research project details
Project Title: Biodegradation of Subsurface Pollutants by Chlorate-Respiring
Microorganisms
Investigator(s): Bruce
Logan
Sponsor: National Science Foundation
Project Summary:
Subsurface bioremediation is limited in part by the availability of suitable electron acceptors
for microorganisms capable of degrading target chemicals. Compounds investigated by others (such
as nitrate, sulfate and hydrogen peroxide) have disadvantages that can include: low solubility,
toxicity, low energy yields (resulting in slow growth rates of targeted microorganisms), and/or
long acclimation times. It is proposed to investigate the potential for using chlorate (ClO3-)
as an alternate electron acceptor for subsurface remediation processes coupled with injection of
chlorate reducing microorganisms (CRMs). CRMs are good candidates for bioaugmentation for several
reasons. Because chlorate is not a naturally occurring compound in nature, it is unlikely that
there is large native chlorate-respiring population in-situ that would compete with injected microbes
for chlorate. CRMs have they have high growth rates and high yields (characteristics which are
more comparable to aerobic than anaerobic microbes) which should help them survive in a competitive
soil community. The only byproduct of chlorate is chloride ion, resulting in no long term adverse
effect of addition of chlorate to subsurface environments. It is hypothesized that simultaneous
injection of chlorate and CRMs (acclimate to specific pollutants) into contaminated soils should
result in high specificity of target pollutant degradation since only these targeted microorganisms
will be capable of chlorate-supported growth.
CRMs are likely a subset of denitrifying microorganisms. From comparison with pollutant degradation abilities of denitrifiers, it is hypothesized chemicals degraded by chlorate reducers could include those degraded under denitrifying conditions. Chemicals selected for study that meet this criterion include: toluene, p-xylene, ethylbenzene, naphthalene, chlorophenol, chlorobenzoate, carbon tetrachloride, and phenol; two additional chemicals selected for study that are thought to be persistent under denitrifying conditions are benzene and pentane. To test our hypothesis that CRMs can be found that degrade these pollutants we will attempt to acclimate mixed cultures of CRMs to these chemicals and, if successful, demonstrate their degradative abilities in batch, chemostat, and column studies.
The physiology of microbial chlorate reduction is not well understood, and we also propose to study chlorate respiration to contrast the pathways for electron transport to chlorate with the pathways for molecular oxygen by using a series of respiratory inhibitors. In both cases, reducing equivalents enter the respiratory chain via dehydrogenases, such as NADH dehydrogenase, and are passed sequentially down a chain or carriers that can include iron-sulfur proteins, quinones, and a series of cytochromes. By inhibiting the action of these different carriers at selected points, it is possible to dissect the electron transport chain and compare components that participate in electron transport to oxygen, nitrate and chlorate.
It is envisioned that this research will show that co-injection of chlorate and chlorate+ pollutant-acclimated microbes is a viable method of selectively stimulating the degradation of target pollutants for subsurface bioremediation. Furthermore, the proposed research will also provide fundamental information on the properties of CRMs and the physiology of microbial chlorate respiration.