High-resolution structure and biochemical properties of the LH1-RC photocomplex from the model purple sulfur bacterium, Allochromatium vinosum.

The mesophilic purple sulfur phototrophic bacterium Allochromatium (Alc.) vinosum (bacterial family Chromatiaceae) has been a favored model for studies of bacterial photosynthesis and sulfur metabolism, and its core light-harvesting (LH1) complex has been a focus of numerous studies of photosynthetic light reactions. However, despite intense efforts, no high-resolution structure and thorough biochemical analysis of the Alc. vinosum LH1 complex have been reported. Here we present cryo-EM structures of the Alc. vinosum LH1 complex associated with reaction center (RC) at 2.24 Å resolution. The overall structure of the Alc. vinosum LH1 resembles that of its moderately thermophilic relative Alc. tepidum in that it contains multiple pigment-binding α- and ß-polypeptides. Unexpectedly, however, six Ca ions were identified in the Alc. vinosum LH1 bound to certain α1/ß1- or α1/ß3-polypeptides through a different Ca2+-binding motif from that seen in Alc. tepidum and other Chromatiaceae that contain Ca2+-bound LH1 complexes. Two water molecules were identified as additional Ca2+-coordinating ligands. Based on these results, we reexamined biochemical and spectroscopic properties of the Alc. vinosum LH1-RC. While modest but distinct effects of Ca2+ were detected in the absorption spectrum of the Alc. vinosum LH1 complex, a marked decrease in thermostability of its LH1-RC complex was observed upon removal of Ca2+. The presence of Ca2+ in the photocomplex of Alc. vinosum suggests that Ca2+-binding to LH1 complexes may be a common adaptation in species of Chromatiaceae for conferring spectral and thermal flexibility on this key component of their photosynthetic machinery.

Hysteretic Pressure Dependence of Ca2+ Binding in LH1 Bacterial Membrane Chromoproteins.

Much of the thermodynamic parameter values that support life are set by the properties of proteins. While the denaturing effects of pressure and temperature on proteins are well documented, their precise structural nature is rarely revealed. This work investigates the destabilization of multiple Ca2+ binding sites in the cyclic LH1 light-harvesting membrane chromoprotein complexes from two Ca-containing sulfur purple bacteria by hydrostatic high-pressure perturbation spectroscopy. The native (Ca-saturated) and denatured (Ca-depleted) phases of these complexes are well distinguishable by much-shifted bacteriochlorophyll a exciton absorption bands serving as innate optical probes in this study. The pressure-induced denaturation of the complexes related to the failure of the protein Ca-binding pockets and the concomitant breakage of hydrogen bonds between the pigment chromophores and protein environment were found cooperative, involving all or most of the Ca2+ binding sites, but irreversible. The strong hysteresis observed in the spectral and kinetic characteristics of phase transitions along the compression and decompression pathways implies asymmetry in the relevant free energy landscapes and activation free energy distributions. A phase transition pressure equal to about 1.9 kbar was evaluated for the complexes from Thiorhodovibrio strain 970 from the pressure dependence of biphasic kinetics observed in the minutes to 100 h time range.

Singlet-triplet Fission of Carotenoid Excitation in the Purple Phototrophic Bacteria Thermochromatium tepidum.

Singlet fission of carotenoid excitation is studied in purple phototrophic bacterium Thermochromatium tepidum. Using time-resolved EPR and magnetic field-induced modulation of fluorescence yield it is shown that the concept of intramolecular excitation fission developed in a number of publications is not supported by the experimental results. The obtained data favor intermolecular fission mechanism involving two carotenoid molecules.

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 ß-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.

Phospholipid distributions in purple phototrophic bacteria and LH1-RC core complexes.

In contrast to plants, algae and cyanobacteria that contain glycolipids as the major lipid components in their photosynthetic membranes, phospholipids are the dominant lipids in the membranes of anoxygenic purple phototrophic bacteria. Although the phospholipid compositions in whole cells or membranes are known for a limited number of the purple bacteria, little is known about the phospholipids associated with individual photosynthetic complexes. In this study, we investigated the phospholipid distributions in both membranes and the light-harvesting 1-reaction center (LH1-RC) complexes purified from several purple sulfur and nonsulfur bacteria. 31P NMR was used for determining the phospholipid compositions and inductively coupled plasma atomic emission spectroscopy was used for measuring the total phosphorous contents. Combining these two techniques, we could determine the numbers of specific phospholipids in the purified LH1-RC complexes. A total of approximate 20-30 phospholipids per LH1-RC were detected as the tightly bound lipids in all species. The results revealed that while cardiolipin (CL) exists as a minor component in the membranes, it became the most abundant phospholipid in the purified core complexes and the sum of CL and phosphatidylglycerol accounted for more than two thirds of the total phospholipids for most species. Preferential association of these anionic phospholipids with the LH1-RC is discussed in the context of the recent high-resolution structure of this complex from Thermochromatium (Tch.) tepidum. The detergent lauryldimethylamine N-oxide was demonstrated to selectively remove phosphatidylethanolamine from the membrane of Tch. tepidum.

Calcite formation induced by Ensifer adhaerens, Microbacterium testaceum, Paeniglutamicibacter kerguelensis, Pseudomonas protegens and Rheinheimera texasensis.

A wide range of bacterial species are able to induce calcium carbonate precipitation. Using our own laboratory-preserved strains, we have newly discovered that Ensifer sp. MY11e, Microbacterium sp. TMd9a1, Paeniglutamicibacter sp. MSa1a, Pseudomonas sp. GTc3, and Rheinheimera sp. ATWe6 can induce the formation of calcite crystals on an agar medium. Type strains of their closely related species (Ensifer adhaerens, Microbacterium testaceum, Paeniglutamicibacter kerguelensis, Pseudomonas protegens, and Rheinheimera texasensis) could also induce calcite formation. Although the initial pH value of the agar medium was 6.1, the pH of the agar media containing calcite, induced by cultivation of the 10 bacterial strains, increased to 8.0-8.4. The ammonification (oxidative deamination) of amino acids may been responsible for this increase in pH. The crystals formed both on and around the bacterial colonies. Furthermore, when these strains (excepting two Microbacterium strains) were cultivated on a cellulose acetate membrane filter (0.20 µm pore size) resting on the surface of the agar medium (i.e., in the membrane filter culture method), the crystals formed on the agar medium separate from the bacterial cells. These results indicate that the bacterial cells did not necessarily become nucleation sites for these crystals. We also investigated whether the studied strains could be applied to the biocementation of sand, and found that only two Ensifer strains were able to form large sand lumps.

Cooperative Photoprotection by Multicompositional Carotenoids in the LH1 Antenna from a Mutant Strain of Rhodobacter sphaeroides.

To explore the photoprotection role of multicompositional carotenoid (Car) in photosynthetic purple bacteria, we investigated, by means of triplet excitation profile (TEP) combined with steady-state optical spectroscopies, the core light-harvesting complex-reaction center of a mutant strain of Rhodobacter sphaeroides (m-LH1-RC) at room temperature. TEP spectra revealed that spheroidene and derivative (Spe) preferentially protect bacteriochlorophylls (BChls) of relatively lower site energy by quenching the triplet excitation (3BChl*); however, spirilloxanthin (Spx) does so irrespective to the site energy of BChls. Triplet excitation results showed the triplet excitation energy-transfer (EET) reaction in a timescale of ∼0.5 µs from Spe and derivatives as a major component (∼85%) to Spx as a minor component (∼8%), suggesting the coexistence of different kinds of Cars in the individual LH1 complex. The nonequivalent quenching potency and the triplet EET reaction between Cars constitute the cooperative photoprotection by multicompositional Cars in bacterial photosynthesis.

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.

Structure of photosynthetic LH1-RC supercomplex at 1.9 Å resolution.

Light-harvesting complex 1 (LH1) and the reaction centre (RC) form a membrane-protein supercomplex that performs the primary reactions of photosynthesis in purple photosynthetic bacteria. The structure of the LH1-RC complex can provide information on the arrangement of protein subunits and cofactors; however, so far it has been resolved only at a relatively low resolution. Here we report the crystal structure of the calcium-ion-bound LH1-RC supercomplex of Thermochromatium tepidum at a resolution of 1.9 Å. This atomic-resolution structure revealed several new features about the organization of protein subunits and cofactors. We describe the loop regions of RC in their intact states, the interaction of these loop regions with the LH1 subunits, the exchange route for the bound quinone QB with free quinone molecules, the transport of free quinones between the inside and outside of the LH1 ring structure, and the detailed calcium-ion-binding environment. This structure provides a solid basis for the detailed examination of the light reactions that occur during bacterial photosynthesis.

Calculation of the excited states properties of LH1 complex of Thermochromatium tepidum.

Calculation of the excited states properties of pigment complexes is one of the key problems in the photosynthesis research. The excited states of LH1 complex of Thermochromatium tepidum were studied by means of the high-precision quantum chemistry methods. The influence of different parameters of the calculation procedure was examined. The optimal scheme of calculation was chosen by comparison of calculated results with the experimental data on absorption, electronic and magnetic circular dichroism spectra. The high importance of the account of the second excited states of bacteriochlorophylls and of site heterogeneity was shown. © 2018 Wiley Periodicals, Inc.

The Influence of the Number of Conjugated Double Bonds in Carotenoid Molecules on the Energy Transfer Efficiency to Bacteriochlorophyll in Light-Harvesting Complexes LH2 from Allochromatium vinosum Strain MSU.

Seven different carotenoids with the number of conjugated double bonds (N) from 5 to 11 were incorporated in vitro into carotenoidless complexes LH2 of the sulfur bacterium Allochromatium vinosum strain MSU. The efficiency of their incorporation varied from 4 to 99%. The influence of N in the carotenoid molecules on the energy transfer efficiency from these pigments to bacteriochlorophyll (BChl) in the modified LH2 complexes was studied for the first time. The highest level of energy transfer was recorded for rhodopin (N = 11) and neurosporene (N = 7) (37 and 51%, respectively). In the other carotenoids, this parameter ranged from 11 to 33%. In the LH2 complexes studied, we found no direct correlation between the decrease in N in carotenoids and increase in the energy transfer efficiency from these pigments to BChl.

Spectrally Selective Spectroscopy of Native Ca-Containing and Ba-Substituted LH1-RC Core Complexes from Thermochromatium tepidum.

The LH1-RC core complex from the thermophilic photosynthetic purple sulfur bacterium Thermochromatium tepidum has recently attracted interest of many researchers because of its several unique properties, such as increased robustness against environmental hardships and the much red-shifted near-infrared absorption spectrum of the LH1 antenna exciton polarons. The known near-atomic-resolution crystal structure of the complex well supported this attention. Yet several mechanistic aspects of the complex prominence remained to be understood. In this work, samples of the native, Ca2+-containing core complexes were investigated along with those destabilized by Ba2+ substitution, using various spectrally selective steady-state and picosecond time-resolved spectroscopic techniques at physiological and cryogenic temperatures. As a result, the current interpretation of exciton spectra of the complex was significantly clarified. Specifically, by evaluating the homogeneous and inhomogeneous compositions of the spectra, we showed that there is little to no effect of cation substitution on the dynamic or kinetic properties of antenna excitons. Reasons of the extra red shift of absorption/fluorescence spectra observed in the Ca-LH1-RC and not in the Ba-LH1-RC complex should thus be searched in subtle structural differences following the inclusion of different cations into the core complex scaffold.

Carotenoid Singlet Fission Reactions in Bacterial Light Harvesting Complexes As Revealed by Triplet Excitation Profiles.

Carotenoids (Cars) in bacterial photosynthesis are known as accessory light harvesters and photoprotectors. Recently, the singlet fission (SF) reaction initiated by Car photoabsorption has been recognized to be an effective excitation deactivation channel disfavoring the light harvesting function. Since the SF reaction and the triplet sensitization reaction underlying photoprotection both yield triplet excited state Cars (3Car*), their contribution to the overall 3Car* photoproduction are difficult to disentangle. To tackle this problem, we resorted to the triplet excitation profiles (TEPs), i.e., the actinic spectra of the overall 3Car* photoproduction. The TEPs combined with the conventional fluorescence excitation spectra allowed us to extract the neat SF contribution, which can serve as a spectroscopic measure for the SF reactivity. This novel spectroscopic strategy was applied to analyze the light harvesting complexes (LHs) from Tch. tepidum and Rba. sphaeroides 2.4.1. The results unambiguously showed that the SF reaction of Cars proceeds with an intramolecular scheme, even in the case of LH1-RC from Rba. sphaeroides 2.4.1 likely binding a secondary pool of Cars. Regarding the SF-reactivity, the geometric distortion in the conjugated backbone of Cars was shown to be the structural determinant, while the length of the Car conjugation was suggested to be relevant to the effective localization of the geminate triplets to avoid being annihilated. The SF reaction scheme and structure-activity relationship revealed herein will be useful not only in deepening our understanding of the roles of Cars in photosynthesis, but also in enlightening the applications of Cars in artificial light conversion systems.

On Light-Induced Photoconversion of B800 Bacteriochlorophylls in the LH2 Antenna of the Purple Sulfur Bacterium Allochromatium vinosum.

The B800-850 LH2 antenna from the photosynthetic purple sulfur bacterium Allochromatium vinosum exhibits an unusual spectral splitting of the B800 absorption band; i.e., two bands are well-resolved at 5 K with maxima at 805 nm (B800R) and 792 nm (B800B). To provide more insight into the nature of the B800 bacteriochlorophyll (BChl) a molecules, high-resolution hole-burning (HB) spectroscopy is employed. Both white light illumination and selective laser excitations into B800R or B800B lead to B800R → B800B phototransformation. Selective excitation into B800B leads to uncorrelated excitation energy transfer (EET) to B800R and subsequent B800R → B800B phototransformation. The B800B → B800R EET time is 0.9 ± 0.1 ps. Excitation at 808.4 nm (into the low-energy side of B800R) shows that the lower limit of B800R → B850 EET is about 2 ps, as the B800R → B800B phototransformation process could contribute to the corresponding zero-phonon hole width. The phototransformation of B800R leads to a ∼ 200 cm-1 average blue-shift of transition energies, i.e., B800R changes into B800B. We argue that it is unlikely that B800-B850 excitonic interactions give rise to a splitting of the B800 band. We propose that the latter is caused by different protein conformations that can lead to both strong or weak hydrogen bond(s) between B800 pigments and the protein scaffolding. Temperature-dependent absorption spectra of B800, which revealed a well-defined isosbestic point, support a two-site model, likely with strongly and weakly hydrogen-bonded B800 BChls. Thus, BChls contributing to B800R and B800B could differ in the position of the proton in the BChl carbonyl-protein hydrogen bond, i.e., proton dynamics along the hydrogen bond may well be the major mechanism of this phototransformation. However, the effective tunneling mass is likely larger than the proton mass.

Engineering of a calcium-ion binding site into the RC-LH1-PufX complex of Rhodobacter sphaeroides to enable ion-dependent spectral red-shifting.

The reaction centre-light harvesting 1 (RC-LH1) complex of Thermochromatium (Tch.) tepidum has a unique calcium-ion binding site that enhances thermal stability and red-shifts the absorption of LH1 from 880nm to 915nm in the presence of calcium-ions. The LH1 antenna of mesophilic species of phototrophic bacteria such as Rhodobacter (Rba.) sphaeroides does not possess such properties. We have engineered calcium-ion binding into the LH1 antenna of Rba. sphaeroides by progressively modifying the native LH1 polypeptides with sequences from Tch. tepidum. We show that acquisition of the C-terminal domains from LH1 α and ß of Tch. tepidum is sufficient to activate calcium-ion binding and the extent of red-shifting increases with the proportion of Tch. tepidum sequence incorporated. However, full exchange of the LH1 polypeptides with those of Tch. tepidum results in misassembled core complexes. Isolated α and ß polypeptides from our most successful mutant were reconstituted in vitro with BChl a to form an LH1-type complex, which was stabilised 3-fold by calcium-ions. Additionally, carotenoid specificity was changed from spheroidene found in Rba. sphaeroides to spirilloxanthin found in Tch. tepidum, with the latter enhancing in vitro formation of LH1. These data show that the C-terminal LH1 α/ß domains of Tch. tepidum behave autonomously, and are able to transmit calcium-ion induced conformational changes to BChls bound to the rest of a foreign antenna complex. Thus, elements of foreign antenna complexes, such as calcium-ion binding and blue/red switching of absorption, can be ported into Rhodobacter sphaeroides using careful design processes.

Emended description of the family Chromatiaceae, phylogenetic analyses of the genera Alishewanella, Rheinheimera and Arsukibacterium, transfer of Rheinheimera longhuensis LH2-2T to the genus Alishewanella and description of Alishewanella alkalitolerans sp. nov. from Lonar Lake, India.

Phylogenetic analyses were performed for members of the family Chromatiaceae, signature nucleotides deduced and the genus Alishewanella transferred to Chromatiaceae. Phylogenetic analyses were executed for the genera Alishewanella, Arsukibacterium and Rheinheimera and the genus Rheinheimera is proposed to be split, with the creation of the Pararheinheimera gen. nov. Furthermore, the species Rheinheimera longhuensis, is transferred to the genus Alishewanella as Alishewanella longhuensis comb. nov. Besides, the genera Alishewanella and Rheinheimera are also emended. Strain LNK-7.1T was isolated from a water sample from the Lonar Lake, India. Cells were Gram-negative, motile rods, positive for catalase, oxidase, phosphatase, contained C16:0, C17:1ω8c, summed feature3 (C16:1ω6c and/or C16:1ω7c) and summed feature 8 (C18:1ω7c) as major fatty acids, PE and PG as the major lipids and Q-8 as the sole respiratory quinone. Phylogenetic analyses using NJ, ME, ML and Maximum parsimony, based on 16S rRNA gene sequences, identified Alishewanella tabrizica RCRI4T as the closely related species of strain LNK-7.1T with a 16S rRNA gene sequence similarity of 98.13%. The DNA-DNA similarity between LNK-7.1T and the closely related species (A. tabrizica) was only 12.0% and, therefore, strain LNK-7.1T was identified as a novel species of the genus Alishewanella with the proposed name Alishewanella alkalitolerans sp. nov. In addition phenotypic characteristics confirmed the species status to strain LNK-7.1T. The type strain of A. alkalitolerans is LNK-7.1T (LMG 29592T = KCTC 52279T), isolated from a water sample collected from the Lonar lake, India.

Metal Cations Induced αß-BChl a Heterogeneity in LH1 as Revealed by Temperature-Dependent Fluorescence Splitting.

Two spectral forms of the core light-harvesting complex (LH1) of the purple bacterium Thermochromatium (Tch.) tepidum, the native Ca2+ -binding and the Ba2+ -substituted one, exhibit different fluorescence (FL) emission spectra at low temperature (T). While Ca-LH1 exhibits one emission band, an unusual splitting of the fluorescence is observed for Ba-LH1. These two sub-bands display the same spectral-width dependence according to T, but their intensity evolves differently with T. Based on the crystal structures, we propose that the FL splitting originates from a large αß-BChl a transition energy heterogeneity, ≈600 cm-1 , which is much larger compared with other LH1 and LH2 complexes (80-200 cm-1 ). This large heterogeneity is induced by the inhomogeneous Coulomb (and possibly hydrogen-bonding) interactions exerted by Ba2+ . The energy levels of the two LH1s were compared using exciton calculations in combination with Redfield theory. To simulate the FL splitting, an electronic transition containing two resonant bands was considered. This work shows how metal cations incorporated into the polypeptide modulate the electronic properties of BChl a aggregates.

Effects of Calcium Ions on the Thermostability and Spectroscopic Properties of the LH1-RC Complex from a New Thermophilic Purple Bacterium Allochromatium tepidum.

The light harvesting-reaction center (LH1-RC) complex from a new thermophilic purple sulfur bacterium Allochromatium (Alc.) tepidum was isolated and characterized by spectroscopic and thermodynamic analyses. The purified Alc. tepidum LH1-RC complex showed a high thermostability comparable to that of another thermophilic purple sulfur bacterium Thermochromatium tepidum, and spectroscopic characteristics similar to those of a mesophilic bacterium Alc. vinosum. Approximately 4-5 Ca2+ per LH1-RC were detected by inductively coupled plasma atomic emission spectroscopy and isothermal titration calorimetry. Upon removal of Ca2+, the denaturing temperature of the Alc. tepidum LH1-RC complex dropped accompanied by a blue-shift of the LH1 Qy absorption band. The effect of Ca2+ was also observed in the resonance Raman shift of the C3-acetyl νC═O band of bacteriochlorophyll-a, indicating changes in the hydrogen-bonding interactions between the pigment and LH1 polypeptides. Thermodynamic parameters for the Ca2+-binding to the Alc. tepidum LH1-RC complex indicated that this reaction is predominantly driven by the largely favorable electrostatic interactions that counteract the unfavorable negative entropy change. Our data support a hypothesis that Alc. tepidum may be a transitional organism between mesophilic and thermophilic purple bacteria and that Ca2+ is one of the major keys to the thermostability of LH1-RC complexes in purple bacteria.

Dependence of the hydration status of bacterial light-harvesting complex 2 on polyol cosolvents.

Low molecular weight (MW) polyols are organic osmolytes influencing protein structure and activity. We have intended to investigate the effects of low MW polyols on the optical and the excited-state properties of the light-harvesting complex 2 (LH2) isolated from the photosynthetic bacterium Thermochromatium (Tch.) tepidum, a thermophile growing at ∼50 °C. Steady state spectroscopy demonstrated that, on increasing glycerol or sorbitol fractions up to 60% (polyol/water, v/v), the visible absorption of carotenoids (Crts) remained unchanged, while the near infrared Qy absorption of bacteriochlorophyll a (BChl) at 800 nm (B800) and 850 nm (B850) varied slightly. Further increasing the fraction of glycerol (but not sorbitol) to 80% (v/v) induced distinct changes of the near infrared absorption and fluorescence spectra. Transient absorption spectroscopy revealed that, following the fast processes of BChl-to-Crt triplet energy transfer, rather weak Qy signals of B800 and B850 remained and evolved in phase with the kinetics of triplet excited state Crt (3Crt*), which are attributed to the Qy band shift as a result of 3Crt*-BChl interaction. The steady state and the transient spectral responses of the Qy bands are found to correlate intimately with the water activity varying against polyol MW and mixing ratio, which are rationalized by the change of the hydration status of the C- and N-termini of LH2. Our results suggest that, with reference to the mesophilic purple bacterium Rhodobacter sphaeroides 2.4.1, Tch. tepidum adopts substantially more robust LH2 hydration against the osmotic effects from the low MW polyols.

Antibacterial low-molecular-weight compounds produced by the marine bacterium Rheinheimera japonica KMM 9513T.

Strain KMM 9513T was isolated from a sediment sample collected from the Sea of Japan seashore and selected due to its ability to inhibit indicator bacterial growth. The strain KMM 9513T has been recently described as a novel species Rheinheimera japonica. This study was undertaken to determine which substances produced by strain KMM 9513T could be responsible for its antimicrobial activity. Eight compounds were obtained from an ethyl acetate extract of R. japonica KMM 9513T. The structures of five diketopiperazines (4-8) and diisobutyl-, dibutyl- and bis(2-ethylhexyl) phthalates (1-3) were established on the basis of detailed interpretation of NMR data, by Marfey method and optical rotation data. The structures of diketopiperazines were determined as cyclo-(L-valyl-L-proline), cyclo-(L-valyl-D-proline), cyclo-(L-phenylalanyl-L-proline), cyclo-(L-leucyl-L-proline), and cyclo-(L-phenylalanyl-D-proline). Compounds 1-3, 5 and 8 revealed antimicrobial activities against Bacillus subtilis and/or Enterococcus faecium and Staphylococcus aureus. In this paper, we describe the isolation and structural elucidation of the isolated compounds 1-8. This is the first report of the characterisation of low molecular weight antibacterial metabolites produced by a member of the genus Rheinheimera.