Pitfalls, artefacts and open questions in chlorophyll thermoluminescence of leaves or algal cells

Redesign of the Chlamydomonas reinhardtii QB binding niche reveals photosynthesis works in the absence of a driving force for QA-QB electron transfer

An in silico redesign of the secondary quinone electron acceptor (QB) binding pocket of the D1 protein of Photosystem II (PSII) suggested that mutations of the F265 residue would affect atrazine binding. Chlamydomonas reinhardtii mutants F265T and F265S were produced to obtain atrazine-hypersensitive strains for biosensor applications, and the mutants were indeed found to be more atrazine-sensitive than the reference strain IL. Fluorescence and thermoluminescence data agree with a weak driving force and confirm slow electron transfer but cannot exclude an additional effect on protonation of the secondary quinone. Both mutants grow autotrophically, indicating that PSII requires strong light for optimal function, as was the case in the ancestral homodimeric reaction center.

A PSII photosynthetic control is activated in anoxic cultures of green algae following illumination

Photosynthetic hydrogen production from microalgae is considered to have potential as a renewable energy source. Yet, the process has two main limitations holding it back from scaling up; (i) electron loss to competing processes, mainly carbon fixation and (ii) sensitivity to O₂ which diminishes the expression and the activity of the hydrogenase enzyme catalyzing H₂ production. Here we report a third, hitherto unknown challenge: We found that under anoxia, a slow-down switch is activated in photosystem II (PSII), diminishing the maximal photosynthetic productivity by three-fold. Using purified PSII and applying in vivo spectroscopic and mass spectrometric techniques on Chlamydomonas reinhardtii cultures, we show that this switch is activated under anoxia, within 10 s of illumination. Furthermore, we show that the recovery to the initial rate takes place following 15 min of dark anoxia, and propose a mechanism in which, modulation in electron transfer at the acceptor site of PSII diminishes its output. Such insights into the mechanism broaden our understanding of anoxic photosynthesis and its regulation in green algae and inspire new strategies to improve bio-energy yields.

Chlamydomonas reinhardtii mutants deficient for Old Yellow Enzyme 3 exhibit increased photooxidative stress

Old Yellow Enzymes (OYEs) are flavin-containing ene-reductases that have been intensely studied with regard to their biotechnological potential for sustainable chemical syntheses. OYE-encoding genes are found throughout the domains of life, but their physiological role is mostly unknown, one reason for this being the promiscuity of most ene-reductases studied to date. The unicellular green alga Chlamydomonas reinhardtii possesses four genes coding for OYEs, three of which we have analyzed biochemically before. Ene-reductase CrOYE3 stood out in that it showed an unusually narrow substrate scope and converted N-methylmaleimide (NMI) with high rates. This was recapitulated in a C. reinhardtii croye3 mutant that, in contrast to the wild type, hardly degraded externally added NMI. Here we show that CrOYE3-mediated NMI conversion depends on electrons generated photosynthetically by photosystem II (PSII) and that the croye3 mutant exhibits slightly decreased photochemical quenching in high light. Non-photochemical quenching is strongly impaired in this mutant, and it shows enhanced oxidative stress. The phenotypes of the mutant suggest that C. reinhardtii CrOYE3 is involved in the protection against photooxidative stress, possibly by converting reactive carbonyl species derived from lipid peroxides or maleimides from tetrapyrrole degradation.

Ascorbate inactivates the oxygen-evolving complex in prolonged darkness

Ascorbate (Asc, vitamin C) is an essential metabolite participating in multiple physiological processes of plants, including environmental stress management and development. In this study, we acquired knowledge on the role of Asc in dark-induced leaf senescence using Arabidopsis thaliana as a model organism. One of the earliest effects of prolonged darkness is the inactivation of oxygen-evolving complexes (OEC) as demonstrated here by fast chlorophyll a fluorescence and thermoluminescence measurements. We found that inactivation of OEC due to prolonged darkness was attenuated in the Asc-deficient vtc2-4 mutant. On the other hand, the severe photosynthetic phenotype of a psbo1 knockout mutant, lacking the major extrinsic OEC subunit PSBO1, was further aggravated upon a 24-h dark treatment. The psbr mutant, devoid of the PSBR subunit of OEC, performed only slightly disturbed photosynthetic activity under normal growth conditions, whereas it showed a strongly diminished B thermoluminescence band upon dark treatment. We have also generated a double psbo1 vtc2 mutant, and it showed a slightly milder photosynthetic phenotype than the single psbo1 mutant. Our results, therefore, suggest that Asc leads to the inactivation of OEC in prolonged darkness by over-reducing the Mn-complex that is probably enabled by a dark-induced dissociation of the extrinsic OEC subunits. Our study is an example that Asc may negatively affect certain cellular processes and thus its concentration and localization need to be highly controlled.

The afterglow photosynthetic luminescence

The afterglow (AG) photosynthetic luminescence is a long-lived chlorophyll fluorescence emitted from PSII after the illumination of photosynthetic materials by FR or white light and placed in darkness. The AG emission corresponds to the fraction of PSII centers in the S_2/3 Q_B non-radiative state immediately after pre-illumination, in which the arrival of an electron transferred from stroma along cyclic/chlororespiratory pathway(s) produces the S_2/3 Q_B ^- radiative state that emits luminescence. This emission can be optimally recorded by a linear temperature gradient as sharp thermoluminescence (TL) band peaking at about 45°C. The AG emission recorded by TL technique has been proposed as a simple non-invasive tool to investigate the chloroplast energetic state and some of its metabolism processes as cyclic transport of electrons around PSI, chlororespiration or photorespiration. On the other hand, this emission has demonstrated to be a useful probe to study the effect of various stress conditions in photosynthetic materials.

Assessment of photosystem II thermoluminescence as a tool to investigate the effects of dehydration and rehydration on the cyclic/chlororespiratory electron pathways in wheat and barley leaves

Thermoluminescence emission from wheat leaves was recorded under various controlled drought stress conditions: (i) fast dehydration (few hours) of excised leaves in the dark (ii) slow dehydration (several days) obtained by withholding watering of plants under a day/night cycle (iii) overnight rehydration of the slowly dehydrated plants at a stage of severe dessication. In fast dehydrated leaves, the AG band intensity was unchanged but its position was shifted to lower temperatures, indicating an activation of cyclic and chlororespiratory pathways in darkness, without any increase of their overall electron transfer capacity. By contrast, after a slow dehydration the AG intensity was strongly increased whereas its position was almost unchanged, indicating respectively that the capacity of cyclic pathways was enhanced but that they remained inactivated in darkness. Under more severe dehydration, the AG band almost disappeared. Rewatering caused its rapid bounce significantly above the control level. No significant differences in AG emission could be found between the two drought-sensitive and drought-tolerant wheat cultivars. The afterglow thermoluminescence emission in leaves provides an additional tool to follow the increased capacity and activation of cyclic electron flow around PSI in leaves during mild, severe dehydration and after rehydration.

Effect of constitutive expression of bacterial phytoene desaturase CRTI on photosynthetic electron transport in Arabidopsis thaliana

The constitutive expression of the bacterial carotene desaturase (CRTI) in Arabidopsis thaliana leads to increased susceptibility of leaves to light-induced damage. Changes in the photosynthetic electron transport chain rather than alterations of the carotenoid composition in the antenna were responsible for the increased photoinhibition. A much higher level of superoxide/hydrogen peroxide was generated in the light in thylakoid membranes from the CRTI expressing lines than in wild-type while the level of singlet oxygen generation remained unchanged. The increase in reactive oxygen species was related to the activity of plastid terminal oxidase (PTOX) since their generation was inhibited by the PTOX-inhibitor octyl gallate, and since the protein level of PTOX was increased in the CRTI-expressing lines. Furthermore, cyclic electron flow was suppressed in these lines. We propose that PTOX competes efficiently with cyclic electron flow for plastoquinol in the CRTI-expressing lines and that it plays a crucial role in the control of the reduction state of the plastoquinone pool.