Carotenoids Do Not Protect Bacteriochlorophylls in Isolated Light-Harvesting LH2 Complexes of Photosynthetic Bacteria from Destructive Interactions with Singlet Oxygen

In-situ construction of biomineralized cadmium sulfide-Rhodopseudomonas palustris hybrid system: Mechanism of synergistic light utilization

Sulfide biomineralization is a microorganism-induced process for transforming the environmentally hazardous cadmium into useful resource utilization. This study successfully constructed cadmium sulfide nanoparticles-Rhodopseudomonas palustris (Bio-CdS NPs-R. palustris) hybrids. For the self-assembling hybrids, Bio-CdS NPs were treated as new artificial-antennas to enhance photosynthesis, especially under low light (LL). Bacterial physiological results of hybrids were significantly increased, particularly for cells under LL, with higher enhancement photon harvesting ability. The enhancement included the pigment contents, and the ratio of the peripheral light-harvesting complex Ⅱ (LH2) to light-harvesting Ⅰ (1.33 ± 0.01 under LL), leading to the improvements of light-harvesting, transfer, and antenna conversion efficiencies. Finally, the stimulated electron chain of hybrids improved bacterial metabolism with increased nicotinamide adenine dinucleotide (NADH, 174.5% under LL) and adenosine triphosphate (ATP, 41.1% under LL). Furthermore, the modified photosynthetic units were induced by the up-regulated expression of fixK, which was activated by reduced oxygen tension of the medium for hybrids. fixK up-regulated genes encoding pigments (crt, and bch) and complexes (puf, pucAB, and pucC), leading to improved light-harvesting and transfer, and transform ability. This study provides a comprehensive understanding of the solar energy utilization mechanism of in-situ semiconductor-phototrophic microbe hybrids, contributing to further theoretical insight into their practical application.

Photogeneration and quenching of singlet molecular oxygen by bacterial C40 carotenoids with long chain of conjugated double bonds

Measurement of photosensitized luminescence of singlet oxygen has been applied to studies of singlet oxygen generation and quenching by C40 carotenoids (neurosporene, lycopene, rhodopin, and spirilloxanthin) with long chain of conjugated double bonds (CDB) using hexafluorobenzene as a solvent. It has been found that neurosporene, lycopene, and rhodopin are capable of the low efficient singlet oxygen generation in aerated solutions upon photoexcitation in the spectral region of their main absorption maxima. The quantum yield of this process was estimated to be (1.5-3.0) × 10-2. This value is near the singlet oxygen yields in solutions of ζ-carotene (7 CDB) and phytoene (3 CDB) and many-fold smaller than in solutions of phytofluene (5 CDB) (Ashikhmin et al. Biochemistry (Mosc) 85:773-780, https://doi.org/10.1134/S0006297920070056 , 2020, Biochemistry (Mosc) 87:1169-1178, 2022, https://doi.org/10.1134/S00062979221001082022 ). Photogeneration of singlet oxygen was not observed in spirilloxanthin solutions. A correlation was found between the singlet oxygen yields and the quantum yields and lifetimes of the fluorescence of the carotenoid molecules. All carotenoids were shown to be strong physical quenchers of singlet oxygen. The rate constants of 1O2 quenching by the carotenoids with long chain of CDB (9-13) were close to the rate constant of the diffusion-limited reactions ≈1010 M-1 s-1, being many-fold greater than the rate constants of 1O2 quenching by the carotenoids with the short chain of CDB (3-7) phytoene, phytofluene, and ζ-carotene studied in prior papers of our group (Ashikhmin et al. 2020, 2022). To our knowledge, the quenching rate constants of rhodopin and spirilloxanthin have been obtained in this paper for the first time. The mechanisms of 1O2 photogeneration by carotenoids in solution and in the LH2 complexes of photosynthetic cells, as well as the efficiencies of their protective action are discussed.

Carotenoid-dependent singlet oxygen photogeneration in light-harvesting complex 2 of Ectothiorhodospira haloalkaliphila leads to the formation of organic hydroperoxides and damage to both pigments and protein matrix

Earlier, it was suggested that carotenoids in light-harvesting complexes 2 (LH2) can generate singlet oxygen, further oxidizing bacteriochlorophyll to 3-acetyl-chlorophyll. In the present work, it was found that illumination of isolated LH2 preparations of purple sulfur bacterium Ectothiorhodospira haloalkaliphila with light in the carotenoid absorption region leads to the photoconsumption of molecular oxygen, which is accompanied by the formation of hydroperoxides of organic molecules in the complexes. Photoformation of two types of organic hydroperoxides were revealed: highly lipophilic (12 molecules per one LH2) and relatively hydrophobic (68 per one LH2). It has been shown that illumination leads to damage to light-harvesting complexes. On the one hand, photobleaching of bacteriochlorophyll and a decrease in its fluorescence intensity are observed. On the other hand, the photoinduced increase in the hydrodynamic radius of the complexes, the reduction in their thermal stability, and the change in fluorescence intensity indicate conformational changes occurring in the protein molecules of the LH2 preparations. Inhibition of the processes described above upon the addition of singlet oxygen quenchers (L-histidine, Trolox, sodium L-ascorbate) may support the hypothesis that carotenoids in LH2 preparations are capable of generating singlet oxygen, which, in turn, damage to protein molecules.

Chlorophylls: A Personal Snapshot

Chlorophylls provide the basis for photosynthesis and thereby most life on Earth. Besides their involvement in primary charge separation in the reaction center, they serve as light-harvesting and light-sensing pigments, they also have additional functions, e.g., in inter-system electron transfer. Chlorophylls also have a wealth of applications in basic science, medicine, as colorants and, possibly, in optoelectronics. Considering that there has been more than 200 years of chlorophyll research, one would think that all has been said on these pigments. However, the opposite is true: ongoing research evidenced in this Special Issue brings together current work on chlorophylls and on their carotenoid counterparts. These introductory notes give a very brief and in part personal account of the history of chlorophyll research and applications, before concluding with a snapshot of this year's publications.