DEHALOCOCCOIDES AND REDUCTIVE DECHLORINATION OF CHLORINATED SOLVENTS - Bioaugmentation for Groundwater Remediation - page 50

thermodynamic constraints (Schink, 1997 ). Hence, the hydrogen concentration is poised at a
level too low to be used by carbon dioxide-reducing homoacetogens and is used only slowly by
methanogens, while Dhc , carrying out thermodynamically more favorable reductive dechlori-
nation reactions, can outcompete the methanogens for electron donor (Fennell et al., 1997 ;
L¨ffler et al., 1999 ). A disadvantage of these electron donors is that thermodynamically
constrained fermentation processes generally are slow and the organisms oxidizing butyrate
or benzoate also grow slowly. This slow growth may result in extended lag times before
dechlorination is observed and requires long-term incubation (several weeks to months) and
monitoring.
2.2.4 Carbon Source
In addition to the electron donor and acceptor, other nutrients are needed to support Dhc
growth. Acetate, a carbon source for Dhc , is produced from any electron donor supporting
reductive dechlorination, including hydrogen, in microcosms and mixed communities (He et al.,
2002 ). Hence, the addition of acetate is not needed, but acetate is commonly included in medium
formulations at 0.5-5 mM concentrations. Vitamin mixtures are a useful addition to enrich-
ments, especially vitamin B 12 (see below). Vitamins are added in trace concentrations, and thus
should not support growth of contaminating organisms. Low concentrations ( < 10 mg/L) of
undefined organic substrates like yeast extract provide diverse nutrients (but not vitamin B 12 ,
which eukaryotes cannot produce), and may help stabilize the mixed culture by allowing
growth of accessory organisms that consume oxygen and/or provide nutrients to Dhc ;
however, these organisms will then become “contaminants” when isolation is attempted,
and such additions will then need to be eliminated. The addition of higher concentrations of
complex organic nutrients should be avoided because Dhc are adapted to oligotrophic (i.e.,
nutrient-poor) environments with dechlorination and growth inhibited under nutrient-rich
conditions (Maym´ -Gatell et al., 1997 ).
2.2.5 Reducing Agent (Reductant)
Another crucial component of growth media for anaerobes is the reducing agent, which
removes traces of oxygen from the system as well as poises the redox potential. In mixed
cultures, oxygen-consuming organisms can serve these purposes and spare the requirement for
a reducing agent, but as cultures are enriched, the reducing agent becomes increasingly
important. Commonly used reductants include sodium sulfide, L-cysteine, DL-dithiothreitol,
iron sulfide and titanium(III) citrate. These reducing agents can be supplied in combination and
will generate redox conditions suitable for Dhc activity and growth; however, it is best to
determine empirically which reducing agents are most suitable to achieve robust dechlorination
activity in a given culture. An important aspect is the supply of sulfur, a required macronutrient
for all organisms. Some of the synthetic, defined medium recipes described for Dhc pure
cultures do not contain any sulfur. Hence, the use of sulfur-containing reducing agents is
pertinent unless another suitable sulfur source is provided. Unfortunately, sulfur metabolism in
Dhc has not been explored and the type and concentration of a sulfur source for optimal
growth is unknown.