Chlorophylls (Chl) play pivotal roles in energy capture, transfer and charge separation in photosynthesis. Among Chls functioning in oxygenic photosynthesis, Chl f is the most red-shifted type first ...found in a cyanobacterium Halomicronema hongdechloris. The location and function of Chl f in photosystems are not clear. Here we analyzed the high-resolution structures of photosystem I (PSI) core from H. hongdechloris grown under white or far-red light by cryo-electron microscopy. The structure showed that, far-red PSI binds 83 Chl a and 7 Chl f, and Chl f are associated at the periphery of PSI but not in the electron transfer chain. The appearance of Chl f is well correlated with the expression of PSI genes induced under far-red light. These results indicate that Chl f functions to harvest the far-red light and enhance uphill energy transfer, and changes in the gene sequences are essential for the binding of Chl f.
Hydrogen (H2) is a renewable, abundant, and nonpolluting source of energy. Photosynthetic organisms capture sunlight very efficiently and convert it into organic molecules. Cyanobacteria produce H2 ...by breaking down organic compounds and water. In this study, biological H2 was produced from various strains of cyanobacteria. Moreover, H2 accumulation by Synechocystis sp. PCC 6803 was as high as 0.037 μmol/mg Chl/h within 120 h in the dark. The wild-type, filamentous, non-heterocystous cyanobacterium Desertifilum sp. IPPAS B-1220 was found to produce a maximum of 0.229 μmol/mg Chl/h in the gas phase within 166 h in the light, which was on par with the maximum yield reported in the literature. DCMU at 10 μM increased H2 production by Desertifilum sp. IPPAS B-1220 by 1.5-fold to 0.348 μmol H2/mg Chl/h. This is the first report on the capability of Desertifilum cyanobacterium to produce H2.
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•Four non-heterocystous cyanobacterial strains were investigated for H2 production.•Best H2 accumulation was observed with Synechocystis sp. PCC 6803 in the dark.•Photoautotrophic H2 production yield of Desertifilum sp. IPPAS B-1220 was 0.229 μmol/mg Chl/h.•H2 yield was increased with the Desertifilum sp. IPPAS B-1220 by 1.5-fold in the presence of DCMU.
Although the photosynthetic reaction center is well conserved among different cyanobacterial species, the modes of metabolism, e.g. respiratory, nitrogen and carbon metabolism and their mutual ...interaction, are quite diverse. To explore such uniformity and diversity among cyanobacteria, here we compare the influence of the light environment on the condition of photosynthetic electron transport through Chl fluorescence measurement of six cyanobacterial species grown under the same photon flux densities and at the same temperature. In the dark or under weak light, up to growth light, a large difference in the plastoquinone (PQ) redox condition was observed among different cyanobacterial species. The observed difference indicates that the degree of interaction between respiratory electron transfer and photosynthetic electron transfer differs among different cyanobacterial species. The variation could not be ascribed to the phylogenetic differences but possibly to the light environment of the original habitat. On the other hand, changes in the redox condition of PQ were essentially identical among different species at photon flux densities higher than the growth light. We further analyzed the response to high light by using a typical energy allocation model and found that 'non-regulated' thermal dissipation was increased under high-light conditions in all cyanobacterial species tested. We assume that such 'non-regulated' thermal dissipation may be an important 'regulatory' mechanism in the acclimation of cyanobacterial cells to high-light conditions.
The fucoxanthin chlorophyll (Chl) a/c-binding protein (FCP) is responsible for excellent light-harvesting strategies that enable survival in fluctuating light conditions. Here, we report the ...light-harvesting and quenching states of two FCP complexes, FCP-A and FCP-B/C, isolated from the diatom Chaetoceros gracilis. Pigment analysis revealed that FCP-A is enriched in Chl c, whereas FCP-B/C is enriched in diadinoxanthin, reflecting differences in low-temperature steady-state absorption and fluorescence spectra of each FCP complex. Time-resolved fluorescence spectra were measured at 77 K, and the characteristic lifetimes were determined using global fitting analysis of the spectra. Tens of picosecond (ps) components revealed energy transfer to low-energy Chl a from Chls a and c, whereas the other components showed only fluorescence decay components with no concomitant rise components. The normalized amplitudes of hundreds of picosecond components were relatively 30% in the total fluorescence, whereas those of longest-lived components were 60%. The hundreds of picosecond components were assigned as excitation energy quenching, whereas the longest-lived components were assigned as fluorescence from the final energy traps. These results suggest that 30% of FCP complex forming quenching state and the other 60% of FCP complex forming light-harvesting state exist heterogeneously in each FCP fraction under continuous low-light condition.
The interactions of chlorophyll (Chl) and bacteriochlorophyll (BChl) pigments with the polypeptides in photosynthetic light‐harvesting proteins are responsible for controlling the absorption energy ...of (B)Chls in protein matrixes. The binding pocket of B800 BChl a in LH2 proteins, which are peripheral light‐harvesting proteins in purple photosynthetic bacteria, is useful for studying such structure–property relationships. We report the reconstitution of Chl f, which has the formyl group at the 2‐position, in the B800 cavity of LH2 from the purple bacterium Rhodoblastus acidophilus. The Qy absorption band of Chl f in the B800 cavity was shifted by 14 nm to longer wavelength compared to that of the corresponding five‐coordinated monomer in acetone. This redshift was larger than that of Chl a and Chl b. Resonance Raman spectroscopy indicated hydrogen bonding between the 2‐formyl group of Chl f and the LH2 polypeptide. These results suggest that this hydrogen bonding contributes to the Qy redshift of Chl f. Furthermore, the Qy redshift of Chl f in the B800 cavity was smaller than that of Chl d. This may have arisen from the different patterns of hydrogen bonding between Chl f and Chl d and/or from the steric hindrance of the 3‐vinyl group in Chl f.
Chlorophyll (Chl) f, which has the formyl group at the 2‐position of the cyclic tetrapyrrole, was reconstituted into the binding site of B800 bacteriochlorophyll a in LH2 protein from the purple photosynthetic bacterium Rhodoblastus acidophilus. The 2‐formyl group of Chl f was hydrogen‐bonded with the LH2 polypeptide. This hydrogen bonding is suggested to contribute to the redshift of the Qy band of Chl f by insertion into the B800 cavity.
Fucoxanthin chlorophyll (Chl) a/c-binding protein (FCP) is a unique light-harvesting antenna in diatoms, which are photosynthesizing algae ubiquitous in aquatic environments. However, it is unknown ...how excitation energy is trapped and quenched in a complex consisting of photosystem II and FCP (PSII–FCPII complex). Here, we report the control mechanism of excitation energy transfer in the PSII–FCPII complexes isolated from a diatom, Chaetoceros gracilis, as revealed by picosecond time-resolved fluorescence spectroscopy. The results showed that Chl-excitation energy is harvested in low-energy Chls near/within FCPII under the 77 K conditions, whereas most of the energy is trapped in reaction center Chls in PSII under the 283 K conditions. Surprisingly, excitation energy quenching was observed in a part of PSII–FCPII complexes with the time constants of hundreds of picosecond, thus indicating the large contribution of FCPII to energy trapping and quenching. On the basis of these results, we discuss the light-harvesting strategy of diatoms.
Extrinsic proteins of photosystem II (PSII) play an important role in optimizing oxygen-evolving reactions in all oxyphototrophs. The currently available crystal structures of cyanobacterial PSII ...core complexes show the binding structures of the extrinsic proteins, PsbO, PsbV, and PsbU; however, how the individual extrinsic proteins affect the structure and the function of the oxygen-evolving center (OEC) in cyanobacterial PSII remains unknown. In this study, we have investigated the effects of the binding of the extrinsic proteins on the protein conformation of the OEC in PSII core complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus, using light-induced Fourier transform infrared (FTIR) difference spectroscopy. Upon removal of the three extrinsic proteins, an S2-minus-S1 FTIR difference spectrum measured in the presence of a high CaCl2 concentration showed a drastic change in amide I bands, reflecting perturbation of the secondary structures of polypeptides, whereas the overall spectral intensity was lost at a low CaCl2 concentration, indicative of inactivation of the Mn4CaO5 cluster. The amide I features as well as the overall intensity were recovered mainly by binding of PsbO, while complete amide I recovery was achieved by further binding of PsbV and PsbU. We thus concluded that PsbO, together with smaller contributions of PsbV and PsbU, plays a role in the maintenance of the proper protein conformation of the OEC in cyanobacterial PSII, which provides the stability of the Mn4CaO5 cluster via the enhanced retention capability of Ca2+ and Cl– ions.
Photosynthesis is one of the most vital process in nature, which consists of two main photoreaction centers called photosystem I and II. The high quantum yield of photosystem I (PSI) makes it ...attractive for bioelectronic applications. However, the native PSI (N-PSI) loses its robust photochemical properties once fabricated into devices. This property degradation results from the difficulty of orientation control of PSI. With the optimal orientation of PSI, photoexcited electrons can easily reach the electrode, yielding good photoelectric conversion efficiency. We developed a novel photoelectrode by integrating a newly designed gene-recombined PSI (G-PSI) with platinum nanoparticles (PtNPs) on substrates using a simple stacking method, which can control the orientation of PSI on the electrode. The target orientation of the attached G-PSI toward the substrate was confirmed by absorption spectra of polarized light. An approximately two-fold increase in the internal quantum yield (IQY) was observed for the G-PSI-attached electrode under 680 nm irradiation compared with that of the N-PSI modified electrode. In addition, a four-fold enhancement of the IQY was detected for cytochrome c (Cyt c) stacking on the G-PSI owing to the electrostatic interaction, suggesting that Cyt c successfully secured the electron transfer pathway.
In algae, light-harvesting complexes contain specific chlorophylls (Chls) and keto-carotenoids; Chl a, Chl c, and fucoxanthin (Fx) in diatoms and brown algae; Chl a, Chl c, and peridinin in ...photosynthetic dinoflagellates; and Chl a, Chl b, and siphonaxanthin in green algae. The Fx-Chl a/c-protein (FCP) complex from the diatom Chaetoceros gracilis contains Chl c1, Chl c2, and the keto-carotenoid, Fx, as antenna pigments, in addition to Chl a. In the present study, we investigated energy transfer in the FCP complex associated with photosystem II (FCPII) of C. gracilis. For these investigations, we analyzed time-resolved fluorescence spectra, fluorescence rise and decay curves, and time-resolved fluorescence anisotropy data. Chl a exhibited different energy forms with fluorescence peaks ranging from 677 nm to 688 nm. Fx transferred excitation energy to lower-energy Chl a with a time constant of 300fs. Chl c transferred excitation energy to Chl a with time constants of 500-600fs (intra-complex transfer), 600-700fs (intra-complex transfer), and 4-6ps (inter-complex transfer). The latter process made a greater contribution to total Chl c-to-Chl a transfer in intact cells of C. gracilis than in the isolated FCPII complexes. The lower-energy Chl a received excitation energy from Fx and transferred the energy to higher-energy Chl a. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.
The phototrophic cyanobacterium
Halomicronema hongdechloris
shows far-red light-induced accumulation of chlorophyll (Chl)
f
, but the involvement of the pigment in photosynthetic energy harvesting by ...photosystem (PS) II is controversially discussed. While
H. hongdechloris
contains negligible amounts of Chl
f
in white-light culture conditions, the ratio of Chl
f
to Chl
a
is reversibly changed up to 1:8 under illumination with far-red light (720–730 nm). We performed UV–Vis absorption spectroscopy, time-integrated and time-resolved fluorescence spectroscopy for the calculation of decay-associated spectra (DAS) to determine excitation energy transfer (EET) processes between photosynthetic pigments in intact
H. hongdechloris
filaments. In cells grown under white light, highly efficient EET occurs from phycobilisomes (PBSs) to Chl
a
with an apparent time constant of about 100 ps. Charge separation occurs with a typical apparent time constant of 200–300 ps from Chl
a
. After 3–4 days of growth under far-red light, robust Chl
f
content was observed in
H. hongdechloris
and EET from PBSs reached Chl
f
efficiently within 200 ps. It is proposed based on mathematical modeling by rate equation systems for EET between the PBSs and PSII and subsequent electron transfer (ET) that charge separation occurs from Chl
a
and excitation energy is funneled from Chl
f
to Chl
a
via an energetically uphill EET driven by entropy, which is effective because the number of Chl
a
molecules coupled to Chl
f
is at least eight- to tenfold larger than the corresponding number of Chl
f
molecules. The long lifetime of Chl
f
molecules in contact to a tenfold larger pool of Chl
a
molecules allows Chl
f
to act as an intermediate energy storage level, from which the Gibbs free energy difference between Chl
f
and Chl
a
can be overcome by taking advantage from the favorable ratio of degeneracy coefficients, which formally represents a significant entropy gain in the Eyring formulation of the Arrhenius law. Direct evidence for energetically uphill EET and charge separation in PSII upon excitation of Chl
f
via anti-Stokes fluorescence in far-red light-adapted
H. hongdechloris
cells was obtained: Excitation by 720 nm laser light resulted in robust Chl
a
fluorescence at 680 nm that was distinctly temperature-dependent and, notably, increased upon DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) treatment in far-red light-adapted cells. Thus, rather than serving as an excitation energy trap, Chl
f
in far-red light-adapted
H. hongdechloris
cells is directly contributing to oxygenic photosynthesis at PSII.
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