Natural niche for organohalide-respiring Chloroflexi

Abstract

The phylum Chloroflexi contains several isolated bacteria that have been found to respire a diverse array of halogenated anthropogenic chemicals. The distribution and role of these Chloroflexi in uncontaminated terrestrial environments, where abundant natural organohalogens could function as potential electron acceptors, have not been studied. Soil samples (116 total, including 6 sectioned cores) from a range of uncontaminated sites were analyzed for the number of Dehalococcoides-like Chloroflexi 16S rRNA genes present. Dehalococcoides-like Chloroflexi populations were detected in all but 13 samples. The concentrations of organochlorine ([organochlorine]), inorganic chloride, and total organic carbon (TOC) were obtained for 67 soil core sections. The number of Dehalococcoides-like Chloroflexi 16S rRNA genes positively correlated with [organochlorine]/TOC while the number of Bacteria 16S rRNA genes did not. Dehalococcoides-like Chloroflexi were also observed to increase in number with a concomitant accumulation of chloride when cultured with an enzymatically produced mixture of organochlorines. This research provides evidence that organohalide-respiring Chloroflexi are widely distributed as part of uncontaminated terrestrial ecosystems, they are correlated with the fraction of TOC present as organochlorines, and they increase in abundance while dechlorinating organochlorines. These findings suggest that organohalide-respiring Chloroflexi may play an integral role in the biogeochemical chlorine cycle.

Fig 1

The distribution of Dehalococcoides-like Chloroflexi (solid symbols) and total [organochlorine]/TOC (open symbols) in the soil cores taken from dominantly oak (A), pine (B), and maple (C) forests in September 2006 (left) and January 2008 (right).

Fig 2

Linear regression fit of the variable log(Chloroflexi) and a model including the independent variables depth (cm), [organochlorine]/TOC, and “pine cover” (samples with pine cover are given a value of 1 and samples with maple and oak cover are given a value of 0).

Fig 3

The increase of 16S rRNA gene sequences of Dehalococcoides-like Chloroflexi (top) and increase in chloride concentrations (bottom) during amendments 1 (left), 2 (center), and 3 (right). Symbols are as follows: ■, organochlorine amended; ▲, organic matter control (received organochlorines only for amendment 2 and organic matter for amendments 1 and 3); ●, unamended. The amount of organic matter used for each of the amendments was equivalent to that used for the others, regardless of whether it was treated with chloroperoxidase. Error bars are the standard errors between mean values of triplicate reactors.