The role of plastocyanin in regulating photosynthetic and high light response remains unclear. Here we found that a larger PC pool effectively increased the linear electron flow while lowering the reduction state of plastoquinone (PQ) and ferredoxin (Fd), subsequently decreasing ROS formation. The biomass yield was increased in PC-rich cells, which, along with a decrease in intracellular starch content, up-regulated the RuBisCO activity, thereby enhancing carbon fixation. Meanwhile, we observed that ADP-glucose pyrophosphorylase activity was down-regulated, probably due to a lower Fd reduction state leading to reduced starch synthesis. During the transition from low to high light, PC accumulates to a substantial level, effectively reducing the reduction state of PQ and restoring Fv/Fm. These findings highlight the critical function of PC as a redox capacitor in affecting PET, CO₂ fixation, and high light acclimation by modulating the redox states of the electron transport chain (ETC) and its potential for optimizing photosynthetic processes.

Fucoxanthin is an oxygenated carotenoid component that has been reported to play important roles in anti-oxidation, anti-obesity and anti-cancer in the human body. Fucoxanthin-chlorophyll protein (FCP) complexes participate in light harvesting and photoprotection in diatom. In order to better understand the change of fucoxanthin content and its role in photoprotection, the growth, fucoxanthin biosynthesis and photosynthetic phenotypes were studied in the diatom Phaeodactylum tricornutum under the treatment of exogenous arachidonic acid (AA). Our results demonstrated that even low concentration of AA at 0.1 mg/L strongly induced fucoxanthin accumulation in algal cells to a maximum of 1.1 mg/g dry weight (DW), which was 36.6% higher than that in the untreated ones. By principal component analysis (PCA), we also identified a close correlation between fucoxanthin accumulation and the expression of genes involved in fucoxanthin biosynthesis, especially phytoene synthase (PSY), suggesting that AA change the metabolism of fucoxanthin by inducing carotenoid metabolic enzymes at the transcriptional level. Furthermore, we found that the exogenous application of AA affected non-photochemical quenching (NPQ) and photoinhibition, which resulted from the changed diadinoxanthin (DD) and diatoxanthin (DT) cycle, and thus played an important role in photoprotection.

The model green alga Chlamydomonas reinhardtii has a pyrenoid within chloroplast for photosynthetic CO₂ fixation under limiting CO₂ conditions and its chloroplast is an ideal chassis for engineering photosynthetic modules. In contrast, carboxysomes are bacterial microcompartments that encapsulate Rubisco for CO₂ fixation. The aim of the present study was to determine if a bacterial carboxysome or its components can replace pyrenoid and function to allow Chlamydomonas growth powered by photosynthesis. We replaced the Chlamydomonas endogenous RbcL gene with cbbL and cbbS from the chemoautotrophic bacterium Halothiobacillus neapolitanus, resulting in proper assembly of the heterologous Rubisco with slightly lower enzymatic activity than the endogenous Rubisco. We next expressed four or six carboxysome-related genes and observed the formation of carboxysome-like structures in chloroplast. Photoautotrophic growth of transformants were enabled under high CO₂ (∼5%) conditions, but not in ambient air (∼0.04% CO₂). Our study serves as a demonstration on how carboxysomes can be partially reconstituted in eukaryotic algae, which have great uses in bioindustry. The single-cell nature of Chlamydomonas allows it to be used as a testbed for optimizing the sequences of the genes required for heterologous carboxysome formation and for future improvements in CO2-fixation efficiency and incorporation of diverse metabolic pathways in chloroplasts.

Photosystem II (PSII) is susceptible to photodamage under high light stress, necessitating an efficient repair cycle that involves the degradation of the damaged D1 protein, primarily by FtsH proteases. While the involvement of FtsH in D1 turnover is established, the regulatory mechanisms ensuring precise degradation remain unclear. In this study, we characterize the function of TEF6, a conserved thylakoid membrane protein with two transmembrane domains in Chlamydomonas reinhardtii. The tef6 exhibits severe growth inhibition, reduced PSII activity, impaired accumulation of PSII supercomplexes, and disorganized thylakoid membranes specifically under high light conditions. Physiological, cellular, biochemical and genetics assays confirmed that loss of TEF6 specifically impairs PSII stability and repair. Furthermore, multiple approaches including co-immunoprecipitation coupled with mass spectrometry, yeast two-hybrid assays, and bimolecular fluorescence complementation (BiFC) experiments demonstrated that TEF6 directly interacts with both the D1 protein and the FtsH proteases (FtsH1/FtsH2). Loss of TEF6 leads to misregulated, excessive accumulation of FtsH under high light, which correlates with accelerated and uncontrolled degradation of the D1 protein, ultimately disrupting the PSII repair cycle and homeostasis. Our findings identify TEF6 functioning as a crucial scaffold-like factor in the PSII repair machinery. TEF6 stabilizes the proper accumulation of the FtsH protease complex in the thylakoid membrane, thereby ensuring the correct and regulated turnover of photo-damaged D1. This study reveals a novel regulatory mechanism, mediated by a GreenCut protein, for maintaining PSII quality control and photosynthetic efficiency under light stress.