No abstract

To explore the relationships between denitrifying bacteria (DB) and sulfate-reducing bacteria (SRB) in H(2)-fed biofilms, we used two H(2)-based membrane biofilm reactors (MBfRs) with or without restrictions on H(2) availability. DB and SRB compete for H(2) and space in the biofilm, and sulfate (SO(4)(2-)) reduction should be out-competed when H(2) is limiting inside the biofilm. With H(2) availability restricted, nitrate (NO(3)(-)) reduction was proportional to the H(2) pressure and was complete at a H(2) pressure of 3 atm; SO(4)(2-) reduction began at H(2) ≥ 3.4 atm. Without restriction on H(2) availability, NO(3)(-) was the preferred electron acceptor, and SO(4)(2-) was reduced only when the NO(3)(-) surface loading was ≤ 0.13 g N/m(2)-day. We assayed DB and SRB by quantitative polymerase chain reaction targeting the nitrite reductases and dissimilatory sulfite reductase, respectively. Whereas DB and SRB increased with higher H(2) pressures when H(2) availability was limiting, SRB did not decline with higher NO(3)(-) removal flux when H(2) availability was not limiting, even when SO(4)(2-) reduction was absent. The SRB trend reflects that the SRB's metabolic diversity allowed them to remain in the biofilm whether or not they were reducing SO(4)(2-). In all scenarios tested, the SRB were able to initiate strong SO(4)(2-) reduction only when competition for H(2) inside the biofilm was relieved by nearly complete removal of NO(3)(-).

No abstract

A novel anaerobic consortium, named DehaloR^2, that performs rapid and complete reductive dechlorination of trichloroethene (TCE) to ethene is described. DehaloR^2 was developed from estuarine sediment from the Back River of the Chesapeake Bay and has been stably maintained in the laboratory for over 2 years. Initial sediment microcosms showed incomplete reduction of TCE to DCE with a ratio of trans- to cis- isomers of 1.67. However, complete reduction to ethene was achieved within 10 days after transfer of the consortium to sediment-free media and was accompanied by a shift to cis-DCE as the prevailing intermediate metabolite. The microbial community shifted from dominance of the Proteobacterial phylum in the sediment to Firmicutes and Chloroflexi in DehaloR^2, containing the genera Acetobacterium, Clostridium, and the dechlorinators Dehalococcoides. Also present were Spirochaetes, possible acetogens, and Geobacter which encompass previously described dechlorinators. Rates of TCE to ethene reductive dechlorination reached 2.83 mM Cl- d(-1) in batch bottles with a Dehalococcoides sp. density of 1.54E+11 gene copies per liter, comparing favorably to other enrichment cultures described in the literature and identifying DehaloR^2 as a promising consortium for use in bioremediation of chlorinated ethene-impacted environments.

A study with H(2)-based membrane biofilm reactors (MBfRs) was undertaken to examine the effectiveness of direct H(2) delivery in ex-situ reductive dechlorination of chlorinated ethenes. Trichloroethene (TCE) could be reductively dechlorinated to ethene with up to 95% efficiency as long as the pH-increase effects of methanogens and homoacetogens were managed and dechlorinators were selected for during start-up by creating H(2) limitation. Based on quantitative PCR, the dominant bacterial groups in the biofilm at the end of reactor operation were Dehalococcoides, Geobacter, and homoacetogens. Pyrosequencing confirmed the dominance of the dechlorinators and identified Acetobacterium as the key homoacetogen. Homoacetogens outcompeted methanogens for bicarbonate, based on the effluent concentration of acetate, by suppressing methanogens during batch start-up. This was corroborated by the methanogenesis functional gene mcrA, which was 1-2 orders of magnitude lower than the FTHFS functional gene for homoacetogens. Imaging of the MBfR fibers using scanning electron microscopy showed a distinct Dehalococcoides-like morphology in the fiber biofilm. These results support that direct addition of H(2) can allow for efficient and complete reductive dechlorination, and they shed light into how H(2)-fed biofilms, when operated to manage methanogenic and homoacetogenic activity, can be used for ex-situ bioremediation of chlorinated ethenes.