Physiology and Molecular Ecology of Microbial Filamentous Sulfur Formation

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

In recent years, we have discovered an unusual marine bacterium that oxidizes hydrogen sulfide into a novel filamentous form of elemental sulfur, while fixing CO2 into cell material. Phylogenetic studies have placed this organism into the genus Arcobacter of the epsilon subdivision of the Proteobacteria. Microbial filamentous sulfur production occurs in high fluid flow marine sulfidic environments, including coastal salt marsh creeks, sulfidic sediments, and marine hydrothermal vents worldwide. During the previous research period, we were able to cultivate this filament-producing organism from coastal sediments (coastal strain putatively named "Candidatus Arcobacter sulfidicus") and deep-sea hydrothermal environments, and have investigated the physiology, morphology and phylogeny of this interesting group. The overall goal of the present project will be to expand studies on the microbial ecology of filamentous sulfur formation. So far, little is known of the abundance, distribution, diversity, and function of "Candidatus Arcobacter sulfidicus"-like organisms in situ. Abundance and distribution will be assessed in environmental samples by employing fluorescent in situ hybridization (FISH) with existing and newly developed fluorescently labeled oligonucleotides of different specificity targeting the 16S rRNA. Genomic DNA and RNA will be extracted from sulfidic marine and hydrothermal environments for diversity analyses using denaturing gradient gel electrophoresis (DGGE), small-subunit rRNA gene cloning and sequencing. These data will form the basis for the design of more refined group-, species-, and strain-specific 16S rRNA probes. Microelectrode measurements will be used in concert with FISH to measure relevant physicochemical parameters in laboratory and environmental samples. "Candidatus Arcobacter sulfidicus" is likely to fix CO2 by an alternative pathway other than the Calvin-Bassham-Benson cycle based on 1) the absence of Rubisco, a key enzyme of a metabolic pathway used by many bacteria and green plants for incorporating CO2 into cell material, and 2) the small d13C fractionation by the cells. Thus, this organism seems to be the first colorless sulfur-oxidizing chemolithoautotrophic bacterium that does not use the widespread Calvin-Bassham-Benson pathway for carbon fixation. This is a finding of evolutionary significance and we propose to elucidate the pathway in our model coastal strain. Evidence also suggests that this organism is capable of fixing nitrogen and this will be confirmed in additional studies. The overall significance of this research lies in the characterization of the parameters and mechanisms by which this unique process of autotrophic sulfide oxidation occurs, as well as the abundance, distribution and genotypic diversity of the responsible microbes in a number of different environments. Finally, the results will enhance our knowledge of the physiology and ecology of a novel, and heretofore-unconsidered component of the sulfur cycle. Filamentous sulfur formation may be an important process at hydrothermal vents, extending into the shallow subsurface biosphere and driven by inorganic nutrients alone (i.e. H2S and CO2, N2).