (Li et al, 2020; Zhang et al, 2015). The measurement for each treatment was repeated for 6 times.…”