Physiology and Phylogeny of Green Sulfur Bacteria Forming a Monospecific Phototrophic Assemblage at a Depth of 100 Meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter⁻¹. High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter⁻¹) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 μmol quanta m⁻² s⁻¹ at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 μmol quanta m⁻² s⁻¹ for photosynthetic ¹⁴CO₂ fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.     

Characterization and in situ carbon metabolism of phototrophic consortia

A dense population of the phototrophic consortium "Pelochromatium roseum" was investigated in the chemocline of a temperate holomictic lake (Lake Dagow, Brandenburg, Germany). Fluorescence in situ hybridization revealed that the brown epibionts of "P. roseum" constituted up to 37% of the total bacterial cell number and up to 88% of all green sulfur bacteria present in the chemocline. Specific amplification of 16S rRNA gene fragments of green sulfur bacteria and denaturing gradient gel electrophoresis fingerprinting yielded a maximum of four different DNA bands depending on the year of study, indicating that the diversity of green sulfur bacteria was low. The 465-bp 16S rRNA gene sequence of the epibiont of "P. roseum" was obtained after sorting of individual consortia by micromanipulation, followed by a highly sensitive PCR. The sequence obtained represents a new phylotype within the radiation of green sulfur bacteria. Maximum light-dependent H(14)CO(3)(-) fixation in the chemocline in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea suggested that there was anaerobic autotrophic growth of the green sulfur bacteria. The metabolism of the epibionts was further studied by determining stable carbon isotope ratios (delta(13)C) of their specific biomarkers. Analysis of photosynthetic pigments by high-performance liquid chromatography revealed the presence of high concentrations of bacteriochlorophyll (BChl) e and smaller amounts of BChl a and d and chlorophyll a in the chemocline. Unexpectedly, isorenieratene and beta-isorenieratene, carotenoids typical of other brown members of the green sulfur bacteria, were absent. Instead, four different esterifying alcohols of BChl e were isolated as biomarkers of green sulfur bacterial epibionts, and their delta(13)C values were determined. Farnesol, tetradecanol, hexadecanol, and hexadecenol all were significantly enriched in (13)C compared to bulk dissolved and particulate organic carbon and compared to the biomarkers of purple sulfur bacteria. The difference between the delta(13)C values of farnesol, the major esterifying alcohol of BChl e, and CO(2) was -7.1%, which provides clear evidence that the mode of growth of the green sulfur bacterial epibionts of "P. roseum" in situ is photoautotrophic.     

Identification of the major chlorosomal bacteriochlorophylls of the green sulfur bacteria Chlorobium vibrioforme and Chlorobium phaeovibrioides; their function in lateral energy transfer

The chlorosomal bacteriochlorophyll (BChl) composition of the green sulfur bacteria Chlorobium vibrioforme and Chlorobium phaeovibrioides was investigated by means of normal-phase high-performance liquid chromatography. From both species a number of homologues was isolated, which were identified by absorption and ²⁵²Cf-plasma desorption mass spectroscopy. Besides BChl d, C. vibrioforme contained a significant amount of BChl c, which may provide an explanation for the previous observation of at least two spectrally different pools of BChl in the chlorosomes of green sulfur bacteria (Otte et al. 1991). C. phaeovibrioides contained various homologues of BChl e only. Absorption spectra in acetone of BChl c, d and e, as well as bacteriopheophytin e are presented. No systematic differences were found for the various homologues of each pigment. In addition to farnesol, the mass spectra revealed the presence of various minor esterifying alcohols in both species, including phytol, oleol, cetol and 4-undecyl-2-furanmethanol, as well as an alcohol of low molecular mass, which is tentatively assumed to be decenol.Abbreviations BChl bacteriochlorophyll - BPh bacteriopheophytin (used as a general name for the Mg-free compound, irrespective of the esterifying alcohol) - HPLC high-performance liquid chromatography     

Spatiotemporal distribution of bacteriochlorophylls in the meromictic Lake Suigetsu, Japan

The spatiotemporal distribution of chlorophyll pigments (chloropigments) in the water column of a meromictic lake, Lake Suigetsu (Fukui, Japan), was investigated. Water samples were collected from the central basin of Lake Suigetsu bimonthly between May 2008 and March 2010 at appropriate depths, including the oxic surface, oxic–anoxic interface, and anoxic bottom layers. Chlorophyll a, related to cyanobacteria and eukaryotic phytoplankton, was detected throughout the water column during the years of the study, whereas bacteriochlorophyll e, related to brown-colored green sulfur bacteria, was detected in the anoxic layers below the chemocline at a maximum concentration of 825 μg L^?1. The concentration of bacteriochlorophyll e was generally maximal at or just below the chemocline of the lake. The cellular content of bacteriochlorophyll e was estimated to be low in the upper part of the chemocline and tended to increase with increasing water depth. Bacteriochlorophyll a, which was presumably related to purple sulfur bacteria, was only detected at the chemocline during summer and autumn at concentrations of 5.4–16.3 μg L^?1. Our analysis of the chloropigment distribution for the two years of the study suggested that brown-colored green sulfur bacteria are the predominant phototroph in the anoxic layers of Lake Suigetsu, and that these play a significant role in the carbon and sulfur cycling of the lake, especially from spring to summer.     

Unexpected Population Distribution in a Microbial Mat Community: Sulfate-Reducing Bacteria Localized to the Highly Oxic Chemocline in Contrast to a Eukaryotic Preference for Anoxia

The distribution and abundance of sulfate-reducing bacteria (SRB) and eukaryotes within the upper 4 mm of a hypersaline cyanobacterial mat community were characterized at high resolution with group-specific hybridization probes to quantify 16S rRNA extracted from 100-μm depth intervals. This revealed a preferential localization of SRB within the region defined by the oxygen chemocline. Among the different groups of SRB quantified, including members of the provisional families “Desulfovibrionaceae” and “Desulfobacteriaceae,” Desulfonema-like populations dominated and accounted for up to 30% of total rRNA extracted from certain depth intervals of the chemocline. These data suggest that recognized genera of SRB are not necessarily restricted by high levels of oxygen in this mat community and the possibility of significant sulfur cycling within the chemocline. In marked contrast, eukaryotic populations in this community demonstrated a preference for regions of anoxia.     

The ecological niche of the consortium “Pelochromatium roseum”

A dense accumulation of the phototrophic consortium “Pelochromatium roseum” in a small, eutrophic, freshwater lake (Dagowsee, Brandenburg, Germany) was investigated. Within the chemocline, the number of epibionts of the consortia represented up to 19% of the total number of bacteria. Per “P. roseum” a mean value of 20 epibionts was determined. Similar to other aquatic habitats, consortia in the Dagowsee were found only at low light intensities (< 7 μmol quanta m^–2 s^–1) and low sulfide concentrations (0–100 μM). In dialysis cultures of “P. roseum”, bacterial cells remained in a stable association only when incubated at light intensities between 5 and 10 μmol quanta m^–2 s^–1. Intact consortia from natural samples had a buoyant density of 1046.8 kg m^–3, which was much higher than that of ambient chemocline water (995.8 kg m^–3). Under environmental conditions and without motility, this density difference would result in rapid sedimentation of consortia toward the lake bottom. Our results indicate that (1) consortia are adapted to a very narrow regime of light intensities and sulfide concentrations, (2) motility and tactic responses must be of ecological significance for the colonization of the free water column of lakes, and (3) phototrophic growth of consortia can be explained only by a cycling of sulfur species in the chemocline, possibly within the consortia themselves. Received: 27 May 1997 / Accepted: 16 September 1997     

Molecular Characterization of the Nonphotosynthetic Partner Bacterium in the Consortium “Chlorochromatium aggregatum”

Phototrophic consortia represent valuable model systems for the study of signal transduction and coevolution between different bacteria. The phototrophic consortium “Chlorochromatium aggregatum” consists of a colorless central rod-shaped bacterium surrounded by about 20 green-pigmented epibionts. Although the epibiont was identified as a member of the green sulfur bacteria, and recently isolated and characterized in pure culture, the central colorless bacterium has been identified as a member of the β-Proteobacteria but so far could not be characterized further. In the present study, “C. aggregatum” was enriched chemotactically, and the 16S rRNA gene sequence of the central bacterium was elucidated. Based on the sequence information, fluorescence in situ hybridization probes targeting four different regions of the 16S rRNA were designed and shown to hybridize exclusively to cells of the central bacterium. Phylogenetic analyses of the 1,437-bp-long sequence revealed that the central bacterium of “C. aggregatum” represents a so far isolated phylogenetic lineage related to Rhodoferax spp., Polaromonas vacuolata, and Variovorax paradoxus within the family Comamonadaceae. The majority of relatives of this lineage are not yet cultured and were found in low-temperature aquatic environments or aquatic environments containing xenobiotica or hydrocarbons. In CsCl-bisbenzimidazole equilibrium density gradients, genomic DNA of the central bacterium of “Chlorochromatium aggregatum” formed a distinct band which could be detected by quantitative PCR using specific primers. Using this method, the G+C content of the central bacterium was determined to be 55.6 mol%.     

Specific detection of green sulfur bacteria by in situ hybridization with a fluorescently labeled oligonucleotide probe

An oligodeoxynucleotide probe (GSB-532) specific for green sulfur bacteria was developed. Highly stringent hybridization conditions were established using whole cells of Chlorobium limicola DSM249 immobilized on glass slides. At a formamide concentration of 10%, the optimum specificity was reached at 47 °C. When a conventional fixation procedure was used, a conspicuous autofluorescence developed within the cells. This autofluorescence was due to the liberation of bacteriochlorophyll by the detergent Triton X-100 and a subsequent conversion to bacteriopheophytin and related compounds. The signal-to-noise ratio could be increased by a final dehydration of the samples with methanol. Finally, the method was adapted to the hybridization of natural samples collected on polycarbonate membrane filters. In situ hybridization of pure cultures, various enrichments, and natural samples from the chemocline of a freshwater lake confirmed that probe GSB-532 hybridized exclusively to cells of green sulfur bacteria. Our protocol allows the highly specific detection of green sulfur bacteria in water samples and a rapid screening of natural bacterial communities. Employing probe GSB-532, the phylogenetic affiliation of the epibionts in “Chlorochromatium aggregatum” and “Pelochromatium roseum” could be demonstrated for the first time. Received: 26 October 1998 / Accepted: 7 January 1999     

Initial characterization of the photosynthetic apparatus of "Candidatus Chlorothrix halophila," a filamentous, anoxygenic photoautotroph

“Candidatus Chlorothrix halophila” is a recently described halophilic, filamentous, anoxygenic photoautotroph (J. A. Klappenbach and B. K. Pierson, Arch. Microbiol. 181:17-25, 2004) that was enriched from the hypersaline microbial mats at Guerrero Negro, Mexico. Analysis of the photosynthetic apparatus by negative staining, spectroscopy, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the photosynthetic apparatus in this organism has similarities to the photosynthetic apparatus in both the Chloroflexi and Chlorobi phyla of green photosynthetic bacteria. The chlorosomes were found to be ellipsoidal and of various sizes, characteristics that are comparable to characteristics of chlorosomes in other species of green photosynthetic bacteria. The absorption spectrum of whole cells was dominated by the chlorosome bacteriochlorophyll c (BChl c) peak at 759 nm, with fluorescence emission at 760 nm. A second fluorescence emission band was observed at 870 nm and was tentatively attributed to a membrane-bound antenna complex. Fluorescence emission spectra obtained at 77 K revealed another complex that fluoresced at 820 nm, which probably resulted from the chlorosome baseplate complex. All of these results suggest that BChl c is present in the chlorosomes of “Ca. Chlorothrix halophila,” that BChl a is present in the baseplate, and that there is a membrane-bound antenna complex. Analysis of the proteins in the chlorosomes revealed an ~6-kDa band, which was found to be related to the BChl c binding protein CsmA found in other green bacteria. Overall, the absorbance and fluorescence spectra of “Ca. Chlorothrix halophila” revealed an interesting mixture of photosynthetic characteristics that seemed to have properties similar to properties of both phyla of green bacteria when they were compared to the photosynthetic characteristics of Chlorobium tepidum and Chloroflexus aurantiacus.     

Long-Term Population Dynamics of Phototrophic Sulfur Bacteria in the Chemocline of Lake Cadagno, Switzerland

Population analyses in water samples obtained from the chemocline of crenogenic, meromictic Lake Cadagno, Switzerland, in October for the years 1994 to 2003 were studied using in situ hybridization with specific probes. During this 10-year period, large shifts in abundance between purple and green sulfur bacteria and among different populations were obtained. Purple sulfur bacteria were the numerically most prominent phototrophic sulfur bacteria in samples obtained from 1994 to 2001, when they represented between 70 and 95% of the phototrophic sulfur bacteria. All populations of purple sulfur bacteria showed large fluctuations in time with populations belonging to the genus Lamprocystis being numerically much more important than those of the genera Chromatium and Thiocystis. Green sulfur bacteria were initially represented by Chlorobium phaeobacteroides but were replaced by Chlorobium clathratiforme by the end of the study. C. clathratiforme was the only green sulfur bacterium detected during the last 2 years of the analysis, when a shift in dominance from purple sulfur bacteria to green sulfur bacteria was observed in the chemocline. At this time, numbers of purple sulfur bacteria had decreased and those of green sulfur bacteria increased by about 1 order of magnitude and C. clathratiforme represented about 95% of the phototrophic sulfur bacteria. This major change in community structure in the chemocline was accompanied by changes in profiles of turbidity and photosynthetically available radiation, as well as for sulfide concentrations and light intensity. Overall, these findings suggest that a disruption of the chemocline in 2000 may have altered environmental niches and populations in subsequent years.     

Novel bacteriochlorophyll <Emphasis Type="BoldItalic">e</Emphasis> structures and species-specific variability of pigment composition in green sulfur bacteria

The relative composition of bacteriochlorophyll (BChl) homologs in five different strains of brown-colored green sulfur bacteria was investigated by HPLC-MS/MS and NMR analyses. In addition, the effect of incubation light intensities on homolog distribution was studied in one of the strains (strain Dagow III). A total of 23 different BChl e structures were detected and comprise four homologous porphyrin ring systems and eight different esterifying alcohols. Several BChl e structures are novel. These include a C-8 ethyl, C-12 methyl [E, M] BChl e(F) homolog which was identified by (1)H-NMR analyses of the isolated, main farnesyl homologs (BChl e(F)). In addition, five previously unknown homolog series with dodecanol, pentadecenol, tetradecanol, hexadecenol and phytol as the esterifying alcohols were detected. The composition of BChl e homologs from the five strains of green sulfur bacteria differed with respect to the relative abundance of the homologs (BChl e(F) : 25.6-67.0% of total BChl e content in stationary cultures). In strain Dagow III, the abundance of BChl e(F) homologs decreased upon entry into the stationary phase. In all free-living strains, the abundance of BChl e(F) was increased when the relative carotenoid content was low. The present results provide a detailed picture of pigment composition in chlorosomes and thus will help to elucidate their structure and function. Furthermore, the newly discovered BChl e molecules are valuable biomarkers for the study of the occurrence and metabolism of green sulfur bacteria in past and present ecosystems.     

Primary Events, Energy Transfer, and Reactions in Photosynthetic Events: Lifetime of the Excited State In Vivo: II. Bacteriochlorophyll in Photosynthetic Bacteria at Room Temperature

Lifetime of the excited state (τ) of bacteriochlorophyll (BChl) in photosynthetic bacteria, measured with a mode-locked argon laser (oscillating at 488 nm; mode locked at 56 MHz) as light source, ranged from 0.3 to 2.5 nsec. These τ values are reported with a precision of ±0.1 nsec. The value of τ at high exciting light intensity (I) was two to three times that at low intensity. For young cultures of green bacterium Chloropseudomonas ethylicum, τ ranged from 0.5 (low I) to 1.0 nsec (high I); for those of the purple bacterium Rhodospirillum rubrum, from 0.4 (low I) to 1.0 nsec (high I); and for those of the BChl b-containing Rhodopseudomonas viridis, from 1.0 (low I) to 2.5 nsec (high I). These data provide information regarding the efficiencies of the photochemical process in these bacteria. Quantum yield (ø) of BChl fluorescence, calculated from ø = τ/τ₀ (where τ₀ is the intrinsic lifetime of fluorescence), ranges from 2-6% at low intensities to 6-14% at high intensities.     

Excitation Energy Transfer Dynamics and Excited-State Structure in Chlorosomes of Chlorobium phaeobacteroides

The excited-state relaxation within bacteriochlorophyll (BChl) e and a in chlorosomes of Chlorobium phaeobacteroides has been studied by femtosecond transient absorption spectroscopy at room temperature. Singlet-singlet annihilation was observed to strongly influence both the isotropic and anisotropic decays. Pump intensities in the order of 10¹¹ photons × pulse⁻¹ × cm⁻² were required to obtain annihilation-free conditions. The most important consequence of applied very low excitation doses is an observation of a subpicosecond process within the BChl e manifold (~200–500 fs), manifesting itself as a rise in the red part of the Q_y absorption band of the BChl e aggregates. The subsequent decay of the kinetics measured in the BChl e region and the corresponding rise in the baseplate BChl a is not single-exponential, and at least two components are necessary to fit the data, corresponding to several BChl e→BChl a transfer steps. Under annihilation-free conditions, the anisotropic kinetics show a generally slow decay within the BChl e band (10–20 ps) whereas it decays more rapidly in the BChl a region (~1 ps). Analysis of the experimental data gives a detailed picture of the overall time evolution of the energy relaxation and energy transfer processes within the chlorosome. The results are interpreted within an exciton model based on the proposed structure.     

Biogeochemical processes in the saline meromictic Lake Kaiike, Japan: implications from molecular isotopic evidences of photosynthetic pigments

Stable carbon and nitrogen isotopic compositions were determined for individual photosynthetic pigments isolated and purified from the saline meromictic Lake Kaiike, Japan, to investigate species-independent biogeochemical processes of photoautotrophs in the natural environment. In the anoxic monimolimnion and benthic microbial mats, the carbon isotopic compositions of BChls e and isorenieratene related to brown-coloured strains of green sulfur bacteria are substantially ( approximately 10 per thousand) depleted in (13)C relative to those found in the chemocline. In conjunction with 16S rDNA evidence reported previously, it strongly suggests that Pelodyctyon luteolum inhabited and photosynthesized in the anoxic monimolimnion and benthic microbial mats by using (13)C-depleted regenerated CO(2). By contrast, both Chl a and BChl a in the monimolimnion and microbial mats have similar isotopic compositions as they do in the chemocline, implying that the source organisms live only in the chemocline. In the chemocline, the nitrogen isotopic compositions of BChl e homologues ranges from -7.7 to-6.5 per thousand, whereas that of BChl a is -2.1 per thousand. These isotopic compositions suggest that green sulfur bacteria Chlorobium phaeovibrioides would conduct nitrogen fixation in the chemocline, whereas purple sulfur bacteria Halochromatium sp. and cyanobacteria Synechococcus sp. may assimilate nitrite.     

Physiology and phylogeny of green sulfur bacteria forming a monospecific phototrophic assemblage at a depth of 100 meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter(-1). High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter(-1)) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 micromol quanta m(-2) s(-1) at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 micromol quanta m(-2) s(-1) for photosynthetic 14CO2 fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.     

Dominance of green sulfur bacteria in the chemocline of the meromictic Lake Suigetsu, Japan, as revealed by dissimilatory sulfite reductase gene analysis

This study investigated the spatiotemporal abundance and diversity of the α-subunit of the dissimilatory sulfite reductase gene (dsrA) in the meromictic Lake Suigetsu for assessing the sulfur-oxidizing bacterial community. The density of dsrA in the chemocline reached up to 3.1 × 10⁶ copies ml^?1 in summer by means of quantitative real-time PCR and it was generally higher than deeper layers. Most of the dsrA clones sequenced were related to green sulfur bacteria such as Chlorobium phaeovibrioides, C. limicola, and C. luteolum. Below the chemocline of the lake, we also detected other dsrA clones related to the purple sulfur bacterium Halochromatium salexigens and some branching lineages of diverse sequences that were related to chemotrophic sulfur bacterial species such as Magnetospirillum gryphiswaldense, Candidatus Ruthia magnifica, and Candidatus Thiobios zoothamnicoli. The abundance and community compositions of sulfur-oxidizing bacteria changed depending on the water depth and season. This study indicated that the green sulfur bacteria dominated among sulfur-oxidizing bacterial population in the chemocline of Lake Suigetsu and that certain abiotic environmental variables were important factors that determined sulfur bacterial abundance and community structure.     

Biogeography, Evolution, and Diversity of Epibionts in Phototrophic Consortia

Motile phototrophic consortia are highly regular associations in which numerous cells of green sulfur bacteria surround a flagellated colorless β-proteobacterium in the center. To date, seven different morphological types of such consortia have been described. In addition, two immotile associations involving green sulfur bacteria are known. By employing a culture-independent approach, different types of phototrophic consortia were mechanically isolated by micromanipulation from 14 freshwater environments, and partial 16S rRNA gene sequences of the green sulfur bacterial epibionts were determined. In the majority of the lakes investigated, different types of phototrophic consortia were found to co-occur. In all cases, phototrophic consortia with the same morphology from the same habitat contained only a single epibiont phylotype. However, morphologically indistinguishable phototrophic consortia collected from different lakes contained different epibionts. Overall, 19 different types of epibionts were detected in the present study. Whereas the epibionts within one geographic region were very similar (Dice coefficient, 0.582), only two types of epibionts were found to occur on both the European and North American continents (Dice coefficient, 0.190). None of the epibiont 16S rRNA gene sequences have been detected so far in free-living green sulfur bacteria, suggesting that the interaction between epibionts and chemotrophic bacteria in the phototrophic consortia is an obligate interaction. Based on our phylogenetic analysis, the epibiont sequences are not monophyletic. Thus, the ability to form symbiotic associations either arose independently from different ancestors or was present in a common ancestor prior to the radiation of green sulfur bacteria and the transition to the free-living state in independent lineages. The present study thus demonstrates that there is great diversity and nonrandom geographical distribution of phototrophic consortia in the natural environment.     

Impact of esterified bacteriochlorophylls on the biogenesis of chlorosomes in Chloroflexus aurantiacus

A chlorosome is an antenna complex located on the cytoplasmic side of the inner membrane in green photosynthetic bacteria that contains tens of thousands of self-assembled bacteriochlorophylls (BChls). Green bacteria are known to incorporate various esterifying alcohols at the C-17 propionate position of BChls in the chlorosome. The effect of these functional substitutions on the biogenesis of the chlorosome has not yet been fully explored. In this report, we address this question by investigating various esterified bacteriochlorophyll c (BChl c) homologs in the thermophilic green non-sulfur bacterium Chloroflexus aurantiacus. Cultures were supplemented with exogenous long-chain alcohols at 52 °C (an optimal growth temperature) and 44 °C (a suboptimal growth temperature), and the morphology, optical properties and exciton transfer characteristics of chlorosomes were investigated. Our studies indicate that at 44 °C Cfl. aurantiacus synthesizes more carotenoids, incorporates more BChl c homologs with unsaturated and rigid polyisoprenoid esterifying alcohols and produces more heterogeneous BChl c homologs in chlorosomes. Substitution of phytol for stearyl alcohol of BChl c maintains similar morphology of the intact chlorosome and enhances energy transfer from the chlorosome to the membrane-bound photosynthetic apparatus. Different morphologies of the intact chlorosome versus in vitro BChl aggregates are suggested by small-angle neutron scattering. Additionally, phytol cultures and 44 °C cultures exhibit slow assembly of the chlorosome. These results suggest that the esterifying alcohol of BChl c contributes to long-range organization of BChls, and that interactions between BChls with other components are important to the assembly of the chlorosome. Possible mechanisms for how esterifying alcohols affect the biogenesis of the chlorosome are discussed.     

Dominating Role of an Unusual Magnetotactic Bacterium in the Microaerobic Zone of a Freshwater Sediment

A combination of polymerase chain reaction-assisted rRNA sequence retrieval and fluorescent oligonucleotide probing was used to identify in situ a hitherto unculturable, big, magnetotactic, rod-shaped organism in freshwater sediment samples collected from Lake Chiemsee. Tentatively named “Magnetobacterium bavaricum,” this bacterium is evolutionarily distant from all other phylogenetically characterized magnetotactic bacteria and contains unusually high numbers of magnetosomes (up to 1,000 magnetosomes per cell). The spatial distribution in the sediment was studied, and up to 7 × 10⁵ active cells per cm³ were found in the microaerobic zone. Considering its average volume (25.8 ± 4.1 μm³) and relative abundance (0.64 ± 0.17%), “M. bavaricum” may account for approximately 30% of the microbial biovolume and may therefore be a dominant fraction of the microbial community in this layer. Its microhabitat and its high content of sulfur globules and magnetosomes suggest that this organism has an iron-dependent way of energy conservation which depends on balanced gradients of oxygen and sulfide.