Search

Cyanobacteria hold great potential to revolutionize conventional industries and farming practices with their light-driven chemical production. To fully exploit their photosynthetic capacity and enhance product yield, it is crucial to investigate their intricate interplay with the environment including the light intensity and spectrum. Mathematical models provide valuable insights for optimizing strategies in this pursuit. In this study, we present an ordinary differential equation-based model for the cyanobacterium Synechocystis sp. PCC 6803 to assess its performance under various light sources, including monochromatic light. Our model can reproduce a variety of physiologically measured quantities, e.g. experimentally reported partitioning of electrons through four main pathways, O2 evolution, and the rate of carbon fixation for ambient and saturated CO2. By capturing the interactions between different components of a photosynthetic system, our model helps in understanding the underlying mechanisms driving system behavior. Our model qualitatively reproduces fluorescence emitted under various light regimes, replicating Pulse-amplitude modulation (PAM) fluorometry experiments with saturating pulses. Using our model, we test four hypothesized mechanisms of cyanobacterial state transitions for ensemble of parameter sets and found no physiological benefit of a model assuming phycobilisome detachment. Moreover, we evaluate metabolic control for biotechnological production under diverse light colors and irradiances. We suggest gene targets for overexpression under different illuminations to increase the yield. By offering a comprehensive computational model of cyanobacterial photosynthesis, our work enhances the basic understanding of light-dependent cyanobacterial behavior and sets the first wavelength-dependent framework to systematically test their producing capacity for biocatalysis., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Pfennig et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Cyanobacteria play a key role in primary production in both oceans and fresh waters and hold great potential for sustainable production of a large number of commodities. During their life, cyanobacteria cells need to acclimate to a multitude of challenges, including shifts in intensity and quality of incident light. Despite our increasing understanding of metabolic regulation under various light regimes, detailed insight into fitness advantages and limitations under shifting light quality remains underexplored. Here, we study photo-physiological acclimation in the cyanobacterium Synechocystis sp. PCC 6803 throughout the photosynthetically active radiation (PAR) range. Using light emitting diodes (LEDs) with qualitatively different narrow spectra, we describe wavelength dependence of light capture, electron transport and energy transduction to main cellular pools. In addition, we describe processes that fine-tune light capture, such as state transitions, or the efficiency of energy transfer from phycobilisomes to photosystems (PS). We show that growth was the most limited under blue light due to inefficient light harvesting, and that many cellular processes are tightly linked to the redox state of the plastoquinone (PQ) pool, which was the most reduced under red light. The PSI-to-PSII ratio was low under blue photons, however, it was not the main growth-limiting factor, since it was even more reduced under violet and near far-red lights, where Synechocystis grew faster compared to blue light. Our results provide insight into the spectral dependence of phototrophic growth and can provide the foundation for future studies of molecular mechanisms underlying light acclimation in cyanobacteria, leading to light optimization in controlled cultivations., (© The Author(s) 2024. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists.)

The atypical unicellular cyanobacterium Gloeobacter violaceus PCC 7421, which diverged very early during the evolution of cyanobacteria, can be regarded as a key organism for understanding many structural, functional, regulatory and evolutionary aspects of oxygenic photosynthesis. In the present work, the performance of two basic photosynthetic adaptation/protection mechanisms, common to all other oxygenic photoautrophs, had been challenged in this ancient cyanobacterium which lacks thylakoid membranes: state transitions and non-photochemical fluorescence quenching. Both low temperature fluorescence spectra and room temperature fluorescence transients show that G. violaceus is capable of performing state transitions similar to evolutionarily more recent cyanobacteria, being in state 2 in darkness and in state 1 upon illumination by weak blue or far-red light. Compared with state 2, variable fluorescence yield in state 1 is strongly enhanced (almost 80%), while the functional absorption cross-section of PSII is only increased by 8%. In contrast to weak blue light, which enhances fluorescence yield via state 1 formation, strong blue light reversibly quenches Chl fluorescence in G. violaceus. This strongly suggests regulated heat dissipation which is triggered by the orange carotenoid protein whose presence was directly proven by immunoblotting and mass spectrometry in this primordial cyanobacterium. The results are discussed in the framework of cyanobacterial evolution.

Exposure of cyanobacterial or red algal cells to high light has been proposed to lead to excitonic decoupling of the phycobilisome antennae (PBSs) from the reaction centers. Here we show that excitonic decoupling of PBSs of Synechocystis sp. PCC 6803 is induced by strong light at wavelengths that excite either phycobilin or chlorophyll pigments. We further show that decoupling is generally followed by disassembly of the antenna complexes and/or their detachment from the thylakoid membrane. Based on a previously proposed mechanism, we suggest that local heat transients generated in the PBSs by non-radiative energy dissipation lead to alterations in thermo-labile elements, likely in certain rod and core linker polypeptides. These alterations disrupt the transfer of excitation energy within and from the PBSs and destabilize the antenna complexes and/or promote their dissociation from the reaction centers and from the thylakoid membranes. Possible implications of the aforementioned alterations to adaptation of cyanobacteria to light and other environmental stresses are discussed., (Copyright © 2011 Elsevier B.V. All rights reserved.)

The γ and ε subunits of F(0)F(1)-ATP synthase from photosynthetic organisms display unique properties not found in other organisms. Although the γ subunit of both chloroplast and cyanobacterial F(0)F(1) contains an extra amino acid segment whose deletion results in a high ATP hydrolysis activity (Sunamura, E., Konno, H., Imashimizu-Kobayashi, M., Sugano, Y., and Hisabori, T. (2010) Plant Cell Physiol. 51, 855-865), its ε subunit strongly inhibits ATP hydrolysis activity. To understand the physiological significance of these phenomena, we studied mutant strains with (i) a C-terminally truncated ε (ε(ΔC)), (ii) γ lacking the inserted sequence (γ(Δ198-222)), and (iii) a double mutation of (i) and (ii) in Synechocystis sp. PCC 6803. Although thylakoid membranes from the ε(ΔC) strain showed higher ATP hydrolysis and lower ATP synthesis activities than those of the wild type, no significant difference was observed in growth rate and in intracellular ATP level both under light conditions and during light-dark cycles. However, both the ε(ΔC) and γ(Δ198-222) and the double mutant strains showed a lower intracellular ATP level and lower cell viability under prolonged dark incubation compared with the wild type. These data suggest that internal inhibition of ATP hydrolysis activity is very important for cyanobacteria that are exposed to prolonged dark adaptation and, in general, for the survival of photosynthetic organisms in an ever-changing environment.

In contrast to eukaryotes, most cyanobacteria contain several isoforms of the Rieske iron-sulfur protein, PetC, resulting in heterogeneity in the composition of the cytochrome b(6)f complexes. Of three isoforms in the mesophilic cyanobacterium Synechocystis PCC 6803, PetC1 is the major Rieske protein in the cytochrome b(6)f complex, whereas the physiological function of PetC2 and PetC3 is still uncertain. Comparison of wild type and various petC-deficient strains under selected light conditions revealed distinct functional differences: high-light exposure of wild type cells resulted in a significantly enhanced petC2 transcript level, whereas a Delta petC1 mutant showed a low cytochrome b(6)f content, low electron flux, and a considerably increased accumulation of cytochrome-bd oxidase. In contrast to wild type and Delta petC1, Delta petC2 and Delta petC3 strains still grew fast under high-light conditions although all three Rieske proteins are required for maximal electron transport rates. Although the presence of PetC3 appears to be required for activation of the cyclic electron transport, state transitions were more effective in the absence of PetC2 and/or PetC3. In summary, our data suggest defined roles of the various PetC proteins in short- and long-term light adaptation.