Alternative electron pathways of photosynthesis drive the algal CO 2 concentrating mechanism

Abstract Global photosynthesis consumes ten times more CO 2 than net anthropogenic emissions, and microalgae account for nearly half of this consumption 1 . The great efficiency of algal photosynthesis relies on a mechanism concentrating CO 2 (CCM) at the catalytic site of the carboxylating enzyme RuBisCO, thus enhancing CO 2 fixation 2 . While many cellular components involved in the transport and sequestration of inorganic carbon (C i ) have been uncovered 3,4 , the way microalgae supply energy to concentrate CO 2 against a thermodynamic gradient remains elusive 4-6 . Here, by monitoring dissolved CO 2 consumption, unidirectional O 2 exchange and the chlorophyll fluorescence parameter NPQ in the green alga Chlamydomonas , we show that the complementary effects of cyclic electron flow and O 2 photoreduction, respectively mediated by PGRL1 and flavodiiron proteins, generate the proton motive force ( pmf ) required by C i transport across thylakoid membranes. We demonstrate that the trans-thylakoid pmf is used by bestrophin-like C i transporters and further establish that a chloroplast-to-mitochondria electron flow contributes to energize non-thylakoid C i transporters, most likely by supplying ATP. We propose an integrated view of the CCM energy supply network, describing how algal cells distribute photosynthesis energy to power different C i transporters, thus paving the way to the transfer of a functional algal CCM in plants towards improving crop productivity. One sentence summary Photosynthetic alternative electron flows and mitochondrial respiration drive the algal CO 2 concentrating mechanis

Alternative photosynthesis pathways drive the algal CO2-concentrating mechanism

International audienceGlobal photosynthesis consumes ten times more CO2 than net anthropogenic emissions, and microalgae account for nearly half of this consumption1. The high efficiency of algal photosynthesis relies on a mechanism concentrating CO2 (CCM) at the catalytic site of the carboxylating enzyme RuBisCO, which enhances CO2 fixation2. Although many cellular components involved in the transport and sequestration of inorganic carbon have been identified3,4, how microalgae supply energy to concentrate CO2 against a thermodynamic gradient remains unknown4–6. Here we show that in the green alga Chlamydomonas reinhardtii, the combined action of cyclic electron flow and O2 photoreduction—which depend on PGRL1 and flavodiiron proteins, respectively—generate a low luminal pH that is essential for CCM function. We suggest that luminal protons are used downstream of thylakoid bestrophin-like transporters, probably for the conversion of bicarbonate to CO2. We further establish that an electron flow from chloroplast to mitochondria contributes to energizing non-thylakoid inorganic carbon transporters, probably by supplying ATP. We propose an integrated view of the network supplying energy to the CCM, and describe how algal cells distribute energy from photosynthesis to power different CCM processes. These results suggest a route for the transfer of a functional algal CCM to plants to improve crop productivity

Coproporphyrin Excretion and Low Thiol Levels Caused by Point Mutation in the Rhodobacter sphaeroides S-Adenosylmethionine Synthetase Gene ▿ †

A spontaneous mutant of Rhodobacter sphaeroides f. sp. denitrificans IL-106 was found to excrete a large amount of a red compound identified as coproporphyrin III, an intermediate in bacteriochlorophyll and heme synthesis. The mutant, named PORF, is able to grow under phototrophic conditions but has low levels of intracellular cysteine and glutathione and overexpresses the cysteine synthase CysK. The expression of molybdoenzymes such as dimethyl sulfoxide (DMSO) and nitrate reductases is also affected under certain growth conditions. Excretion of coproporphyrin and overexpression of CysK are not directly related but were both found to be consequences of a diminished synthesis of the key metabolite S-adenosylmethionine (SAM). The wild-type phenotype is restored when the gene metK encoding SAM synthetase is supplied in trans. The metK gene in the mutant strain has a mutation leading to a single amino acid change (H145Y) in the encoded protein. This point mutation is responsible for a 70% decrease in intracellular SAM content which probably affects the activities of numerous SAM-dependent enzymes such as coproporphyrinogen oxidase (HemN); uroporphyrinogen III methyltransferase (CobA), which is involved in siroheme synthesis; and molybdenum cofactor biosynthesis protein A (MoaA). We propose a model showing that the attenuation of the activities of SAM-dependent enzymes in the mutant could be responsible for the coproporphyrin excretion, the low cysteine and glutathione contents, and the decrease in DMSO and nitrate reductase activities

Accumulation of plastid lipid‐associated proteins (fibrillin/CDSP34) upon oxidative stress, ageing and biotic stress in Solanaceae and in response to drought in other species

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Elevated Expression of PGR5 and NDH-H in Bundle Sheath Chloroplasts in C4Flaveria Species

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ACCLIMATION OF PHOTOSYNTHESIS TO THE ENVIRONMENT 1 regulates Photosystem II Supercomplex dynamics in response to light in Chlamydomonas reinhardtii

Photosynthetic organisms require acclimation mechanisms to regulate photosynthesis in response to light conditions. Here, two mutant alleles of ACCLIMATION OF PHOTOSYNTHESIS TO THE ENVIRONMENT 1 ( ape1 ) have been characterized in Chlamydomonas reinhardtii. The ape1 mutants are photosensitive and show PSII photoinhibition during high light acclimation or under high light stress. The ape1 mutants retain more PSII super-complexes and have changes to thylakoid stacking relative to control strains during photosynthetic growth at different light intensities. The APE1 protein is found in all oxygenic phototrophs and encodes a 25 kDa thylakoid protein that interacts with the Photosystem II core complex as monomers, dimers and supercomplexes. We propose a model where APE1 bound to PSII supercomplexes releases core complexes and promotes PSII heterogeneity influencing the stacking of Chlamydomonas thylakoids. APE1 is a regulator in light acclimation and its function is to reduce over-excitation of PSII centres and avoid PSII photoinhibition to increase the resilience of photosynthesis to high light