Energy transfer and charge separation in the purple non-sulfur bacterium Roseospirillum parvum

The Thermal-Stable LH1-RC Complex of a Hot Spring Purple Bacterium Powers Photosynthesis with Extremely Low-Energy Near-Infrared Light

Blastochloris (Blc.) tepida is a hot spring purple nonsulfur phototrophic bacterium that contains bacteriochlorophyll (BChl) b. Here, we present a 2.21 Å cryo-EM structure of the thermostable light-harvesting 1-reaction center (LH1-RC) complex from Blc. tepida. The LH1 ring comprises 16 circularly arranged αβγ-subunits plus one αβ-subunit that surround the RC complex composed of C-, H-, L-, and M-subunits. In a comparative study, the Blc. tepida LH1-RC showed numerous electrostatic and hydrophobic interactions both within the LH1 complex itself and between the LH1 and the RC complexes that are absent from the LH1-RC complex of its mesophilic counterpart, Blc. viridis. These additional interactions result in a tightly packed LH1-RC architecture with a reduced accessible surface area per volume that enhances the thermal stability of the Blc. tepida complex and allows the light reactions of photosynthesis to proceed at hot spring temperatures. Moreover, based on high-resolution structural information combined with spectroscopic evidence, the unique photosynthetic property of the Blc. tepida LH1-RC─absorption of energy-poor near-infrared light beyond 1000 nm─can be attributed to strong hydrogen-bonding interactions between the C3-acetyl C═O of the LH1 BChl b and two LH1 α-Trp residues, structural rigidity of the LH1, and the enhanced exciton coupling of the LH1 BChls of this thermophile.

Insights into the divergence of the photosynthetic LH1 complex obtained from structural analysis of the unusual photocomplexes of Roseospirillum parvum

Purple phototrophic bacteria produce two kinds of light-harvesting complexes that function to capture and transmit solar energy: the core antenna (LH1) and the peripheral antenna (LH2). The apoproteins of these antennas, encoded respectively by the genes pufBA and pucBA within and outside the photosynthetic gene cluster, respectively, exhibit conserved amino acid sequences and structural topologies suggesting they were derived from a shared ancestor. Here we present the structures of two photosynthetic complexes from Roseospirillum (Rss.) parvum 930I: an LH1-RC complex and a variant of the LH1 complex also encoded by pufBA that we designate as LH1'. The LH1-RC complex forms a closed elliptical structure consisting of 16 pairs of αβ-polypeptides that surrounds the RC. By contrast, the LH1' complex is a closed ring structure composed of 14 pairs of αβ-polypeptides, and it shows significant similarities to LH2 complexes both spectrally and structurally. Although LH2-like, the LH1' complex is larger than any known LH2 complexes, and genomic analyses of Rss. parvum revealed the absence of pucBA, genes that encode classical LH2 complexes. Characterization of the unique Rss. parvum photocomplexes not only underscores the diversity of such structures but also sheds new light on the evolution of light-harvesting complexes from phototrophic bacteria.

Remembering Jan Amesz (1934-2001): a great gentleman, a major discoverer, and an internationally renowned biophysicist of both oxygenic and anoxygenic photosynthesisa

We present here the research contributions of Jan Amesz (1934-2001) on deciphering the details of the early physico-chemical steps in oxygenic photosynthesis in plants, algae and cyanobacteria, as well as in anoxygenic photosynthesis in purple, green, and heliobacteria. His research included light absorption and the mechanism of excitation energy transfer, primary photochemistry, and electron transfer steps until the reduction of pyridine nucleotides. Among his many discoveries, we emphasize his 1961 proof, with L. N. M. Duysens, of the "series scheme" of oxygenic photosynthesis, through antagonistic effects of Light I and II on the redox state of cytochrome f. Further, we highlight the following research on oxygenic photosynthesis: the experimental direct proof that plastoquinone and plastocyanin function at their respective places in the Z-scheme. In addition, Amesz's major contributions were in unraveling the mechanism of excitation energy transfer and electron transport steps in anoxygenic photosynthetic bacteria (purple, green and heliobacteria). Before we present his research, focusing on his key discoveries, we provide a glimpse of his personal life. We end this Tribute with reminiscences from three of his former doctoral students (Sigi Neerken; Hjalmar Pernentier, and Frank Kleinherenbrink) and from several scientists (Suleyman Allakhverdiev; Robert Blankenship; Richard Cogdell) including two of the authors (G. Garab and A. Stirbet) of this Tribute.

Energy Transfer and Exciton Relaxation in B880-B800-RC Complex through Two-Dimensional Electronic Spectroscopy

The light-harvesting (LH) and reaction center (RC) core complex of purple bacterium Roseiflexus castenholzii, B880-B800-RC, are different from those of the typical photosynthetic unit, (B850-B800)x-B880-RC. To investigate the excitation flowing dynamics in this unique complex, two-dimensional electronic spectroscopy is employed. The obtained time constants for the exciton relaxation in B880, exciton relaxation in B800, B800 → B880 energy transfer (EET), and B880 → closed RC EET are 43 fs, 177 fs, 1.9 ps, and 205 ps, respectively. These time constants result in an overall EET efficiency similar to that of the typical photosynthetic unit. Analysis of the oscillatory signals reveals that while several vibronic coherences are involved in the exciton relaxation process, only one prominent vibronic coherence, with a frequency of 27 cm-1 and coupled to the B880 electronic transition, may contribute to the B800 → B880 EET process.

A Dual Role for Ca2+ in Expanding the Spectral Diversity and Stability of Light-Harvesting 1 Reaction Center Photocomplexes of Purple Phototrophic Bacteria

The light-harvesting 1 reaction center (LH1-RC) complex in the purple sulfur bacterium Thiorhodovibrio ( Trv.) strain 970 cells exhibits its LH1 Q _y transition at 973 nm, the lowest-energy Q _y absorption among purple bacteria containing bacteriochlorophyll a (BChl a). Here we characterize the origin of this extremely red-shifted Q _y transition. Growth of Trv. strain 970 did not occur in cultures free of Ca²⁺, and elemental analysis of Ca²⁺-grown cells confirmed that purified Trv. strain 970 LH1-RC complexes contained Ca²⁺. The LH1 Q _y band of Trv. strain 970 was blue-shifted from 959 to 875 nm upon Ca²⁺ depletion, but the original spectral properties were restored upon Ca²⁺ reconstitution, which also occurs with the thermophilic purple bacterium Thermochromatium ( Tch.) tepidum. The amino acid sequences of the LH1 α- and β-polypeptides from Trv. strain 970 closely resemble those of Tch. tepidum; however, Ca²⁺ binding in the Trv. strain 970 LH1-RC occurred more selectively than in Tch. tepidum LH1-RC and with a reduced affinity. Ultraviolet resonance Raman analysis indicated that the number of hydrogen-bonding interactions between BChl a and LH1 proteins of Trv. strain 970 was significantly greater than for Tch. tepidum and that Ca²⁺ was indispensable for maintaining these bonds. Furthermore, perfusion-induced Fourier transform infrared analyses detected Ca²⁺-induced conformational changes in the binding site closely related to the unique spectral properties of Trv. strain 970. Collectively, our results reveal an ecological strategy employed by Trv. strain 970 of integrating Ca²⁺ into its LH1-RC complex to extend its light-harvesting capacity to regions of the near-infrared spectrum unused by other purple bacteria.

Energy transfer in purple bacterial photosynthetic units from cells grown in various light intensities

Three photosynthetic membranes, called intra-cytoplasmic membranes (ICMs), from wild-type and the ∆pucBA_abce mutant of the purple phototrophic bacterium Rps. palustris were investigated using optical spectroscopy. The ICMs contain identical light-harvesting complex 1-reaction centers (LH1-RC) but have various spectral forms of light-harvesting complex 2 (LH2). Spectroscopic studies involving steady-state absorption, fluorescence, and femtosecond time-resolved absorption at room temperature and at 77 K focused on inter-protein excitation energy transfer. The studies investigated how energy transfer is affected by altered spectral features of the LH2 complexes as those develop under growth at different light conditions. The study shows that LH1 → LH2 excitation energy transfer is strongly affected if the LH2 complex alters its spectroscopic signature. The LH1 → LH2 excitation energy transfer rate modeled with the Förster mechanism and kinetic simulations of transient absorption of the ICMs demonstrated that the transfer rate will be 2-3 times larger for ICMs accumulating LH2 complexes with the classical B800-850 spectral signature (grown in high light) compared to the ICMs from the same strain grown in low light. For the ICMs from the ∆pucBA_abce mutant, in which the B850 band of the LH2 complex is blue-shifted and almost degenerate with the B800 band, the LH1 → LH2 excitation energy transfer was not observed nor predicted by calculations.

Biochemical and Spectroscopic Characterizations of a Hybrid Light-Harvesting Reaction Center Core Complex

The light-harvesting 1 reaction center (LH1-RC) complex from Thermochromatium tepidum exhibits a largely red-shifted LH1 Q _y absorption at 915 nm due to binding of Ca²⁺, resulting in an "uphill" energy transfer from LH1 to the reaction center (RC). In a recent study, we developed a heterologous expression system (strain TS2) to construct a functional hybrid LH1-RC with LH1 from Tch. tepidum and the RC from Rhodobacter sphaeroides [Nagashima, K. V. P., et al. (2017) Proc. Natl. Acad. Sci. U. S. A. 114, 10906]. Here, we present detailed characterizations of the hybrid LH1-RC from strain TS2. Effects of metal cations on the phototrophic growth of strain TS2 revealed that Ca²⁺ is an indispensable element for its growth, which is also true for Tch. tepidum but not for Rba. sphaeroides. The thermal stability of the TS2 LH1-RC was strongly dependent on Ca²⁺ in a manner similar to that of the native Tch. tepidum, but interactions between the heterologous LH1 and RC became relatively weaker in strain TS2. A Fourier transform infrared analysis demonstrated that the Ca²⁺-binding site of TS2 LH1 was similar but not identical to that of Tch. tepidum. Steady-state and time-resolved fluorescence measurements revealed that the uphill energy transfer rate from LH1 to the RC was related to the energy gap in an order of Rba. sphaeroides, Tch. tepidum, and strain TS2; however, the quantum yields of LH1 fluorescence did not exhibit such a correlation. On the basis of these findings, we discuss the roles of Ca²⁺, interactions between LH1 and the RC from different species, and the uphill energy transfer mechanisms.

New Insights into the Mechanism of Uphill Excitation Energy Transfer from Core Antenna to Reaction Center in Purple Photosynthetic Bacteria

The uphill excitation energy transfer (EET) from the core antenna (LH1) to the reaction center (RC) of purple photosynthetic bacteria was investigated at room temperature by comparing the native LH1-RC from Thermochromatium ( Tch.) tepidum with the hybrid LH1-RC from a mutant strain of Rhodobacter ( Rba.) sphaeroides. The latter protein with chimeric Tch-LH1 and Rba-RC exhibits a substantially larger RC-to-LH1 energy difference (Δ E = 630 cm^-1) of 3-fold thermal energy (3 k_B T). The spectroscopic and kinetics results are discussed on the basis of the newly reported high-resolution structures of Tch. tepidum LH1-RC, which allow us to propose the existence of a passage formed by LH1 BChls that facilitates the LH1 → RC EET. The semilogarithmic plot of the EET rate against Δ E was found to be linear over a broad range of Δ E, which consolidates the mechanism of thermal activation as promoted by the spectral overlap between the LH1 fluorescence and the special pair absorption of RC.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

The origin of the unusual Qy red shift in LH1-RC complexes from purple bacteria Thermochromatium tepidum as revealed by Stark absorption spectroscopy

Native LH1-RC of photosynthetic purple bacteria Thermochromatium (Tch.) tepidum, B915, has an ultra-red BChl a Qy absorption. Two blue-shifted complexes obtained by chemical modification, B893 and B882, have increasing full widths at half maximum (FWHM) and decreasing transition dipole oscillator strength. 77K Stark absorption spectroscopy studies were employed for the three complexes, trying to understand the origin of the 915 nm absorption. We found that Tr(∆α) and |∆μ| of both Qy and carotenoid (Car) bands are larger than for other purple bacterial LH complexes reported previously. Moreover, the red shifts of the Qy bands are associated with (1) increasing Tr(∆α) and |∆μ| of the Qy band, (2) the red shift of the Car Stark signal and (3) the increasing |∆μ| of the Car band. Based on the results and the crystal structure, a combined effect of exciton-charge transfer (CT) states mixing, and inhomogeneous narrowing of the BChl a site energy is proposed to be the origin of the 915 nm absorption. CT-exciton state mixing has long been found to be the origin of strong Stark signal in LH1 and special pair, and the more extent of the mixing in Tch. tepidum LH1 is mainly the consequence of the shorter BChl-BChl distances. The less flexible protein structure results in a smaller site energy disorder (inhomogeneous narrowing), which was demonstrated to be able to influence |∆μ| and absorption.

The discovery of stromatolites developing at 3570 m above sea level in a high-altitude volcanic lake Socompa, Argentinean Andes

We describe stromatolites forming at an altitude of 3570 m at the shore of a volcanic lake Socompa, Argentinean Andes. The water at the site of stromatolites formation is alkaline, hypersaline, rich in inorganic nutrients, very rich in arsenic, and warm (20-24°C) due to a hydrothermal input. The stromatolites do not lithify, but form broad, rounded and low-domed bioherms dominated by diatom frustules and aragonite micro-crystals agglutinated by extracellular substances. In comparison to other modern stromatolites, they harbour an atypical microbial community characterized by highly abundant representatives of Deinococcus-Thermus, Rhodobacteraceae, Desulfobacterales and Spirochaetes. Additionally, a high proportion of the sequences that could not be classified at phylum level showed less than 80% identity to the best hit in the NCBI database, suggesting the presence of novel distant lineages. The primary production in the stromatolites is generally high and likely dominated by Microcoleus sp. Through negative phototaxis, the location of these cyanobacteria in the stromatolites is controlled by UV light, which greatly influences their photosynthetic activity. Diatoms, dominated by Amphora sp., are abundant in the anoxic, sulfidic and essentially dark parts of the stromatolites. Although their origin in the stromatolites is unclear, they are possibly an important source of anaerobically degraded organic matter that induces in situ aragonite precipitation. To the best of our knowledge, this is so far the highest altitude with documented actively forming stromatolites. Their generally rich, diverse and to a large extent novel microbial community likely harbours valuable genetic and proteomic reserves, and thus deserves active protection. Furthermore, since the stromatolites flourish in an environment characterized by a multitude of extremes, including high exposure to UV radiation, they can be an excellent model system for studying microbial adaptations under conditions that, at least in part, resemble those during the early phase of life evolution on Earth.

Excitation energy transfer and trapping dynamics in the core complex of the filamentous photosynthetic bacterium Roseiflexus castenholzii

The light-harvesting core complex of the thermophilic filamentous anoxygenic phototrophic bacterium Roseiflexus castenholzii is intrinsic to the cytoplasmic membrane and intimately bound to the reaction center (RC). Using ultrafast transient absorption and time-resolved fluorescence spectroscopy with selective excitation, energy transfer, and trapping dynamics in the core complex have been investigated at room temperature in both open and closed RCs. Results presented in this report revealed that the excited energy transfer from the BChl 800 to the BChl 880 band of the antenna takes about 2 ps independent of the trapping by the RC. The time constants for excitation quenching in the core antenna BChl 880 by open and closed RCs were found to be 60 and 210 ps, respectively. Assuming that the light harvesting complex is generally similar to LH1 of purple bacteria, the possible structural and functional aspects of this unique antenna complex are discussed. The results show that the core complex of Roseiflexus castenholzii contains characteristics of both purple bacteria and Chloroflexus aurantiacus.

A hypersaline microbial mat from the Pacific Atoll Kiritimati: insights into composition and carbon fixation using biomarker analyses and a 13C-labeling approach

Modern microbial mats are widely recognized as useful analogs for the study of biogeochemical processes relevant to paleoenvironmental reconstruction in the Precambrian. We combined microscopic observations and investigations of biomarker composition to investigate community structure and function in the upper layers of a thick phototrophic microbial mat system from a hypersaline lake on Kiritimati (Christmas Island) in the Northern Line Islands, Republic of Kiribati. In particular, an exploratory incubation experiment with (13)C-labeled bicarbonate was conducted to pinpoint biomarkers from organisms actively fixing carbon. A high relative abundance of the cyanobacterial taxa Aphanocapsa and Aphanothece was revealed by microscopic observation, and cyanobacterial fatty acids and hydrocarbons showed (13)C-uptake in the labeling experiment. Microscopic observations also revealed purple sulfur bacteria (PSB) in the deeper layers. A cyclic C(19:0) fatty acid and farnesol were attributed to this group that was also actively fixing carbon. Background isotopic values indicate Calvin-Benson cycle-based autotrophy for cycC(19:0) and farnesol-producing PSBs. Biomarkers from sulfate-reducing bacteria (SRB) in the top layer of the mat and their (13)C-uptake patterns indicated a close coupling between SRBs and cyanobacteria. Archaeol, possibly from methanogens, was detected in all layers and was especially abundant near the surface where it contained substantial amounts of (13)C-label. Intact glycosidic tetraether lipids detected in the deepest layer indicated other archaea. Large amounts of ornithine and betaine bearing intact polar lipids could be an indicator of a phosphate-limited ecosystem, where organisms that are able to substitute these for phospholipids may have a competitive advantage.

Excitation dynamics of two spectral forms of the core complexes from photosynthetic bacterium Thermochromatium tepidum

The intact core antenna-reaction center (LH1-RC) core complex of thermophilic photosynthetic bacterium Thermochromatium (Tch.) tepidum is peculiar in its long-wavelength LH1-Q(y) absorption (915 nm). We have attempted comparative studies on the excitation dynamics of bacteriochlorophyll (BChl) and carotenoid (Car) between the intact core complex and the EDTA-treated one with the Q(y) absorption at 889 nm. For both spectral forms, the overall Car-to-BChl excitation energy transfer efficiency is determined to be approximately 20%, which is considerably lower than the reported values, e.g., approximately 35%, for other photosynthetic purple bacteria containing the same kind of Car (spirilloxanthin). The RC trapping time constants are found to be 50 approximately 60 ps (170 approximately 200 ps) for RC in open (closed) state irrespective to the spectral forms and the wavelengths of Q(y) excitation. Despite the low-energy LH1-Q(y) absorption, the RC trapping time are comparable to those reported for other photosynthetic bacteria with normal LH1-Q(y) absorption at 880 nm. Selective excitation to Car results in distinct differences in the Q(y)-bleaching dynamics between the two different spectral forms. This, together with the Car band-shift signals in response to Q(y) excitation, reveals the presence of two major groups of BChls in the LH1 of Tch. tepidum with a spectral heterogeneity of approximately 240 cm(-1), as well as an alteration in BChl-Car geometry in the 889-nm preparation with respect to the native one.

Unexpected diversity and complexity of the Guerrero Negro hypersaline microbial mat

We applied nucleic acid-based molecular methods, combined with estimates of biomass (ATP), pigments, and microelectrode measurements of chemical gradients, to map microbial diversity vertically on a millimeter scale in a hypersaline microbial mat from Guerrero Negro, Baja California Sur, Mexico. To identify the constituents of the mat, small-subunit rRNA genes were amplified by PCR from community genomic DNA extracted from layers, cloned, and sequenced. Bacteria dominated the mat and displayed unexpected and unprecedented diversity. The majority (1,336) of the 1,586 bacterial 16S rRNA sequences generated were unique, representing 752 species (> or =97% rRNA sequence identity) in 42 of the main bacterial phyla, including 15 novel candidate phyla. The diversity of the mat samples differentiated according to the chemical milieu defined by concentrations of O(2) and H(2)S. Bacteria of the phylum Chloroflexi formed the majority of the biomass by percentage of bulk rRNA and of clones in rRNA gene libraries. This result contradicts the general belief that cyanobacteria dominate these communities. Although cyanobacteria constituted a large fraction of the biomass in the upper few millimeters (>80% of the total rRNA and photosynthetic pigments), Chloroflexi sequences were conspicuous throughout the mat. Filamentous Chloroflexi bacteria were identified by fluorescence in situ hybridization within the polysaccharide sheaths of the prominent cyanobacterium Microcoleus chthonoplastes, in addition to free living in the mat. The biological complexity of the mat far exceeds that observed in other polysaccharide-rich microbial ecosystems, such as the human and mouse distal guts, and suggests that positive feedbacks exist between chemical complexity and biological diversity. The sequences determined in this study have been submitted to the GenBank database and assigned accession numbers DQ 329539 to DQ 331020, and DQ 397339 to DQ 397511.

Rhodospirillum rubrum possesses a variant of the bchP gene, encoding geranylgeranyl-bacteriopheophytin reductase

The bchP gene product of Rhodobacter sphaeroides is responsible for the reduction of the isoprenoid moiety of bacteriochlorophyll (Bchl) from geranylgeraniol (GG) to phytol; here, we show that this enzyme also catalyzes the reduction of the isoprenoid moiety of bacteriopheophytin (Bphe). In contrast, we demonstrate that a newly identified homolog of this gene in Rhodospirillum rubrum encodes an enzyme, GG-Bphe reductase, capable of reducing the isoprenoid moiety of Bphe only. We propose that Rhodospirillum rubrum is a naturally occurring bchP mutant and that an insertion mutation may have been the initial cause of a partial loss of function. Normal BchP function can be restored to Rhodospirillum rubrum, creating a new transconjugant strain possessing Bchl esterified with phytol. We speculate on the requirement of Rhodospirillum rubrum for phytylated Bphe and on a potential link between the absence of LH2 and of phytylated Bchl from the wild-type bacterium. The identification of a second role for the fully functional BchP in catalyzing the synthesis of phytylated Bphe strongly suggests that homologs of this enzyme may be similarly responsible for the synthesis of phytylated pheophytin in organisms possessing photosystem 2. In addition to bchP, other members of a photosynthesis gene cluster were identified in Rhodospirillum rubrum, including a bchG gene, demonstrated to encode a functional Bchl synthetase by complementation of a Rhodobacter sphaeroides mutant.

Absence of carotenes and presence of a tertiary methoxy group in a carotenoid from a thermophilic filamentous photosynthetic bacterium Roseiflexus castenholzii

We identified pigments in a thermophilic filamentous photosynthetic bacterium Roseiflexus castenholzii strain HL08. We detected neither bacteriochlorophyll (BChl) c nor carotenes in this bacterium cultured under the aerobic dark and the anaerobic light conditions, which may correspond to its lack of chlorosomes. In the cells cultured under the aerobic dark conditions, the carotenoids were derivatives of keto-gamma-carotene, and the major ones were methoxy-keto-myxocoxanthin and keto-myxocoxanthin glucoside fatty acid ester. Although the tertiary methoxy group at C-1' and the double bond at C-3',4' in the psi end group of carotenoid, such as spirilloxanthin, have only been found in purple bacteria, this was the first such report in other bacterial groups. The fatty acid moiety was composed of iso fatty acids, which were rare in the cellular lipids. In the cells cultured under the anaerobic light conditions, in addition to these keto-carotenoids, we also found non-oxidized carotenoids (derivatives of gamma-carotene). Concerning the esterifying alcohol of BChl a, we found a substantial amount of geranylgeraniol, although the major component was phytol. The existence of these pigments makes this bacterium unique among the known species in CHLOROFLEXACEAE.