Photosynthetic light reactions establish electron flow in the chloroplast's thylakoid membranes, leading to the production of the ATP and NADPH that participate in carbon fixation. Two modes of electron flow exist-linear electron flow (LEF) from water to NADP(+) via photosystem (PS) II and PSI in series and cyclic electron flow (CEF) around PSI (ref. 2). Although CEF is essential for satisfying the varying demand for ATP, the exact molecule(s) and operational site are as yet unclear. In the green alga Chlamydomonas reinhardtii, the electron flow shifts from LEF to CEF on preferential excitation of PSII (ref. 3), which is brought about by an energy balancing mechanism between PSII and PSI (state transitions). Here, we isolated a protein supercomplex composed of PSI with its own light-harvesting complex (LHCI), the PSII light-harvesting complex (LHCII), the cytochrome b(6)f complex (Cyt bf), ferredoxin (Fd)-NADPH oxidoreductase (FNR), and the integral membrane protein PGRL1 (ref. 5) from C. reinhardtii cells under PSII-favouring conditions. Spectroscopic analyses indicated that on illumination, reducing equivalents from downstream of PSI were transferred to Cyt bf, whereas oxidised PSI was re-reduced by reducing equivalents from Cyt bf, indicating that this supercomplex is engaged in CEF (Supplementary Fig. 1). Thus, formation and dissociation of the PSI-LHCI-LHCII-FNR-Cyt bf-PGRL1 supercomplex not only controlled the energy balance of the two photosystems, but also switched the mode of photosynthetic electron flow.

Absorption of light in excess of the capacity for photosynthetic electron transport is damaging to photosynthetic organisms. Several mechanisms exist to avoid photodamage, which are collectively referred to as nonphotochemical quenching. This term comprises at least two major processes. State transitions (qT) represent changes in the relative antenna sizes of photosystems II and I. High energy quenching (qE) is the increased thermal dissipation of light energy triggered by lumen acidification. To investigate the respective roles of qE and qT in photoprotection, a mutant (npq4 stt7-9) was generated in Chlamydomonas reinhardtii by crossing the state transition-deficient mutant (stt7-9) with a strain having a largely reduced qE capacity (npq4). The comparative phenotypic analysis of the wild type, single mutants, and double mutants reveals that both state transitions and qE are induced by high light. Moreover, the double mutant exhibits an increased photosensitivity with respect to the single mutants and the wild type. Therefore, we suggest that besides qE, state transitions also play a photoprotective role during high light acclimation of the cells, most likely by decreasing hydrogen peroxide production. These results are discussed in terms of the relative photoprotective benefit related to thermal dissipation of excess light and/ or to the physical displacement of antennas from photosystem II.

In plants and algae, light serves both as the energy source for photosynthesis and a biological signal that triggers cellular responses via specific sensory photoreceptors. Red light is perceived by bilin-containing phytochromes and blue light by the flavin-containing cryptochromes and/or phototropins (PHOTs), the latter containing two photosensory light, oxygen, or voltage (LOV) domains. Photoperception spans several orders of light intensity, ranging from far below the threshold for photosynthesis to values beyond the capacity of photosynthetic CO assimilation. Excess light may cause oxidative damage and cell death, processes prevented by enhanced thermal dissipation via high-energy quenching (qE), a key photoprotective response. Here we show the existence of a molecular link between photoreception, photosynthesis, and photoprotection in the green alga Chlamydomonas reinhardtii. We show that PHOT controls qE by inducing the expression of the qE effector protein LHCSR3 (light-harvesting complex stress-related protein 3) in high light intensities. This control requires blue-light perception by LOV domains on PHOT, LHCSR3 induction through PHOT kinase, and light dissipation in photosystem II via LHCSR3. Mutants deficient in the PHOT gene display severely reduced fitness under excessive light conditions, indicating that the sensing, utilization, and dissipation of light is a concerted process that plays a vital role in microalgal acclimation to environments of variable light intensities.

Plants and green algae have a low pH-inducible mechanism in photosystem II (PSII) that dissipates excess light energy, measured as the nonphotochemical quenching of chlorophyll fluorescence (qE). Recently, nonphotochemical quenching 4 (npq4), a mutant strain of the green alga Chlamydomonas reinhardtii that is qE-deficient and lacks the light-harvesting complex stress-related protein 3 (LHCSR3), was reported [Peers G, et al. (2009) Nature 462(7272):518-521]. Here, applying a newly established procedure, we isolated the PSII supercomplex and its associated light-harvesting proteins from both WT C. reinhardtii and the npq4 mutant grown in either low light (LL) or high light (HL). LHCSR3 was present in the PSII supercomplex from the HLgrown WT, but not in the supercomplex from the LL-grown WT or mutant. The purified PSII supercomplex containing LHCSR3 exhibited a normal fluorescence lifetime at a neutral pH (7.5) by single-photon counting analysis, but a significantly shorter lifetime at pH 5.5, which mimics the acidified lumen of the thylakoid membranes in HL-exposed chloroplasts. The switch from light-harvesting mode to energy-dissipating mode observed in the LHCSR3-containing PSII supercomplex was sensitive to dicyclohexylcarbodiimide, a protein-modifying agent specific to protonatable amino acid residues. We conclude that the PSII-LHCII-LHCSR3 supercomplex formed in the HL-grown C. reinhardtii cells is capable of energy dissipation on protonation of LHCSR3.high light stress | light acclimation | photosynthesis | time-resolved fluorescence P hotosynthetic reactions in plants and algae fix CO 2 by converting solar energy into electrochemical energy. In nature, unexpected changes in light intensity could lead to overexcitation of the photosystems, resulting in the accumulation of harmful reactive oxygen species (1). Plants and green algae have developed protective nonphotochemical quenching (NPQ) mechanisms that alleviate such photo-oxidative stress. Among these mechanisms, quenching of chlorophyll (Chl) fluorescence (qE)-a feedback process of photosystem II (PSII) regulation-dissipates excess light energy captured by PSII as heat on luminal acidification of the thylakoid membranes, which occurs along with elevated electron flow.Numerous previous studies have focused on elucidating the molecular mechanism of qE quenching (see refs. 1-3 for reviews). In higher plants, qE induction depends on activation of the xanthophyll cycle (4-6) and the sensing of luminal acidification by PsbS, a protein homologous to light harvesting complex (LHC) proteins (7,8). Protonation of PsbS induces a change in the macroorganization of the thylakoid membranes (3, 9, 10) that results in the aggregation of LHCII proteins (11) and/or induces conformational change(s) in LHCII (12), allowing for the formation of energyquenching sites (13). Although both PsbS protein and zeaxanthin are thought to have crucial roles in qE quenching in higher plants, such molecular effectors have not been studied in depth in other photosynthetic organisms. The ...

Background: Light-harvesting complex II (LHCII) proteins associate with photosystem II (PSII) to form a supercomplex. Results: We isolated an active PSII supercomplex from thylakoids solubilized with dodecyl-␣-D-maltoside and visualized it in a large projection map with an electron microscope. Conclusion:The novel PSII supercomplex harbored three LHCII trimers on each side. Significance: The large and active PSII-LHCII supercomplex is engaged in green algal photosynthesis.