Dehalococcoides' gene transcripts as quantitative bioindicators of tetrachloroethene, trichloroethene, and cis-1,2-dichloroethene dehalorespiration rates

Gene transcripts corresponding to 16S rRNA, the hydrogenase (H(2)ase) Hup, a sequence annotated as formate dehydrogenase (Fdh) and reductive dehalogenases (RDases)TceA, PceA, DET1559, and DET1545 in Dehalococcoides ethenogenes strain 195 (DET) hold promise as potential bioindicators of the dehalorespiration of chlorinated ethenes. Here, we present quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) data taken from DET-containing mixed culture microcosms (4.4 x 10(8) DET 16S rRNA gene copies/mL) operated under continuous-feed conditions, with the aim of clarifying the relationships between pseudosteady-state abundances of bioindicator transcripts and respiration rate of various substrates: tetra chloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (cDCE). Results from PCE-fed microcosms suggested an induction threshold for transcription of some bioindicator genes between chloroethene respiration rates of 2.1 and 9.5 mu eeq/L/hr. Putative RDase genes DET1559 and DET1545, however, were up-regulated at low PCE respiration rates and may be functionally significant when substrate levels are low. Data from PCE-fed microcosms operated at saturation kinetics indicated that a high respiration rate was not necessarily associated with a correspondingly high bioindicator transcript abundance. From these microcosms we calculated an approximate yield value of 1.6 x 10(8) 16S rRNA gene copies (cells) per mu mol Cl- released and estimated a k(max) Of PCE respiration of 3 x 10(-9) mu mol Cl- per 16S rRNA gene copy per day. TCE- and cDCE-fed microcosm studies indicated that Fdh, Hup, and TceA were the most abundant transcripts and could make suitable choices as bioindicators of activity for these substrates. Hup transcripts could be positively correlated to respiration rate (between approximately 8 and 45 mu eeq/L/hr) regardless of chloroethene substrate, with transcript levels predicted to increase by 1.8 x 10(9) copies/mL culture for every mu eeq/L/hr increase in respiration rate (R-2 = 0.90). Although RDase transcripts may provide information on substrate range, H(2)ase transcripts may be better indicators of per cell respiration rates.