Per- and polyﬂuoroalkyl substances (PFAS), called “forever chemicals”, have strong carbon−ﬂuorine bonds that under typical environmental conditions are not broken down. So, they accumulate. PFAS in contaminated water is typically removed by activated carbon filters that are then incinerated.
Biodegradation as a means to remediate PFAS is currently being explored. However, no organism capable of complete defluorination of PFAS has yet been found. Nonetheless, researchers hope the so-called infallibility principle — that microbes can evolve to degrade essentially any substance — will hold.
One example of the infallibility principle is the degradation of polychlorinated biphenyls (PCB). For years scientists assumed that biodegradation of these tough compounds was impossible until the late 80’s when PCB dechlorination was attributed to bacteria. Dehalococcoides is capable of transferring electrons to PCB, kicking out chloride ions in the process; however, it is much easier to break carbon–chlorine bonds than carbon–fluorine bonds. Also, Dehalococcoides have evolved in the presence of chlorinated compounds, but naturally occurring fluorinated compounds are rare. One naturally occurring fluorinated compound is fluoroacetate, a chemical that can be produced by certain plants. However, fluoroacetate contains only one fluorine atom while PFAS compounds are enveloped by carbon-fluorine bonds.
Investigations into PFAS biotransformation have found microbes that can exploit vulnerable sites in PFAS such as a carbon−hydrogen bonds. For example, aerobic Gordonia can consume the sulfonated portion of a class of PFAS, ﬂuorotelomers, and then follow up by removing a portion of the fluorinated tail of the molecule leaving non-sulfonated perﬂuorinated products that are one or two carbons shorter than the original. Why the defluorination stops after a cycle or two is not known; computations indicate that the defluorination is energetically favorable. Therefore, investigations are now focused on getting the bacterium to repeat the removal of fluorine atoms.
Other vulnerable sites in perﬂuorinated PFAS could serve as sites of biodegradation, such as double bonds. For example, one commercially available microbial culture that includes Dehalococcoides and other bacteria, was able to degrade 90% of two different unsaturated perﬂuorinated molecules within 130 days but left behind all saturated perfluorinated compounds.
As for PFAS compounds that lack vulnerable sites (e.g., perfluorooctanoic acid, PFOA; perfluorooctanesulfonic acid, PFOS), an Acidimicrobium strain, A6, appears capable of their defluorination. The strain’s genome contains some quite novel dehalogenases, and when fed either PFOA or PFOS as the sole carbon source, the molecules steadily disappeared over 100 days while as dissolved organic carbon and fluoride ions appeared in their place. The mechanisms of A6’s defluorinating power is currently being investigated.
To make the possibility of PFAS bioremediation a reality, the challenges of the slow growth of microbes that inefficiently defluorinate must be overcome. But investigators, and we at Microbial Insights, are hopeful: bacteria evolve fast and have huge potential.
Adapted from: Lim, X., Can Microbes Save Us from PFAS? ACS Cent Sci, 2021. 7(1): p. 3-6.