Perfluoroalkyl acids and microorganisms: implications for subsurface transport and microbial processes

Abstract

Perfluoroalkyl acids (PFAAs) are contaminants of emerging concern found throughout the environment. The interactions between subsurface microbiological process and PFAAs are largely unknown. Similarly, the effects of active microorganisms on PFAA transport are not well understood. This work explores these interactions by assessing co-contaminant biodegradation in the presence of PFAAs, shifts in microbial ecosystems, and stress-related effects on select microorganisms. Additionally, transport characteristics of PFAAs in the presence of pure cellular material and active microbiology are addressed. PFAAs are often found in aqueous film-forming foams (AFFF) used for fire suppression and often co-occur in groundwater with chlorinated solvents and BTEX compounds (benzene, toluene, ethylbenzene, and xylene). Here we show that reductive dechlorination by a methanogenic, mixed culture was significantly inhibited when exposed to concentrations representative of PFAA source zones (>66 mg/L total of 11 PFAA analytes, 6 mg/L each). Significant repression (8-fold decrease in abundance) of the pivotal reductive dechlorinator Dehalococcoides corresponded to an enhancement of methane-generating Archaea within the community (9-fold increase). Growth and dechlorination by axenic cultures of Dehalococcoides mccartyi strain 195, which can completely dechlorinate TCE to non-toxic ethene, were similarly repressed under these conditions. These results suggest that enhanced reductive dechlorination of chlorinated solvents could be impeded in subsurface environments. This work also addresses the effects of PFAAs on biodegradation of toluene. No effect on toluene degradation rate or induction time was observed when active cells of Rhodococcus jostii strain RHA1 were exposed to toluene and a mixture of PFAAs at concentrations of 110 mg/L total PFAAs. However, exposure to aqueous PFAA concentrations above 2 mg/L each was associated with enhanced aggregation of bacterial cells and extracellular polymeric substance production. This behavior was accompanied by two- to three-fold upregulation of stress-associated genes, sigF3 and prmA, during growth of this Rhodococcus in the presence of PFAAs. These results suggest that biological responses, such as microbial stress and biofilm formation, could be more prominent than suppression of BTEX biodegradation in subsurface locations where PFAAs occur with hydrocarbon fuels. To address the impacts of microbiological presence of PFAA transport, this dissertation evaluated aqueous sorption coefficients for PFAAs onto cellular material. Calculated logarithmic distribution coefficients (logKd) generally increase with increasing carbon chain length within the range of 2.3 to 4.7, exceeding the published sorption coefficients for sorption to cellular organic matter by nearly an order of magnitude. Microcosms containing soil amended with inactivated bacterial cells at quantities representative of subsurface growth in a biostimulated zone revealed changes in organic carbon normalized distribution coefficients as a function of biomass and analyte. These results demonstrate that PFAAs preferentially sorb to intact bacterial cells over soil-associated organic carbon and that traditional normalization techniques to bulk organic carbon may not accurately predict sorption of all PFAAs in microbially active zones. This phenomenon is a function of carbon chain length: … This information may have significant effects on our ability to predict subsurface fate and transport of PFAAs. Future research is proposed that focuses on upscaling sorption behavior and community ecology by assessing controlled flow-through column scenarios. These systems will evaluate changes in PFAA sorption and desorption as a function of biomass growth, coupled with the monitoring of biofilm production and corresponding advective shifts as a function of PFAA concentration.