Three strains of Rhodopseudomonas palustris were isolated from phototrophic enrichment cultures containing 3-chlorobenzoate (3-CBA) and benzoate (BA). These new strains as well as several previously described strains of R. palustris were tested in this study and shown to degrade 3-CBA if grown in media that contained BA as a co-substrate. All of the pure cultures that originally required BA for the degradation of 3-CBA acquired the ability to degrade 3-CBA as the sole carbon source after long periods of incubation that ranged from 1 to 3 months. After this adaptation period, the 3-CBA-degrading capabilities of all variants were stable, and the rates of 3-CBA degradation were significantly enhanced as compared to the parental strains. Furthermore, the variants had also acquired the ability to metabolize 2- and 4-CBA as sole carbon sources indicating that the enhanced ability to metabolize 3-CBA was accompanied by an expanded ability to metabolize chlorinated benzoates. These data indicate that acquisition of the ability to degrade 3-CBA may be rather common among strains of R. palustris and mutations that confer the ability to metabolize 3-CBA may provide a selective advantage to R. palustris under specific environmental conditions.
A two‐member co‐culture consisting of the dehalorespiring Desulfitobacterium frappieri TCE1 and the sulphate‐reducing Desulfovibrio sp. strain SULF1 was obtained via anaerobic enrichment from soil contaminated with tetrachloroethene (PCE). In this co‐culture, PCE dechlorination to cis ‐dichloroethene was due to the activity of the dehalorespiring bacterium only. Chemostat experiments with lactate as the primary electron donor for both strains along with varying sulphate and PCE concentrations showed that the sulphate‐reducing strain outnumbered the dehalogenating strain at relatively high ratios of sulphate/PCE. Stable co‐cultures with both organisms present at similar cell densities were observed when both electron acceptors were supplied in the reservoir medium in nearly equimolar amounts. In the presence of low sulphate/PCE ratios, the Desulfitobacterium sp. became the numerically dominant strain within the chemostat co‐culture. Surprisingly, in the absence of sulphate, strain SULF1 did not disappear completely from the co‐culture despite the fact that there was no electron acceptor provided with the medium to be used by this sulphate reducer. Therefore, we propose a syntrophic association between the sulphate‐reducing and the dehalorespiring bacteria via interspecies hydrogen transfer. The sulphate reducer was able to sustain growth in the chemostat co‐culture by fermenting lactate and using the dehalogenating bacterium as a ‘biological electron acceptor’. This is the first report describing growth of a sulphate‐reducing bacterium in a defined two‐member continuous culture by syntrophically coupling the electron and hydrogen transfer to a dehalorespiring bacterium.
