Photosynthesis Research最新文献

The initial electron transfer (ET) processes in reaction centers (RCs) of Chloroflexus (Cfl.) aurantiacus were studied at 295 K using femtosecond transient absorption (TA) difference spectroscopy. Particular attention was paid to the decay kinetics of the primary electron donor excited state (P^*) and the formation/decay of the absorption band of the monomeric bacteriochlorophyll a anion (B_A^-) at ~ 1035 nm, which reflects the dynamics of the charge-separated state P⁺B_A^-. It was found that in Q_A-depleted RCs containing native bacteriopheophytin a (BPheo) molecules at the H_A and H_B binding sites, the decay of P^* to form the P⁺H_A^- state contains a fast (4 ps; relative amplitude 70%) and a slow (13 ps; relative amplitude 30%) kinetic components. The B_A^- absorption band at ~ 1035 nm was detected only for the fast component. Based on global analysis of the TA data, the results are discussed in terms of the presence of two P^* populations: in one, P^* decays in 4 ps via a dominant two-step activationless P^* → P⁺B_A^- → P⁺H_A^- ET with a contribution of 70% to the overall primary charge separation process, and in the other, P^* decays in 13 ps via a one-step superexchange P^* → P⁺H_A^- ET (contribution of 30%). Similar femtosecond TA measurements on Q_A-depleted-Pheo_A-modified RCs, in which the charge separation energetics was changed by replacing BPheo H_A with plant pheophytin a, suggest the presence of a P^* population where P⁺H_A^- formation can occur via a thermally activated two-step ET process.

Excitation energy transfer between the photochemically active protein complexes is key for photosynthetic processes. Phototrophic organisms like cyanobacteria experience subtle changes in irradiance under natural conditions. Such changes need adjustments to the excitation energy transfer between the photosystems for sustainable growth. Spectroscopic assessments on purified photosystems usually fail to capture these subtle changes. In this study, we examined whole cells from two model cyanobacteria, Synechocystis sp. PCC 6803 and Synechococcus elongatus UTEX 2973, grown under high and low light conditions to decode the high light tolerance of the latter. This allowed us to study photosynthetic machinery in the native state and in this work we particularly focused on the excitation energy transfer within PSII and PSI manifold. Understanding the high-light tolerance mechanism is imperative as it can help design strategies for increasing the light tolerance of cyanobacteria used for carbon neutral bioproduction. Our observations suggest that Synechococcus 2973 employs an uncommon photoprotection strategy, and the absence of hydroxy-echinenone pigment in this strain opens the possibility of an orange carotenoid protein homolog utilizing zeaxanthin as a scavenger of reactive oxygen species to provide photoprotection. Furthermore, the adjustments to the high-light adaptation mechanism involve downregulating the phycobilisome antenna in Synechococcus 2973, but not in Synechocystis 6803. Additionally, the stoichiometric changes to PSII/PSI are more tightly regulated in Synechococcus 2973.

The femtosecond dynamics of energy transfer from light-excited spirilloxanthin (Spx) to bacteriochlorophyll (BChl) a in the reaction centers (RCs) of purple photosynthetic bacteria Rhodospirillum rubrum was studied. According to crio-electron microscopy data, Spx is located near accessory BChl a in the B-branch of cofactors. Spx was excited by 25 fs laser pulses at 490 nm, and difference absorption spectra were recorded in the range 500-700 nm. To reveal the dynamics of individual states, we applied global analysis using different kinetic schemes. We found that the energy transfer Spx → BChl a occurs during 0.22 ps with a low efficiency of ~ 31%. The monomeric BChl a acts as the primary energy acceptor, presumably in the B-branch of cofactors. Then the energy is transferred to the BChl a dimer within 0.25 ps and subsequently used for charge separation. As a result of internal conversion in Spx, the majority (~ 69%) of the excitation energy transfers in 0.2 ps from the singlet-excited state S₂ to the states S₁ and S*, which, in turn, relax to the ground state in 1.5 and 9 ps, ​​respectively. We showed that the S₁ and S* states in Spx are not involved in energy transfer to BChl a. The found parameters of energy transfer Spx→BChl a turned out to be close to those in the light-harvesting complexes LH1 of Rhodospirillum rubrum. The sequence of events in Spx after its excitation is discussed.

Red algae are photosynthetic eukaryotes whose light-harvesting complexes (LHCs) associate with photosystem I (PSI). In this study, we examined characteristics of PSI-LHCI, PSI, and LHCI isolated from the red alga Galdieria sulphuraria NIES-3638. The PSI-LHCI supercomplexes were purified using anion-exchange chromatography followed by hydrophobic-interaction chromatography, and finally by trehalose density gradient centrifugation. PSI and LHCI were similarly prepared following the dissociation of PSI-LHCI with Anzergent 3-16. Polypeptide analysis of PSI-LHCI revealed the presence of PSI and LHC proteins, along with red-lineage chlorophyll a/b-binding-like protein (RedCAP), which is distinct from LHC proteins within the LHC protein superfamily. RedCAP, rather than LHC proteins, exhibited tight binding to PSI. Carotenoid analysis of LHCI identified zeaxanthin, β-cryptoxanthin, and β-carotene, with zeaxanthin particularly enriched, which is consistent with other red algal LHCIs. A Qy peak of chlorophyll a in the LHCI absorption spectrum was blue-shifted compared with those of PSI-LHCI and PSI, and a fluorescence emission peak was similarly shifted to shorter wavelengths. Based on these results, we discuss the diversity of LHC proteins and RedCAP in red algal PSI-LHCI supercomplexes.

Maize (Zea mays L.) performs highly efficient C₄ photosynthesis by dividing photosynthetic metabolism between mesophyll and bundle sheath cells. In vivo physiological measurements are indispensable for C₄ photosynthesis research as photosynthetic activities are easily interrupted by leaf section or cell isolation. Yet, direct in vivo observation regarding bundle sheath cells in the delicate anatomy of the C₄ leaf is still challenging. In the current work, we used two-photon fluorescence-lifetime imaging microscopy (two-photon-FLIM) to access the photosynthetic properties of bundle sheath cells on intact maize leaves. The results provide spectroscopic evidence for the diminished total PSII activity in bundle sheath cells at its physiological level and show that the single PSIIs could undergo charge separation as usual. We also report an acetic acid-induced chlorophyll fluorescence quenching on intact maize leaves, which might be a physiological state related to the nonphotochemical quenching mechanism.

The Orange Carotenoid Protein (OCP) is a unique water-soluble photoactive protein that plays a critical role in regulating the balance between light harvesting and photoprotective responses in cyanobacteria. The challenge in understanding OCP´s photoactivation mechanism stems from the heterogeneity of the initial configurations of its embedded ketocarotenoid, which in the dark-adapted state can form up to two hydrogen bonds to critical amino acids in the protein's C-terminal domain, and the extremely low quantum yield of primary photoproduct formation. While a series of experiments involving point mutations within these contacts helped us to identify these challenges, they did not resolve them. To overcome this, we shifted from classical mutagenesis to the translational introduction of non-canonical amino acid residues into the OCP structure. In this work, we demonstrate that replacing a single meta-hydrogen in tyrosine-201 with a halogen atom (chlorine, bromine, or iodine) leads to targeted modifications in the keto-carotenoid-protein matrix interaction network, both in the dark-adapted state and upon photoactivation. We found that such atomic substitutions allow us to effectively weaken key hydrogen bonds without disrupting protein folding, thereby increasing the yield of OCP photoactivation products. Such genetically encoded chemical modification of individual atoms and their systematic in situ variation in complex protein structures establishes a foundation for transforming OCP into a practical tool for optogenetics and other applications.

The perennially ice-covered Lake Bonney in Antarctica has been deemed a natural laboratory for studying life at the extreme. Photosynthetic algae dominate the lake food webs and are adapted to a multitude of extreme conditions including perpetual shading even at the height of the austral summer. Here we examine how the unique light environment in Lake Bonney influences the physiology of two Chlamydomonas species. Chlamydomonas priscui is found exclusively in the deep photic zone where it receives very low light levels biased in the blue part of the spectrum (400-500 nm). In contrast, Chlamydomonas sp. ICE-MDV is represented at various depths within the water column (including the bright surface waters), and it receives a broad range of light levels and spectral wavelengths. The psychrophilic character of both species makes them an ideal system to study the effects of light quality and quantity on chlorophyll biosynthesis and photosynthetic performance in extreme conditions. We show that the shade-adapted C. priscui exhibits a decreased ability to accumulate chlorophyll and severe photoinhibition when grown under red light compared to blue light. These effects are particularly pronounced under red light of higher intensity, suggesting a loss of capability to acclimate to varied light conditions. In contrast, ICE-MDV has retained the ability to synthesize chlorophyll and maintain photosynthetic efficiency under a broader range of light conditions. Our findings provide insights into the mechanisms of photosynthesis under extreme conditions and have implications on algal survival in changing conditions of Antarctic ice-covered lakes.

Pheophytin-a derivatives possessing plastoquinone and phylloquinone analogs in the peripheral 3-substituent were prepared by Friedel-Crafts reactions of a 3-hydroxymethyl-chlorin as one of the chlorophyll-a derivatives with benzo- and naphthohydroquinones, respectively, and successive oxidation of the 1,4-dihydroxy-aryl groups in the resulting dehydration products. The 3-quinonylmethyl-chlorins exhibited ultraviolet-visible absorption and circular dichroism spectra in acetonitrile, which were composed of those of the starting 3-hydroxymethyl-chlorin and the corresponding methylated benzo- and naphthoquinones. No intramolecular interaction between the chlorin and quinone π-systems was observed in the solution owing to the methylene spacer. The first reduction potentials of the quinone moieties in the synthetic conjugates were determined by cyclic voltammetry and shifted positively from those of the reference quinones. The former quinonyl groups were reduced more readily by approximately 0.1 V than the latter quinones, which was ascribable to the stabilization of the quinonyl anion radical by the nearby macrocyclic chlorin π-chromophore. This observation implied that the reduction potentials of quinones were regulated by the close pheophytin-a derivative by through-space interaction. Considering the charge shift from pheophytin-a anion radical to plastoquinone and phylloquinone in reaction centers of photosystems II and I, respectively, the reduction potentials of these quinones as a determinant factor of the rapid electron transfer process would be dependent on the pheophytin-a in the photosynthetic reaction centers of oxygenic phototrophs as well as on the neighboring peptides.

Seed priming and plant growth-promoting bacteria (PGPB) may alleviate salt stress effects. We exposed a salt-sensitive variety of melon to salinity following seed priming with NaCl and inoculation with Bacillus. Given the sensitivity of photosystem II (PSII) to salt stress, we utilized dark- and light-adapted chlorophyll fluorescence alongside analysis of leaf stomatal conductance of water vapour (G_sw). Priming increased total seed germination by 15.5% under salt-stress. NaCl priming with Bacillus inoculation (PB) increased total leaf area (LA) by 45% under control and 15% under stress. Under the control condition, priming (P) reduced membrane permeability (RMP) by 36% and PB by 55%, while under stress Bacillus (BS) reduced RMP by 10%. Although Bacillus inoculation (B) and priming (P) treatments did not show significant effects on some PSII efficiency parameters (F_V/F_M, ABS/RC, PI_ABS, F_M), the BS treatment induced a significantly higher quantum efficiency of PSII (ΦPSII) and increased G_sw by 159% in the final week of the experiment. The BS treatment reduced electron transport rate per reaction center (ET_O/RC) by 10% in comparison to the salt treatment, which showed less reaction centre damage. Bacillus inoculation and seed priming treatment under the stressed condition (PBS) induced an increase in electron transport rate of 40%. Salt stress started to show significant effects on PSII after 12 days, and adversely impacted all morphological and photosynthetic parameters after 22 days. Salt priming and PGPB mitigated the negative impacts of salt stress and may serve as effective tools in future-proofing saline agriculture.