Rhodospirillum salexigens, spec. nov., an obligatory halophilic phototrophic bacterium

Soil Chemistry and Nutrients Influence the Distribution of Aerobic Anoxygenic Phototrophic Bacteria and Eukaryotic Phototrophic Microorganisms of Physical Soil Crusts at Different Elevations on the Tibetan Plateau

Photosynthetic microorganisms are widely distributed in the soil and play an important role in plant-free soil crusts. However, the distribution and environmental drivers of phototrophic microbial communities in physical soil crusts, where the abundance of cyanobacteria is low, are scarcely understood. Here, we performed high-throughput sequencing of pufM and 18S rRNA genes in soil crusts at different elevations on the Tibetan Plateau and used the data combined with environmental variables to analyze the diversity and structure of phototrophic microbial communities. We found that the dominant taxa of aerobic anoxygenic phototrophic bacteria (AAPB) and eukaryotic phototrophic microorganisms (EPM) were shown to shift with elevation. The phototrophic microbial diversity showed a single-peak pattern, with the lowest diversity of AAPB and highest diversity of EPM at middle elevations. Moreover, the elevation and soil property determined the phototrophic microbial community. Soil salts, especially Cl^-, were the most important for AAPB. Likewise, soil nutrients, especially carbon, were the most important for EPM. The relationship between high-abundance taxa and environmental variables showed that Rhizobiales was significantly negatively correlated with salt ions and positively correlated with chlorophyll. Rhodobacterales showed the strongest and significant positive associations with Cl^-. Chlorophyceae and Bacillariophyceae were positively correlated with CO₃^2-. These results indicated that salinity and soil nutrients affected the diversity and structure of microbial communities. This study contributes to our understanding of the diversity, composition, and structure of photosynthetic microorganisms in physical soil crusts and helps in developing new approaches for controlling desertification and salinization and improving the desert ecological environment.

The Majority of Active Rhodobacteraceae in Marine Sediments Belong to Uncultured Genera: A Molecular Approach to Link Their Distribution to Environmental Conditions

General studies on benthic microbial communities focus on fundamental biogeochemical processes or the most abundant constituents. Thereby, minor fractions such as the Rhodobacteraceae are frequently neglected. Even though this family belongs to the most widely distributed bacteria in the marine environment, their proportion on benthic microbial communities is usually within or below the single digit range. Thus, knowledge on these community members is limited, even though their absolute numbers might exceed those from the pelagic zone by orders of magnitudes. To unravel the distribution and diversity of benthic, metabolically active Rhodobacteraceae, we have now analyzed an already existing library of bacterial 16S rRNA transcripts. The dataset originated from 154 individual sediment samples comprising seven oceanic regions and a broad variety of environmental conditions. Across all samples, a total of 0.7% of all 16S rRNA transcripts was annotated as Rhodobacteraceae. Among those, Sulfitobacter, Paracoccus, and Phaeomarinomonas were the most abundant cultured representatives, but the majority (78%) was affiliated to uncultured family members. To define them, the 45 most abundant Rhodobacteraceae-OTUs assigned as "uncultured" were phylogenetically assembled in new clusters. Their next relatives particularly belonged to different subgroups other than the Roseobacter group, reflecting a large part of the hidden diversity within the benthic Rhodobacteraceae with unknown functions. The general composition of active Rhodobacteraceae communities was found to be specific for the geographical location, exhibiting a decreasing richness with sediment depth. One-third of the Rhodobacteraceae-OTUs significantly responded to the prevailing redox regime, suggesting an adaption to anoxic conditions. A possible approach to predict their physiological properties is to identify the metabolic capabilities of their nearest relatives. Those need to be proven by physiological experiments, as soon an isolate is available. Because many uncultured members of these subgroups likely thrive under anoxic conditions, in future research, a molecular-guided cultivation strategy can be pursued to isolate novel Rhodobacteraceae from sediments.

Elevated seawater temperature disrupts the microbiome of an ecologically important bioeroding sponge

Bioeroding sponges break down calcium carbonate substratum, including coral skeleton, and their capacity for reef erosion is expected to increase in warmer and more acidic oceans. However, elevated temperature can disrupt the functionally important microbial symbionts of some sponge species, often with adverse consequences for host health. Here, we provide the first detailed description of the microbial community of the bioeroding sponge Cliona orientalis and assess how the community responds to seawater temperatures incrementally increasing from 23°C to 32°C. The microbiome, identified using 16S rRNA gene sequencing, was dominated by Alphaproteobacteria, including a single operational taxonomic unit (OTU; Rhodothalassium sp.) that represented 21% of all sequences. The "core" microbial community (taxa present in >80% of samples) included putative nitrogen fixers and ammonia oxidizers, suggesting that symbiotic nitrogen metabolism may be a key function of the C. orientalis holobiont. The C. orientalis microbiome was generally stable at temperatures up to 27°C; however, a community shift occurred at 29°C, including changes in the relative abundance and turnover of microbial OTUs. Notably, this microbial shift occurred at a lower temperature than the 32°C threshold that induced sponge bleaching, indicating that changes in the microbiome may play a role in the destabilization of the C. orientalis holobiont. C. orientalis failed to regain Symbiodinium or restore its baseline microbial community following bleaching, suggesting that the sponge has limited ability to recover from extreme thermal exposure, at least under aquarium conditions.

Limibacillus halophilus gen. nov., sp. nov., a moderately halophilic bacterium in the family Rhodospirillaceae isolated from reclaimed land

A Gram-stain-negative, aerobic, non-motile, non-spore-forming and short rod-shaped bacterial strain, designated CAU 1121T, was isolated from reclaimed land in the Republic of Korea and its taxonomic position was investigated using a polyphasic approach. The bacterium grew optimally at 37 °C, at pH 6.5 and in the presence of 2 % (w/v) NaCl. Based on 16S rRNA gene sequence similarity, the novel isolate belonged to the family Rhodospirillaceae within the class Alphaproteobacteria and formed an independent lineage within the evolutionary radiation encompassed by the phylum Proteobacteria. Strain CAU 1121T exhibited very low levels of 16S rRNA gene sequence similarity with its phylogenetic neighbours Pelagibius litoralis (similarity, 92.5 %), Fodinicurvata fenggangensis (similarity, 91.4 %), Fodinicurvata sediminis (similarity, 90.7 %) and Tistlia consotensis (similarity, 91.0 %). Strain CAU 1121T contained ubiquinone-10 as the only respiratory quinone and C18 : 1ω7c as the major cellular fatty acid. The DNA G+C content of the strain was 65 mol%. On the basis of phylogenetic inference, and physiological and chemotaxonomic data, it is proposed that strain CAU 1121T represents a novel genus and novel species in the family Rhodospirillaceae, for which the name Limibacillus halophilus gen. nov., sp. nov. is suggested. The type strain is CAU 1121T ( = KCTC 42420T = CECT 8803T = NBRC 110928T).

Molecular and lipid biomarker analysis of a gypsum-hosted endoevaporitic microbial community

Modern evaporitic microbial ecosystems are important analogs for understanding the record of earliest life on Earth. Although mineral-depositing shallow-marine environments were prevalent during the Precambrian, few such environments are now available today for study. We investigated the molecular and lipid biomarker composition of an endoevaporitic gypsarenite microbial mat community in Guerrero Negro, Mexico. The 16S ribosomal RNA gene-based phylogenetic analyses of this mat corroborate prior observations indicating that characteristic layered microbial communities colonize gypsum deposits world-wide despite considerable textural and morphological variability. Membrane fatty acid analysis of the surface tan/orange and lower green mat crust layers indicated cell densities of 1.6 × 10(9) and 4.2 × 10(9)  cells cm(-3) , respectively. Several biomarker fatty acids, ∆7,10-hexadecadienoic, iso-heptadecenoic, 10-methylhexadecanoic, and a ∆12-methyloctadecenoic, correlated well with distributions of Euhalothece, Stenotrophomonas, Desulfohalobium, and Rhodobacterales, respectively, revealed by the phylogenetic analyses. Chlorophyll (Chl) a and cyanobacterial phylotypes were present at all depths in the mat. Bacteriochlorophyl (Bchl) a and Bchl c were first detected in the oxic-anoxic transition zone and increased with depth. A series of monomethylalkanes (MMA), 8-methylhexadecane, 8-methylheptadecane, and 9-methyloctadecane were present in the surface crust but increased in abundance in the lower anoxic layers. The MMA structures are similar to those identified previously in cultures of the marine Chloroflexus-like organism 'Candidatus Chlorothrix halophila' gen. nov., sp. nov., and may represent the Bchl c community. Novel 3-methylhopanoids were identified in cultures of marine purple non-sulfur bacteria and serve as a probable biomarker for this group in the lower anoxic purple and olive-black layers. Together microbial culture and environmental analyses support novel sources for lipid biomarkers in gypsum crust mats.

Defluvicoccus vanus gen. nov., sp. nov., a novel Gram-negative coccus/coccobacillus in the 'Alphaproteobacteria' from activated sludge

A novel Gram-negative coccus/coccobacillus, strain Ben 114(T), growing in tetrads, clusters or aggregates, was isolated from activated sludge by micromanipulation. 16S rRNA gene sequence analysis revealed that it belonged to the 'Alphaproteobacteria', with no close relatives among cultured bacterial isolates. On the basis of phylogenetic data, this organism is considered to belong to a new genus, Defluvicoccus, represented by the species Defluvicoccus vanus sp. nov., a name chosen because of the distinctive staining properties of this organism; only the cell wall stained strongly with a wide range of stains, giving the cell a hollow and empty appearance. No intracellular polyphosphate granules could be detected after staining, but poly-beta-hydroxyalkanoate inclusions were detected using Nile blue A staining. Because of its taxonomic distance from its closest relatives among the 'Alphaproteobacteria', namely members of the genera Azospirillum, Phaeospirillum, Rhodospirillum, Rhodocista, Magnetospirillum and Rhodospira, D. vanus is considered to represent a new phylogenetic lineage within subgroup 1 of the 'Alphaproteobacteria', the D. vanus subgroup. The type strain is Ben 114(T) (=NCIMB 13612(T)=CIP 107350(T)).

Isolation of iodide-oxidizing bacteria from iodide-rich natural gas brines and seawaters

Iodide-oxidizing bacteria (IOB), which oxidize iodide (I-) to molecular iodine (I2), were isolated from iodide-rich (63 microM to 1.2 mM) natural gas brine waters collected from several locations. Agar media containing iodide and starch were prepared, and brine waters were spread directly on the media. The IOB, which appeared as purple colonies, were obtained from 28 of the 44 brine waters. The population sizes of IOB in the brines were 10(2) to 10(5) colony-forming units (CFU) mL(-1). However, IOB were not detected in natural seawaters and terrestrial soils (fewer than 10 CFU mL(-1) and 10(2) CFU g wet weight of soils(-1), respectively). Interestingly, after the enrichment with 1 mM iodide, IOB were found in 6 of the 8 seawaters with population sizes of 10(3) to 10(5) CFU mL(-1). 16S rDNA sequencing and phylogenetic analyses showed that the IOB strains are divided into two groups within the alpha-subclass of the Proteobacteria. One of the groups was phylogenetically most closely related to Roseovarius tolerans with sequence similarities between 94% and 98%. The other group was most closely related to Rhodothalassium salexigens, although the sequence similarities were relatively low (89% to 91%). The iodide-oxidizing reaction by IOB was mediated by an extracellular enzyme protein that requires oxygen. Radiotracer experiments showed that IOB produce not only I2 but also volatile organic iodine, which were identified as diiodomethane (CH2I2) and chloroiodomethane (CH2ClI). These results indicate that at least two types of IOB are distributed in the environment, and that they are preferentially isolated in environments in which iodide levels are very high. It is possible that IOB oxidize iodide in the natural environment, and they could significantly contribute to the biogeochemical cycling of iodine.

Isolation and characterization of spirilloid purple phototrophic bacteria forming red layers in microbial mats of Mediterranean salterns: description of Halorhodospira neutriphila sp. nov. and emendation of the genus Halorhodospira

Microbial mats developing in the hypersaline lagoons of a commercial saltern in the Salin-de-Giraud (Rhône delta) were found to contain a red layer fully dominated by spirilloid phototrophic purple bacteria underlying a cyanobacterial layer. From this layer four strains of spirilloid purple bacteria were isolated, all of which were extremely halophilic. All strains were isolated by using the same medium under halophilic photolithoheterotrophic conditions. One of them, strain SG 3105 was a purple non-sulfur bacterial strain closely related to Rhodovibrio sodomensis with a 16S rDNA sequence similarity of 98.8%. The three other isolated strains, SG 3301T, SG 3302 and SG 3304, were purple sulfur bacteria and were found to be very similar. The cells were motile by a polar tuft of flagella. Photosynthetic intracytoplasmic membranes of the lamellar stack type contained BChl a and spirilloxanthin as the major carotenoid. Phototrophic growth with sulfide as electron donor was poor; globules of elemental sulfur were present outside the cells. In the presence of sulfide and CO2 good growth occurred with organic substrates. Optimum growth occurred in the presence of 9-12% (w/v) NaCl at neutral pH (optimal pH 6.8-7) and at 30-35 degrees C. The DNA base composition of strains SG 3301T and SG 3304 were 74.5 and 74.1 mol% G + C, respectively. According to the 16S rDNA sequences, strains SG 3301T and SG 3304 belonged to the genus Halorhodospira, but they were sufficiently separated morphologically, physiologically and genetically from other recognized Halorhodospira species to be described as a new species of the genus. They are, therefore, described as Halorhodospira neutriphila sp. nov. with strain SG 3301T as the type strain (=DSM 15116T).

The phylogeny of purple bacteria: the alpha subdivision

The technique of oligonucleotide cataloging shows the purple photosynthetic eubacteria to comprise three major subdivisions, temporarily called alpha, beta, and gamma--previously designated groups I-III by Gibson et al. (1979). Each subdivision contains a number of non-photosynthetic genera in addition to the photosynthetic ones. The alpha subdivision, the subject of the present report, contains most but not all of the species that fall into the classically defined genera Rhodospirillum, Rhodopseudomonas and Rhodomicrobium. Intermingled with these are a variety of non-photosynthetic species from genera such as Agrobacterium, Rhizobium, Azospirillum, Nitrobacter, Erythrobacter, Phenylobacterium, Aquaspirillum, and Paracoccus. The phylogenetic substructure of the alpha subdivision is presented and the evolutionary significance of the admixture of biochemical phenotypes is discussed.

Plasmid transfer and susceptibility to antibiotics in the halophilic phototrophs Rhodovibrio salinarum and Rhodothalassium salexigens

The present study defines a series of genetic procedures to be used for molecular studies in photosynthetic halophilic species such as Rhodovibrio salinarum and Rhodothalassium salexigens. In both species, the minimal inhibitory concentrations for the antibiotics tetracycline, rifampicin, chloramphenicol, spectinomycin, streptomycin, and kanamycin were determined. In addition, conjugal transfer of IncP and IncQ plasmids from Escherichia coli was demonstrated and the resistance markers expressed in these halophiles were determined. Finally, Rth. salexigens growth dependence on variable salt concentrations was measured: maximal growth rates were seen at 6% and 4% NaCl under phototrophic and chemotrophic conditions, respectively. To the best of our knowledge, this is the first report analyzing the genetic properties of two representative species of halophilic purple non-sulfur phototrophs.

Forty-five years of developmental biology of photosynthetic bacteria

Developmental biology and cell differentiation of photosynthetic prokaryotes are less noticed fields than the showpieces of eukaryotes, e.g. Drosophila melanogaster. The large metabolic versatility of the facultative purple bacteria and their great capability to adapt to different ecological conditions, however, aroused the inquisitiveness to investigate the process of cell differentiation and to use these bacteria as model system to study structure, function and biosynthesis of the photosynthetic apparatus. The great progress in research in this field paved the way to study principal mechanisms of cellular organization and differentiation in these bacteria. In this article, the history of the research on membrane structure and development of anoxygenic photosynthetic prokaryotes during the last 45 years is described. A personal account of how I entered the field through research on the phototaxis of cyanobacteria is given. Intracytoplasmic membranes (ICM) were detected by electron microscopy in cyanobacteria and in purple non-sulfur bacteria. The formation of ICM by invagination of the cytoplasmic membrane in purple bacteria was observed for the first time. Investigations on the effect of changes in oxygen tension and light intensity on the formation of pigments and intracytoplasmic membranes followed. The isolation, purification, and analysis of light-harvesting complexes and of pigment-binding proteins was the next step of our research. Lipopolysaccharides and peptidoglycans were detected and analyzed in the outer membrane of photosynthetic bacteria. Functional membrane differentiation includes variations in the rates of photophosphorylation and electron transport. Molecular genetic approaches have initiated the investigation of transcriptional regulation and the analysis of correlation between pigment and protein synthesis. Molecular analysis of assembly of light-harvesting complexes and membrane differentiation are the present aspects of our research. Cell differentiation has been considered under evolutionary view.

Occurrence and purification of the photoactive yellow protein of Ectothiorhodospira halophila (PYP) and of immunologically related proteins of Rhodospirillum salexigens and Chromatium salexigens and intracellular localization of PYP

The photoactive yellow protein of Ectothiohodospira halophila (PYP) was purified to homogeneity by an advanced method and applied as an affinity ligand for the isolation of an anti-PYP IgG fraction which was used for immunoscreening. The distribution of proteins immunologically related to PYP was investigated in protein fractions of 51 strains from 38 species of non-halophilic and halophilic phototrophic and chemotrophic eubacteria and archaeobacteria. Strong immunoreactive bands indicating the presence of authentic PYP on Western blots (apparent mass 17.8 kDa) was only found in the strains of E. halophila. Additionally, two soluble proteins of Chromatium salexigens and Rhodospirillum salexigens (apparent molecular masses 16.4 and 19 kDa, respectively) cross-reacted to approx. 6% and 4%. Analyses of cell fractions of E. halophila revealed that PYP is a cytoplasmic protein.

Lipopolysaccharide of Rhodospirillum salinarum 40: structural studies on the core and lipid A region

The structural elucidation of lipid A of the cell wall lipopolysaccharide (LPS) of Rhodospirillum salinarum 40 by chemical methods and laser desorption mass spectrometry revealed the presence of a mixed lipid A composed of three different 1,4'bisphosphorylated beta (1 --> 6)-linked backbone hexosaminyl-hexosamine disaccharides, i.e. those composed of GlcN --> GlcN, 2,3-diamino-2,3-dideoxy-D-Glc-(DAG --> DAG, and DAG --> GlcN. Lipid A of R. salinarum contained preferentially 3-OH-18:0 and 3-OH-14:0 as amide-linked and cis delta 11-18:1 and c19:0 as ester-linked fatty acids. The mass spectra of the liberated acyl-oxyacyl residues proved the concomitant presence of 3-O-(cis delta 11-18:1)-18:0 and 3-O-(c19:0)-14:0 as the predominating diesters in this mixed lipid A. The glycosidically linked and the ester-linked phosphate groups of the backbone disaccharide were neither substituted by ethanolamine, phosphorylethanolamine, nor by 4-amino-4-deoxy-L-arabinose, in contrast to most of the enterobacterial lipid As. In the core oligosaccharide fraction, a HexA (1 --> 4)HexA(1 --> 5)Kdo-trisaccharide was identified by methylation analysis. The terminal HexA (hexuronic acid) is possibly 4-OMe-GalA, a component described here as an LPS constituent for the first time. LPS of R. salinarum showed a lethality in C57BL/10 ScSN (LPS-responder)-mice) of an order of 10(-1)-10(-2) of that reported for Salmonella abortus equi LPS, and it was also capable of inducing TNF alpha and IL6 in macrophages of C57BL/10ScSN mice.

Amino acid sequences of cytochromes c2 and c' from the moderately halophilic purple phototrophic bacterium Rhodospirillum salexigens

Rhodospirillum salexigens is a moderately halophilic purple phototrophic bacterium which grows optimally in 8% NaCl. The amino acid sequences of the two principal soluble cytochromes c have been determined. One of these is a cytochrome c2, similar in size to mitochondrial cytochrome c. While clearly of the same sequence class as mitochondrial cytochrome c and the proteins from several other Gram-negative bacteria, it does not show particular affinity to any already known sequence in terms of the percentage sequence identity. The other protein is a cytochrome c', but is also a divergent member of this widespread group. The lack of appreciable sequence identity to other species is probably due to a limit of divergence which has been reached for the majority of purple bacterial species. However, the numbers of insertions and deletions and their locations in cytochromes c2 and c' suggest that R salexigens may be related to Rhodospirillum molischianum. Like other electron transport proteins from halophiles, both of these cytochromes are notable for their high content of arginine as compared with lysine and both are acidic. However, they do not show any particular sequence homology to electron transport proteins that have been characterized from the extremely halophilic phototrophes of the genus Ectothiorhodospira. Thus, it appears that adaptation to halophilic habitats has independently occurred more than once in purple bacteria.

Soluble cytochromes and a photoactive yellow protein isolated from the moderately halophilic purple phototrophic bacterium, Rhodospirillum salexigens

Three soluble cytochromes were found in two strains of the halophilic non-sulfur purple bacterium Rhodospirillum salexigens. These are cytochromes C2, C and c-551. Cytochrome C2 was recognized by the presence of positive charge at the site of electron transfer (measured by laser flash photolysis), although the protein has an overall negative charge (pI = 4.7). Cytochrome C2 has a high redox potential (300 mV) and is monomeric (13 kDa). Cytochrome c was recognized from its characteristic absorption spectrum. It has a redox potential of 95 mV, an isoelectric point of 4.3, and is isolated as a dimer (33 kDa) of identical subunits (14 kDa), a property which is typical of this family of proteins. R. salexigens cytochrome c-551 has an absorption spectrum similar to the low redox potential Rb. sphaeroides cytochrome c-551.5. It also has a low redox potential (-170 mV), is very acidic (pI = 4.5), and is monomeric (9 kDa), apparently containing 1 heme per protein. The existence of abundant membrane-bound cytochromes c-558 and c-551 which are approximately half reduced by ascorbate and completely reduced by dithionite suggests the presence of a tetraheme reaction center cytochrome in R. salexigens, although reaction centers purified in a previous study (Wacker et al., Biochim. Biophys. Acta (1988) 933, 299-305) did not contain a cytochrome. The most interesting observation is that R. salexigens contains a photoactive yellow protein (PYP), previously observed only in the extremely halophilic purple sulfur bacterium Ectothiorhodospira halophila. The R. salexigens PYP appears to be slightly larger than that of Ec. halophila (16 kDa vs. 14 kDa). Otherwise, these two yellow proteins have similar absorption spectra, chromatographic properties and kinetics of photobleaching and recovery.

Variation of environmental features and microbial populations with salt concentrations in a multi-pond saltern

A multi-pond saltern that creates a gradient of salt concentrations has been studied with respect to some characteristics of the resulting environments and their microbial populations. The increase in salt concentration was correlated with increase in diurnal temperature and biomass present and with decrease in oxygen concentrations. Many types of organisms below 15% (w/v) total salts, were found, many of them normal inhabitants of seawater and even freshwater. Most organisms over 15% salts were halophilic. The salt concentrations comprised two ranges, each characterized by different microbial populations. First, between 15 and 30% salts, the populations ofDunaliella increased, reaching large numbers; moderately halophilic eubacteria and some fast-growing halobacteria predominated as heterotrophic microorganisms and, among the first, thePseudomonas-Alteromonas-Alcaligenes group andVibrio were the more abundant taxonomic groups; and gram-positive cocci appeared mainly over 25% salts. Phototrophic bacteria, both oxygenic and anoxygenic, were also found in this range, and among the anoxygenic type,Chromatium species andRodospirillum salexigens were probably predominant. Second, over 30% salts the diversity decreased greatly, all organisms found at the lower salt concentrations disappeared, and instead large populations of halobacteria developed. Over 50% salts, only three species of halobacteria were found.

Protein on the cell surface of the moderately halophilic phototrophic bacterium Rhodospirillum salexigens

A cell surface protein (Mr 68,000) of the moderately but obligately halophilic phototrophic bacterium Rhodospirillum salexigens was identified by two independent methods: first, by labeling the cell surface with radioactive iodine and lactoperoxidase, and second, by washing cells in 30% sucrose to remove proteins attached to the cell surface by ionic bonds. The identified protein very likely represents the outermost layer of the cell envelope of R. salexigens as observed by electron microscopy. The protein was isolated. Its isoelectric point was determined to be 4.4; the excess of acidic over basic amino acids was found to be 18.3 mol%; and its average hydrophobicity was 2.26 kJ per residue.

Nitrogen fixation and nitrogenase activities in members of the family Rhodospirillaceae

Strains of all 18 species of the family Rhodospirillaceae (nonsulfur photosynthetic bacteria) were studied for their comparative nitrogen-fixing abilities. All species, with the exception of Rhodocyclus purpureus, were capable of growth with N2 as the sole nitrogen source under photosynthetic (anaerobic) conditions. Most rapid growth on N2 was observed in strains of Rhodopseudomonas capsulata. Within the genus Rhodopseudomonas, the species R. capsulata, R. sphaeroides, R. viridis, R. gelatinosa, and R. blastica consistently showed the highest in vivo nitrogenase rates (with the acetylene reduction technique); nitrogenase rates in other species of Rhodopseudomonas and in most species of Rhodospirillum were notably lower. Chemotrophic (dark microaerobic) nitrogen fixation occurred in all species with the exception of one strain of Rhodospirillum fulvum; oxygen requirements for dark N2 fixation varied considerably among species and even within strains of the same species. We conclude that the capacity to fix molecular nitrogen is virtually universal among members of the Rhodospirillaceae but that the efficacy of the process varies considerably among species.

Taxonomy of phototrophic green and purple bacteria: a review

The presently existing classification for the green and purple bacteria comprises physiological-ecological assemblages of phototrophic bacteria with anoxygenic photosynthesis. The taxonomic units of the different levels were based entirely on common phenotypic traits, including morphological, cytological, physiological and biochemical characteristics. Since degrees of resemblance form the basis of the grouping, this classification cannot reflect the genetic or evolutionary relatedness of these bacteria, neither among themselves nor with other bacteria. The advantage of the artificial system, however, is the use of features which can be established in most laboratories and which allow the comparison and identification of newly isolated strains with those already studied and described. The four existing families correspond to the four major recognized, ecophysiological groups, the Chlorobiaceae and Chloroflexaceae among the green bacteria, and the Chromatiaceae and Rhodospirillaceae among the purple bacteria. Our knowledge of all these groups is incomplete; this is reflected by the fact that seven new species have been described during the past three years (6th Newsletter on phot. bacteria, Trüper and Hansen, 1982). The description of the new genus and species Erythrobacter longus (Shiba and Simidu, 1982) is also interesting, as it comprises aerobic chemoorganotrophic marine bacteria which form bacteriochlorophyll a and carotenoids; however, no strains were able to grow phototrophilcally. Significant success is currently being obtained in the different approaches toward elucidating the genetic relationships within and outside of the purple and green bacteria. Detailed studies of the lipopolysaccharides of several species and genera of the Rhodospirillaceae (Weckesser et al., 1979, and more recent paper) have proven to be very useful for the recognition of relationships or dissimilarities between the species of a genus or between different genera. Amino acid sequence studies of cytochromes c from Rhodospirillaceae, other bacteria and eukaryotic organisms (Dickerson, 1980) have led to the recognition of four different groups of cytochrome c molecules (long, medium and two groups of short protein chains). The subdivision of the Rhodospirillaceae into three species groups, each possessing one of the three types of cytochrome c, proved to be in total agreement with the results of oligonucleotide cataloging of the 16 S ribosomal RNA of these bacteria (Gibson et al., 1979). The latter method also revealed that several chemotropic bacteria, including the nitrifying bacteria, are more closely related to certain purple bacteria than different species of the purple bacteria among themselves (Seewaldt et al., 1982).(ABSTRACT TRUNCATED AT 400 WORDS)