Microbes Below and Above: Donna Fennell’s Legacy of Remediation Science
- Ray Sullivan
- 3 days ago
- 4 min read
At the 2025 Theobald Smith Society Spring Symposium, Professor Donna Fennell, Chair of the Department of Environmental Sciences at Rutgers University, delivered an inspiring and wide-ranging 72nd Waksman Honorary Lecture. With warmth, humor, and scientific rigor, Dr. Fennell walked the audience through a remarkable career focused on microbial remediation—from chlorinated solvents in groundwater to dioxins in river sediments and even microbial activity in the atmosphere.
The Ground Beneath Our Feet: Chlorinated Solvents and Groundwater Remediation
Fennell opened with the concept of the “malenclave”—pockets of groundwater too contaminated for use, a term coined in 1965 by Henry Legrand. These zones, she explained, are disturbingly common, even beneath and around Rutgers' Cook and Douglass campuses. Chlorinated solvents like trichloroethylene (TCE) and perchloroethylene (PCE), often used in dry cleaning and metal degreasing, are key culprits. Because these chemicals are denser than water, they sink deep into aquifers, dissolve slowly, and migrate through fractures in rock or clay, making them difficult to detect and remove.
In the 1980s, conventional wisdom held that chlorinated ethenes were recalcitrant. Early remediation strategies like pump-and-treat systems using activated carbon often failed, largely due to biofouling by microbial communities—ironically, a sign that biology might offer solutions.
Harnessing Microbial Power: Anaerobes and Methanotrophs
Fennell recounted how microbial degradation of chlorinated solvents was first discovered in methanotrophic and anaerobic organisms. While methanotrophs can co-metabolize compounds like TCE under aerobic conditions, it was the discovery of obligate organohalide-respiring bacteria, such as Dehalococcoides mccartyi, that changed the remediation landscape.
These organisms use chlorinated compounds as electron acceptors and hydrogen as an electron donor—essentially “breathing” pollutants. Fennell's doctoral work helped reveal how fermentation of specific organic substrates (like butyrate or lactate) could supply just the right level of hydrogen to stimulate dechlorinators without encouraging methane production, which could be dangerous in confined environments.
She emphasized the importance of Dehalococcoides strain 195, the first isolate shown to completely dechlorinate TCE to ethene—a non-toxic end product. This discovery laid the foundation for modern bioremediation practices, including commercial products that deliver electron donors or microbial cultures to polluted aquifers.
Tackling Tougher Pollutants: Dioxins and Furans
Shifting from groundwater to sediments, Fennell described her work with Max Häggblom on the microbial degradation of polychlorinated dibenzo-para-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). These highly toxic, hydrophobic compounds accumulate in river sediments and bioaccumulate up the food chain.
One of the most infamous sites is the lower Passaic River in Newark, NJ—contaminated by Agent Orange byproducts during herbicide manufacturing. Incredibly, sediment stored for years in a refrigerator still harbored microbes capable of dechlorinating TCE and model dioxins. Fennell’s lab showed that even ultra-toxic congeners like 2,3,7,8-tetrachlorodioxin could be slowly transformed by sediment microbial communities. Key organisms included Dehalogenimonas, trace amounts of Dehalococcoides, and a possibly novel organohalide-respiring bacterium.
This research continues through a Superfund project with Gerben Zylstra and Michigan State University, targeting the Tittabawassee and Saginaw Rivers, where similar compounds persist.
From the Ground to the Sky: Microbes in the Atmosphere
In a striking pivot, Fennell described her foray into atmospheric microbiology in collaboration with Gedi Mainelis and Lee Kerkhof. Inspired by the question, “What do airborne microbes actually do?”, her lab began investigating microbial activity in air.
They built rotating “air microcosms” from 55-gallon drums to suspend aerosolized bacteria and studied whether these airborne communities remained metabolically active. Their work showed that some organisms, like Sphingomonas aurantiaca, could synthesize ribosomes in air when fed, indicating potential for metabolic activity.
Most notably, using stable isotope probing, her team demonstrated that airborne methanotrophs can incorporate methane-derived carbon into their DNA—direct evidence that microbes are active not only in soil and water, but also in the open atmosphere. These findings suggest that atmospheric microbial communities may play an underappreciated role in global methane cycling.
Reflections and Future Directions
Fennell’s talk was also a meditation on scientific adaptability. She shared stories of mentors like Bill Jewell and Jim Gossett, who pivoted research focus in response to shifting funding and environmental needs—lessons in resilience for today’s students and early-career scientists.
Her work spans the micro and the macro, the buried and the airborne. Whether tackling carcinogenic compounds in groundwater or studying bioaerosols in clouds, Dr. Fennell’s career exemplifies the power of environmental microbiology to solve complex real-world problems. As she reminded the audience, the microbes are always working—above us, beneath us, and often in ways we’ve only just begun to understand.
You can watch Professor Fennell’s talk at: https://youtu.be/EnDFhQ9MhNM
References
Dillon KP, Krumins V, Deshpande A, Kerkhof LJ, Mainelis G, Fennell DE. Characterization and DNA Stable-Isotope Probing of Methanotrophic Bioaerosols. Microbiol Spectr. 2022 Dec 21;10(6):e0342122. doi: 10.1128/spectrum.03421-22. Epub 2022 Nov 21. PMID: 36409096; PMCID: PMC9769660.
Dean RK, Schneider CR, Almnehlawi HS, Dawson KS, Fennell DE. 2,3,7,8-Tetrachlorodibenzo-p-dioxin Dechlorination is Differentially Enhanced by Dichlorobenzene Amendment in Passaic River, NJ Sediments. Environ Sci Technol. 2020 Jul 7;54(13):8380-8389. doi: 10.1021/acs.est.0c00876. Epub 2020 Jun 11. PMID: 32432863.
Rodenburg LA, Dewani Y, Häggblom MM, Kerkhof LJ, Fennell DE. Forensic Analysis of Polychlorinated Dibenzo-p-Dioxin and Furan Fingerprints to Elucidate Dechlorination Pathways. Environ Sci Technol. 2017 Sep 19;51(18):10485-10493. doi: 10.1021/acs.est.7b02705. Epub 2017 Aug 30. PMID: 28796943.
Krumins V, Fennell DE. Identifying the Correct Biotransformation Model from Polychlorinated Biphenyl and Dioxin Dechlorination Batch Studies. Environ Eng Sci. 2014 Oct 1;31(10):548-555. doi: 10.1089/ees.2013.0463. PMID: 25317036; PMCID: PMC4188385.
Zhen H, Du S, Rodenburg LA, Mainelis G, Fennell DE. Reductive dechlorination of 1,2,3,7,8-pentachlorodibenzo-p-dioxin and Aroclor 1260, 1254 and 1242 by a mixed culture containing Dehalococcoides mccartyi strain 195. Water Res. 2014 Apr 1;52:51-62. doi: 10.1016/j.watres.2013.12.038. Epub 2014 Jan 8. PMID: 24462927.