Characterization of a deep-sea microbial mat from an active cold seep at the Milano mud volcano in the Eastern Mediterranean Sea

A white, filamentous microbial mat at the Milano mud volcano in the Eastern Mediterranean Sea was sampled during the Medinaut cruise of the R/V Nadir in 1998. The composition of the mat community was characterized using a combination of phylogenetic and lipid biomarker methods. The mat sample was filtered through 0.2 and 5-microm filters to coarsely separate unicellular and filamentous bacteria. Analyses of 16S rRNA gene sequences amplified from the total community DNA from these fractions showed that similar archaeal populations were present in both fractions. However, the bacterial populations in the fractions differed from one another, and were more diverse than the archaeal ones. Lipid analysis showed that bacteria were the dominant members of the mat microbial community and the relatively low delta(13)C carbon isotope values of bulk bacterial lipids suggested the occurrence of methane- and sulfide-based chemo(auto)trophy. Consistent with this, the bacterial populations in the fractions were related to Alpha-, Gamma- and Epsilonproteobacteria, most of which were chemoautotrophic bacteria that utilize hydrogen sulfide (or reduced sulfur compounds) and/or methane. The most common archaeal 16S rRNA gene sequences were related to those of previously identified Archaea capable of anaerobic methane oxidation. Although the filamentous organisms observed in the mat were not conclusively identified, our results indicated that the Eastern Mediterranean deep-sea microbial mat community might be sustained on a combination of methane- and sulfide-driven chemotrophy.     

Growth yield increase linked to reductive dechlorination in a defined 3-chlorobenzoate degrading methanogenic coculture

The microbially mediated reductive dehalogenation of aromatic compounds is potentially important in removal of chlorinated aromatic compounds from the environment. Thermodynamic data are presented which show that the reductive dechlorination of 3-chlorobenzoate to benzoate is exergonic, which led to the hypothesis that reductive elimination of chlorine from 3-chlorobenzoate yields biologically useful energy. In the present paper this hypothesis is tested. Experimental data were obtained with a defined 3-chlorobenzoate degrading methanogenic consortium. These data showed that (i) the molar growth yield of a defined 3-chlorobenzoate degrading consortium increased from 4.9 g protein per mol benzoate metabolized to 6.8 g protein per mol 3-chlorobenzoate when 3-chlorobenzoate replaced benzoate as energy source, and that (ii) the ATP level in starved consortium cells was twice as high when the cells were fed 3-chlorobenzoate than when fed benzoate. These observations show that the electrochemical potential between the redox partners of the H⁺/H₂ (electron-donating) and 3-chlorobenzoate/benzoate (electron-accepting) couples is a potential source of energy and are consistent with the hypothesis that reductive dechlorination of aromatic compounds is coupled to a novel type of microbial chemotrophy.     

Reductive dechlorination of 3-chlorobenzoate is coupled to ATP production and growth in an anaerobic bacterium,strain DCB-1

Thermodynamic data that the reductive dechlorination of 3-chlorobenzoate is exergonic have led to the hypothesis that this reaction yields biologically useful energy. This hypothesis was tested with strain DCB-1, a dehalogenating bacterium. The organism was grown under strictly anaerobic conditions in vitamin-amended mineral medium with formate plus acetate as electron donor and 3-chlorobenzoate as electron acceptor. The cell yield increased stoichiometrically to the amount of 3-chlorobenzoate dechlorinated. No growth was observed in the absence of 3-chlorobenzoate, or when 3-chlorobenzoate was replaced by benzoate. To obtain further evidence on that energy is derived from dechlorination, 3-chlorobenzoate was added to starved cells. This amendment resulted in an increase in the ATP level of the cells at 10 nmol per mg protein versus 3 nmol per mg protein in non-amended controls. These data indicate that the reductive dehalogenation of chlorinated aromatic compounds can be coupled to a novel type of chemotrophy.