#FEMSmicroBlog: Humic acids help microbes degrade toxins


Perchloroethene is an environmental contaminant toxic to humans. Several microbes can degrade this chloride-containing toxin, albeit with suboptimal efficiency. The study “Enhanced perchloroethene dechlorination by humic acids via increasing the dehalogenase activity of Dehalococcoides strains” published in FEMS Microbiology Ecology explores microbial remediation of this toxin with the help of humic acids. Chaofeng Shen explains for the #FEMSmicroBlog how these compounds impact the microbial enzyme responsible for the effective dechlorination of perchloroethene. #FascinatingMicrobes


Microbial reductive dechlorination

To degrade toxic chlorinated organic compounds, certain species of bacteria use reductive dechlorination. In this reaction, they break C-Cl bonds to release chloride ions under anaerobic conditions.

So far, microbes such as Dehalococcoides were described to be capable of degrading chlorinated chemicals. Interestingly, these microbes can be found in diverse environments. For example, microbial reductive dechlorination has been observed in many soil and aquatic systems as a natural process that does not cause any secondary pollution.


The impact of humic acids on microbial reductive dechlorination

Although microbial remediation is environmentally friendly and cost-effective, its low efficiency limits its application. Previously, dechlorinating organisms could be stimulated in situ with compounds that support their growth and activities. Hence, thanks to microbial remediation, thousands of contaminated sites could be treated and improved.

Yet, the most challenging sites need efficient remediation due to complicated geologies or complex contaminant mixtures. To tackle these contaminations, we need to develop better application approaches.

One possible strategy is to use humic acids. These natural organic mixtures are harmless and highly reactive compounds. Since they represent interesting options for the remediation of chlorinated organic compounds, the study “Enhanced perchloroethene dechlorination by humic acids via increasing the dehalogenase activity of Dehalococcoides strains” published in FEMS Microbiology Ecology explores their impact on microbial reductive dechlorination.


Humic acids affect the reductive dechlorination by Dehalococcoides

The genus Dehalococcoides is the only known bacterium with the ability to reductively dechlorinate the groundwater pollutants perchloroethene and trichloroethene. Now, this bacterium is considered an important player in the restoration of chloroethene-contaminated sites.

This is why the study focuses on the effects of humic acids on mixed Dehalococcoides as well as on pure cultures of Dehalococcoides mccartyi strain CBDB1. Results show that humic acids do not intervene with the dechlorination pathway, however, they accelerate the dechlorination reaction.

The effects of humic acids (HA) and anthraquinone-2,6-disulfonic acid (AQDS) on perchloroethene dechlorination and microbial remediation..
The effects of humic acids (HA) and anthraquinone-2,6-disulfonic acid (AQDS) on perchloroethene remediation. From Wan et al. (2022).

Humic acids transfer electrons between organisms and chlorinated pollutants using their quinone functional groups. To investigate their electron-shuttling properties, this study selected synthetic quinones with the same electron-shuttling effects as model compounds for humic acids.

However, results show that the synthetic quinones inhibit the reductive dechlorination in both mixed and pure cultures of Dehalococcoides. Humic acids and synthetic quinones have thus opposite effects. These findings imply that humic acids do not shuttle electrons from organisms to chlorinated pollutants via their quinone moieties.

Notably, humic acids show no effect on the growth of Dehalococcoides. This led to the hypothesis that humic acids might influence the reductive dehalogenase, which is the key enzyme to reduce chlorinated pollutants. Interestingly, an enzymatic activity assay proved indeed that humic acids increase the activity of reductive dehalogenase.

In summary, the study shows that humic acids accelerate the reductive dechlorination of Dehalococcoides mccartyi via increasing its enzyme activity. This novel strategy could provide inspiration for enhancing the microbial remediation of organic pollution in our environment.


About the authors of this blog

Chaofeng Shen is an associate professor in Department of Environmental Engineering at Zhejiang University. He got his Ph.D. degree in Environmental Engineering with joint training at Zhejiang University and Research Center for Eco-Environmental Science, Chinese Academy of Sciences. Once we visited the Technologiezentrum Wasser (TZW) in Germany as a senior visiting scholar. He is committed to solving the problem of environmental pollution with some nature-friendly technologies.


About this blog section

The section #FascinatingMicrobes for the #FEMSmicroBlog explains the science behind a paper and highlights the significance and broader context of a recent finding. One of the main goals is to share the fascinating spectrum of microbes across all fields of microbiology.

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