Biology subjects

Calcinoni, A.

Publications and source records attributed to Calcinoni, A..

3 recordsLinked to original sources

bioRxiv2026

Bidirectional Electron Transfer in Far-Red-Light Adapted Photosystem I. Implications for the Photosystem's Functionality

Far-Red (FR) Light Photoacclimation (FaRLiP) enables cyanobacteria to extend photosynthetic activity into the far-red region by extensively remodelling Photosystem I (PSI), including the replacement of several core subunits with paralogs that coordinate the red-shifted chlorophyll f (Chl f). The binding positions of Chls f are still a matter of debate, with the most recent structural findings supporting the location of a single Chl f molecule within the reaction centre (RC) at the so-called A-1B site. This was in turn suggested to strongly affect electron transfer (ET) directionality leading to an almost monodirectional transfer along the B branch in FR-PSI RC. Here, we directly probe ET in FR-PSI by characterising the photogenerated [P700A1-] spin-correlated radical pair using complementary pulse and Time-Resolved (TR) Electron Paramagnetic Resonance (EPR) spectroscopy at cryogenic temperature. Electron spin-echo decay kinetics are distinctly biexponential, indicating the formation of two charge-separated states. Consistently, out-of-phase ESEEM traces are quantitatively described by two modulation frequencies arising from different dipolar interactions, while TR-EPR spectra are accurately simulated by the combined contributions of [P700A1A-] and [P700A1B-] radical pairs. These results provide direct spectroscopic evidence that both the A and B branches remain photochemically active in FR-PSI. The conservation of bidirectional ET, even when considering the presence of a single Chl f molecule in the RC, further implies that the two radical pairs originate from a common primary electron donor. This finding identifies P700 as the most likely primary donor and argues against a mechanism in which the RC Chl f initiates charge separation.

biophysics↗

bioRxiv2025

Enhanced carotenoid photoprotection in Far-Red light acclimated Chroococcidiopsis thermalis

Oxygenic photosynthesis is mainly driven by visible light in most photosynthetic organisms. However, some cyanobacteria strains can reversibly remodel their photosynthetic apparatus in order to rely exclusively on far-red photons. This acclimation, known as FaRLiP, requires the synthesis of red-shifted pigments, chlorophyll d, chlorophyll f and far-red allophycocyanin, that are incorporated in paralog subunits of the main photosynthetic complexes, namely photosystem II and photosystem I as well as phycobilisomes. In addition, some far-red-acclimating strains were also observed to show a rise in the carotenoid content and in the expression of genes involved in the biosynthesis of UV-shielding molecules, suggesting a counterintuitive photoprotective response concomitant with acclimation to lower energy wavelengths. In this work we investigated the response of Chroococcidiopsis thermalis to far-red light, and identified a robust set of photoprotective mechanisms associated with acclimation. Enhanced carotenoid/chlorophyll content ratios correlated with stronger carotenoid-chlorophyll triplet quenching, particularly pronounced for red-shifted chlorophylls. Non-photochemical quenching was higher and activated more rapidly than in cells grown under simulated solar light, and this was accompanied by enhanced antioxidant activity. Collectively, these findings indicate that the far-red acclimated cells, whose photosynthetic apparatus is not optimal under visible light, exhibit a strong photoprotected state, that we propose to be crucial under fluctuating irradiance conditions and during transitions from shaded to non-shaded environments. HighlightsO_LIFar-red light photoacclimation (FaRLiP) in Chroococcidiopsis thermalis is associated with an increased carotenoid-to-chlorophyll ratio. The relative abundance of myxol-2 fucoside, echinenone and {beta}-carotene increased in cells acclimated to far-red light compared with cells acclimated to simulated solar irradiation. C_LIO_LIUpon establishment of FaRLiP an exceptionally strong chlorophyll triplet quenching by carotenoids was observed, and it was specifically involving red-shifted chlorophylls. C_LIO_LIWhen exposed to the same light treatment, non-photochemical quenching was higher in far-red-acclimated cells than in solar-acclimated. Moreover, following high visible-light stress, far-red-acclimated cells exhibited lower levels of reactive oxygen species. The FaRLiP photosynthetic apparatus is safeguarded by robust mechanisms of photoprotection. C_LI

plant biology↗

bioRxiv2025

The Recombination Triplet State in the Far-Red Light Adapted Photosystem II is Located at the ChlD1 Site and Resides on the Red-Most Chlorophyll of the Reaction Center.

The energetic limits of Photosystem II (PSII) photochemical reactivity required reconsideration after the discovery of far-red light acclimation responses in cyanobacteria. Insights on PSII functionality following the inclusion of the red shifted Chlorophylls d and f can be obtained by extending the current knowledge on spectroscopic and structural properties of its reaction center (RC). The photo-induced triplet states, that represent selective endogenous probes, were therefore investigated in far-red adapted PSII by magnetic resonance techniques. Zero-field splitting tensor analysis combined with spin-polarization dynamics arising from radical pair recombination unambiguously identify an intrinsically low-energy-absorbing chlorophyll participating to charge separation reactions. The triplet-minus-singlet (T-S) spectrum associated to the recombination triplet state, obtained by microwave selection, showed a sharp 725 nm bleaching demonstrating the dominant involvement of this red-shifted chlorophyll in the lowest RC exciton. Moreover, spectral simulations provided strong evidence in favor of its localization at the ChlD1 position, making it the most likely site of primary photochemistry.

biochemistry↗