Nitrifying Microorganisms Linked to Biotransformation of Perfluoroalkyl Sulfonamido Precursors from Legacy Aqueous Film-Forming Foams

Drinking water supplies
across the United States have
been contaminated
by firefighting and fire-training activities that use aqueous film-forming
foams (AFFF) containing per- and polyfluoroalkyl substances (PFAS).
Much of the AFFF is manufactured using electrochemical fluorination
by 3M. Precursors with six perfluorinated carbons (C6) and non-fluorinated
amine substituents make up approximately one-third of the PFAS in
3M AFFF. C6 precursors can be transformed through nitrification (microbial
oxidation) of amine moieties into perfluorohexane sulfonate (PFHxS),
a compound of regulatory concern. Here, we report biotransformation
of the most abundant C6 sulfonamido precursors in 3M AFFF with available
commercial standards (FHxSA, PFHxSAm, and PFHxSAmS) in microcosms
representative of the groundwater/surface water boundary. Results
show rapid (<1 day) biosorption to living cells by precursors but
slow biotransformation into PFHxS (1–100 pM day–1). The transformation pathway includes one or two nitrification steps
and is supported by the detection of key intermediates using high-resolution
mass spectrometry. Increasing nitrate concentrations and total abundance
of nitrifying taxa occur in parallel with precursor biotransformation.
Together, these data provide multiple lines of evidence supporting
microbially limited biotransformation of C6 sulfonamido precursors
involving ammonia-oxidizing archaea (Nitrososphaeria) and nitrite-oxidizing bacteria (Nitrospina). Further elucidation of interrelationships between precursor biotransformation
and nitrogen cycling in ecosystems would help inform site remediation
efforts.