The overall objective of this project was to uncover novel ecological relationships among aerobic and anaerobic vinyl chloride (VC)-degrading bacteria, and oxygen-producing bacteria in VC-contaminated groundwater environments. VC, a known human carcinogen, is frequently found at chlorinated ethene contaminated sites because of incomplete anaerobic bioremediation of chlorinated solvents such as tetrachloroethene and trichloroethene. VC-oxidizing bacteria could play a significant role in VC removal in contaminated groundwater systems by either fortuitously oxidizing VC or utilizing VC directly as a carbon and energy source when enough oxygen is present to use as the electron acceptor and reactant. Because anaerobic VC-reducing bacteria generate products useful as substrates for VC-oxidizers, there are possible ecological relationships between these microbial groups near sources of low-level oxygen flux in high VC concentration zones at contaminated sites. We hypothesized that VC oxidation and reduction can occur nearly simultaneously under low dissolved oxygen concentrations, possibly in biofilms containing anaerobic VC-degraders at the core and aerobic VC-degraders at the surface. Furthermore, we also postulated the oxygen-producing bacteria could serve as a source of oxygen for VC-oxidizers in otherwise highly anaerobic environments that favor anaerobic VC-degrading bacteria.

To test these hypotheses, we aimed to demonstrate simultaneous oxidation and reduction of VC in laboratory microcosms using a mixed culture of anaerobic and aerobic VC-degrading bacteria. Next, we investigated spatial relationships between VC-oxidizers, anaerobic VC-dechlorinators, and potential oxygen-producing bacteria in sediment samples from a VC-contaminated site. Finally, we searched for bacteria that convert nitric oxide into nitrogen and oxygen gas in the environment as a way of exploring potential novel oxygen transfer relationships.

There were several significant outcomes of this work. First, by using oxygen permeation tubes we showed that bacteria in contaminated aquifer materials oxidized VC under conditions that are nominally anaerobic. That is, there is little to no measurable oxygen. (https://doi.org/10.1007/s00253-022-12147-y). This is an important ability for VC-oxidizing bacteria to survive under low oxygen conditions in contaminated habitats dominated by anaerobic bacteria. Next, we discovered that a bacterial strain (Nocardioides sp. strain JS614) oxidized VC and several different organic acids (e.g., acetate, propionate, butyrate) simultaneously, while not responding to the presence of lactate (https://doi.org/10.1007/s11356-022-19755-1). This outcome is important because anaerobic microbial communities that dehalogenate VC and other chlorinated ethenes produce organic acids, and lactate is often used to biostimulate these anaerobic communities as a bioremediation strategy. Thus, VC-oxidizing bacteria like JS614 will not be adversely affected by the presence of organic acids. Next, we showed that VC oxidation and reduction occurred simultaneously in a mixed culture of anaerobic VC-degrading bacteria and VC-oxidizing strain JS614 fed VC and low amounts of oxygen (0-1 mg/L). This major finding - that these two bacterial groups can co-exist - holds significant intellectual merit because the current paradigm is that these bacterial groups are incompatible and are well separated from each other in the environment.

We developed a fluorescence microscopy technique for VC-oxidizing bacteria that allows us to see these bacteria in laboratory cultures and environmental samples (https://doi.org/10.1016/j.mimet.2021.106147). This new microbiological method, when combined with other fluorescent microscopy techniques, can be used to visualize spatial relationships between aerobic and anaerobic VC-degrading bacteria in laboratory suspended cultures and/or biofilms and in environmental samples where both groups are known to be present.

The chlorinated solvent bioremediation field is focused on anaerobic reductive dechlorination processes, and the contributions of aerobic bacteria are generally under-recognized by scientists and consultants. Applying the science of microbial ecology in this project has advanced the bioremediation field with respect to clean-up of chlorinated ethene contamination and could ultimately affect bioremediation outcomes. This improved understanding of ecological relationships between VC dechlorinators and VC oxidizers has the potential to foster novel and more effective VC bioremediation strategies and possibly develop new commercial products for bioremediation. For instance, this understanding could lead to decreasing the amount of electron donor added to treatment zones and/or the development of new bioaugmentation cultures containing both aerobic and anaerobic bacteria for use in chlorinated solvent remediation.

The project outcomes described here, because they address anthropogenic deterioration of water quality by xenobiotic compounds, has the potential to benefit society by providing a deeper understanding of the role microorganisms play in ameliorating the negative consequences. Further broader impacts of the work were realized by coordinating with existing educational programs at the University of Iowa to achieve the following objectives: (1) Train undergraduate and graduate environmental engineering students to apply microbial ecology tools to environmental problems, (2) establish bacterial cultures with the potential for commercial applications, (3) develop strategies to more broadly disseminate the research among the bioremediation community and (4) foster the participation of high school students interested in pursuing Science, Technology, Engineering and Math (STEM) education.

Last Modified: 12/20/2022
Modified by: Timothy E Mattes