A coastal marine sulfide-oxidizing autotrophic bacterium produces hydrophilic filamentous sulfur as a novel metabolic end product. Phylogenetic analysis placed the organism in the genus Arcobacter in the epsilon subdivision of the Proteobacteria. This motile vibrioid organism can be considered difficult to grow, preferring to grow under microaerophilic conditions in flowing systems in which a sulfide-oxygen gradient has been established. Purified cell cultures were maintained by using this approach. Essentially all 4,6-diamidino-2-phenylindole dihydrochloride-stained cells in a flowing reactor system hybridized with Arcobacter-specific probes as well as with a probe specific for the sequence obtained from reactor-grown cells. The proposed provisional name for the coastal isolate is "Candidatus Arcobacter sulfidicus." For cells cultured in a flowing reactor system, the sulfide optimum was higher than and the CO 2 fixation activity was as high as or higher than those reported for other sulfur oxidizers, such as Thiomicrospira spp. Cells associated with filamentous sulfur material demonstrated nitrogen fixation capability. No ribulose 1,5-bisphosphate carboxylase/oxygenase could be detected on the basis of radioisotopic activity or by Western blotting techniques, suggesting an alternative pathway of CO 2 fixation. The process of microbial filamentous sulfur formation has been documented in a number of marine environments where both sulfide and oxygen are available. Filamentous sulfur formation by "Candidatus Arcobacter sulfidicus" or similar strains may be an ecologically important process, contributing significantly to primary production in such environments.In the marine environment, hydrogen sulfide is a ubiquitous end product of anaerobic processes of organic matter remineralization (5,23,29,30). At ridge crest sites on the ocean floor, it is produced from the geothermal transformation of sulfate and elemental sulfur leaching via seawater-basaltic rock interaction (26,40,62). When brought into contact with the aerobic biosphere, hydrogen sulfide becomes an energy-yielding substrate for chemosynthetic colorless sulfur-oxidizing bacteria. Members of this group include free-living rods or ovoids of the genera Thiobacillus, Thiomonas, Acidiphilium, Thiomicrospira, and Thiovulum (31,32,34,42) as well as the morphologically conspicuous gliding and nongliding filamentous forms of the genera Beggiatoa, Thioploca, and Thiothrix (19,47,48,69). These organisms are characterized by their ability to catalyze the oxidation of sulfide and its chemically and biologically mediated partial oxidation products (polysulfides, S n 2Ϫ ; elemental sulfur, S 0 ; sulfane monosulfonic acids, HSS n O 3 2Ϫ ]; and polythionates, S n O 6 2Ϫ ) (14, 33, 66) coupled to the fixation of carbon dioxide to organic carbon by utilizing the same CalvinBassham-Benson cycle enzymes as those used by oxygenic phototrophs.Because of the differential rates of oxidation of hydrogen sulfide and derived oxidation products, intermediates may substantially accu...

Chemosynthetic production of microbial biomass, determined by 14CO2 fixation and enzymatic (RuBisCo) activity, at the Mid‐Atlantic Ridge (MAR) 23° and 26°N vent sites was found in various niches: warm water emissions, loosely rock‐attached flocculent material, dense morphologically diverse bacterial mats covering the surfaces of polymetal sulfide deposits, and filamentous microbes on the carapaces of shrimp (Rimicaris exoculata). The bacterial mats on polymetal sulfide surfaces contained unicellular and filamentous bacteria which appeared to use as their chemolithotrophic electron or energy source either dissolved reduced minerals from vent emissions, mainly sulfur compounds, or solid metal sulfide deposits, mainly pyrite. Moderately thermophilic Chemosynthetic activity was observed in carbon dioxide fixation experiments and in enrichments, but no thermophilic aerobic sulfur oxidizers could be isolated. Both obligate and facultative chemoautotrophs growing at mesophilic temperatures were isolated from all chemosynthetically active surface scrapings. The obligate autotrophs could oxidize sterilized MAR natural sulfide deposits as well as technical pyrite at near neutral pH, in addition to dissolved reduced sulfur compounds. While the grazing by shrimp on the surface mats of MAR metal sulfide deposits was observed and deemed important, the animals’ primary occurrence in dense swarms near vent emissions suggests that they were feeding at these sites, where conditions for Chemosynthetic growth of their filamentous microbial epiflora were optimal. The data show that the transformation of geothermal energy at the massive polymetal sulfide deposits of the MAR is based on the lithoautotrophic oxidation of soluble sulfides and pyrites into microbial biomass.

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Autotrophic carbon fixation was characterized in representative members of the three lineages of the bacterial phylum Aquificae. Enzyme activity measurements and the detection of key genes demonstrated that Aquificae use the reductive tricarboxylic acid (TCA) cycle for autotrophic CO(2) fixation. This is the first time that strains of the Hydrogenothermaceae and 'Desulfurobacteriaceae' have been investigated for enzymes of autotrophic carbon fixation. Unexpectedly, two different mechanisms of citrate cleavage could be identified within the Aquificae. Aquificaceae use citryl-CoA synthetase and citryl-CoA lyase, whereas Hydrogenothermaceae and 'Desulfurobacteriaceae' use ATP citrate lyase. The first mechanism is likely to represent the ancestral version of the reductive TCA cycle. Sequence analyses further suggest that ATP citrate lyase formed by a gene fusion of citryl-CoA synthetase and citryl-CoA lyase and subsequently became involved in a modified version of this pathway. However, rather than having evolved within the Aquificae, our phylogenetic analyses indicate that Aquificae obtained their ATP citrate lyase through lateral gene transfer. Aquificae play an important role in biogeochemical processes in a variety of high-temperature habitats. Thus, these findings substantiate the hypothesis that autotrophic carbon fixation through the reductive TCA cycle is widespread and contributes significantly to biomass production particularly in hydrothermal habitats.

The chemical stress factors for microbial life at deep-sea hydrothermal vents include high concentrations of heavy metals and sulfide. Three hyperthermophilic vent archaea, the sulfur-reducing heterotrophs Thermococcus fumicolans and Pyrococcus strain GB-D and the chemolithoautotrophic methanogen Methanocaldococcus jannaschii, were tested for survival tolerance to heavy metals (Zn, Co, and Cu) and sulfide. The sulfide addition consistently ameliorated the high toxicity of free metal cations by the formation of dissolved metal-sulfide complexes as well as solid precipitates. Thus, chemical speciation of heavy metals with sulfide allows hydrothermal vent archaea to tolerate otherwise toxic metal concentrations in their natural environment.Hyperthermophilic archaea grow in the steep thermal and chemical gradients in hydrothermal vent chimney rock; in this habitat, they are exposed to metal-and sulfide-rich vent fluid, which is transported through the porous chimney matrix with concomitant precipitation of metal-sulfides and -sulfates and in some chimney silica (15) upon mixing with seawater. Although metal and sulfide concentrations in the rock matrix have not been measured in situ, end-member concentrations represent upper-limit approximations. Sulfide concentrations are typically in the millimolar range and can be higher than 12 mM. Metal concentrations are typically in the range of 10 to 40 M for Cu, 20 to 220 nM for Co, and 40 to 780 M for Zn (8, 30). Site-specific peak concentrations can reach 1 to 2 M for Co (22) and 1,000 to 3,000 M for Zn (30). The chemical speciation of metals and sulfide, in particular metal-sulfide complex formation, might play a critical role in shaping the environmental niches and survival strategies of hydrothermal vent microorganisms. Metal-sulfide complexes play an important role in biological contexts (23), for example, by relieving cadmium toxicity to amphipods in marine sediments (6) and by influencing the distribution of hydrothermal vent invertebrates such as Riftia pachyptila and Alvinella pompeiana (19).For hydrothermal vent archaea, efforts to characterize their growth conditions and survival capabilities have focused on extremes of temperature and pH, oxygen sensitivity, and electron acceptor and donor range (13, 27). Genomic information on metal tolerance and metabolism is limited to tentatively identified metal transport proteins (primarily Co, Cu, and Fe transporters) in the genomes of Pyrococcus furiosus, P. abyssi, P. horikoshii (http://www.ncbi.nlm.nih.gov), and Methanocaldococcus jannaschii (4, 26). As a consequence, the tolerances of vent archaea to the high concentrations of metals in their native habitat, their response to the chemical speciation of those metals, and their defense mechanisms against these environmental stress factors have remained largely obscure. Silver (26) has pointed out that a surprisingly small number of detoxicification genes appear to be present in the genome of M. jannaschii given its high-metal habitat in hydrothermal vent chimneys.In or...