Correlation of Dehalococcoides 16S rRNA and chloroethene-reductive dehalogenase genes with geochemical conditions in chloroethene-contaminated groundwater

Abstract

Quantitative analysis of genes that code for Dehalococcoides 16S rRNA and chloroethene-reductive dehalogenases TceA, VcrA, and BvcA was done on groundwater sampled from 150 monitoring wells spread over 11 chlorinated ethene polluted European locations. Redundancy analysis was used to relate molecular data to geochemical conditions. Dehalococcoides 16S rRNA- and vinyl chloride (VC)-reductase genes were present at all tested locations in concentrations up to 10(6) gene copies per ml of groundwater. However, differences between and also within locations were observed. Variation in Dehalococcoides 16S rRNA gene copy numbers were most strongly correlated to dissolved organic carbon concentration in groundwater and to conditions appropriate for biodegradation of chlorinated ethenes (U.S. Environmental Protection Agency score). In contrast, vcrA gene copy numbers correlated most significantly to VC and chlorinated ethene concentrations. Interestingly, bvcA and especially tceA were more correlated with oxidizing conditions. In groundwater microcosms, dechlorination of 1 mM VC was correlated to an increase of vcrA and/or bvcA gene copies by 2 to 4 orders of magnitude. Interestingly, in 34% of the monitoring wells and in 40% of the active microcosms, the amount of individual VC-reductase gene copies exceeded that of Dehalococcoides 16S rRNA gene copies. It is concluded that the geographical distribution of the genes was not homogeneous, depending on the geochemical conditions, whereby tceA and bvcA correlated to more oxidized conditions than Dehalococcoides 16S rRNA and vcrA. Because the variation in VC-reductase gene numbers was not directly correlated to variation in Dehalococcoides spp., VC-reductase genes are better monitoring parameters for VC dechlorination capacity than Dehalococcoides spp.

FIG. 1.

Total amount of Dehalococcoides 16S rRNA gene and specific reductase vcrA (A), bvcA (B), or tceA (C) gene copies, respectively, in groundwater of the individual monitoring wells. Symbols represent the data of monitoring wells from the different locations (⧫, location A; ▪, location B; ▴, location C; •, location D; *, location E; ⋄, location F; +, location G; -, location H; □, location I; ▵, location J; ○, location K). x- and y-axis error bars indicate the standard deviations of the Dehalococcoides 16S rRNA or reductase genes, respectively, and were only visualized when larger than the symbols. Regressions lines (A, y = 13.19x^0.7526 [R² = 0.62]; B, y = 5.64x^0.4977 [R² = 0.33]; C, y = 31.29x^0.2777 [R² = 0.06]) were given for monitoring wells with >10³ 16S rRNA gene copies/ml of groundwater.

FIG. 2.

RDA of geochemical parameters and gene copy numbers. Multivariate analysis was used to explain correlation between geochemical parameters on the concentration of Dehalococcoides 16S rRNA (DHC 16S), vcrA, bvcA, and tceA gene copy numbers. The data are presented as vectors, whereby the bold arrows represent the geochemical parameters (EPA, EPA score; methane, methane concentration; T, temperature; pH, pH; EC, conductivity; VC, VC concentration; ethene, ethene concentration; CAH, total concentration of PCE, TCE, DCE, and VC in units of potential available ethene; SO₄, sulfate; NO₃, nitrate; DOC, dissolved organic carbon; E_h, redox potential; Fe²⁺, concentration of Fe²⁺). The angle between vectors represents correlation of those vectors, whereby vectors pointing in the same direction (angle < 90°) are positively correlated while vectors in opposite direction are negatively correlated. The eigen values of the first (x) and second (y) canonical axes are 0.238 and 0.114, respectively.