Correlation of Respiratory Gene Expression Levels and Pseudo-Steady-State PCE Respiration Rates in Dehalococcoides ethenogenes

DNA and RNA transcripts, particularly of genes of functional importance in the reductively dechlorinating microbe Dehalococcoides, are increasingly being studied as potential molecular bioindicators of reductive dechlorination. Ideally, mRNA bioindicators would be informative both qualitatively (with respect to dechlorination end point and substrate range) and quantitatively (with respect to activity rates). Here, we examined pseudo-steady-state mRNA levels in Dehalococcoides-containing microcosms continuously fed PCE at various loading rates. We characterized gene transcript abundance of potential Dehalococcoides bioindicators of reductive dechlorination, including 16S rRNA, and genes encoding an annotated formate dehydrogenase (Fdh), the hydrogenase (H₂ase) Hup, and the reductive dehalogenases (RDases) TceA, DET1559, PceA, and DET1545. Increases in steady PCE loading rate led to corresponding increases in PCE respiration rate (1.5 ± 0.1, 2.5 ± 0.3, 4.8 ± 0.1, and 9.2 ± 0.5 µmol/L/hr). We also observed that pseudo-steady-state expression levels of most functional targets increase linearly over PCE respiration rates of 1.5−4.8 µmol/L/hr, with Fdh, Hup, and TceA transcripts increasing by approximately 2 × 10¹⁰ copies per mL of culture for every µmol/L/hr increase in chloroethene respiration rate, and DET1559 and PceA transcripts increasing by approximately 9 × 10⁹ copies per mL of culture, but that increased respiration rates of 9.2 µmol/L/hr did not necessarily lead to corresponding increases in transcript levels.