Shifting biogeochemical conditions and capping media impact composition and behavior of PAH-enriched microbial communities

Purpose Release of polycyclic aromatic hydrocarbons (PAH) can impact surface water and overlaying sediments. Contaminant control strategies include in situ containment technologies like capping, which consists of placing layers of inert or reactive media on top of the impacted sediment. Cap placement alters the sediment environment and can influence natural processes such as biodegradation of PAH. This study examined the combined influence of capping media (sand, clay, and granular activated carbon (GAC)) and reducing conditions on the PAH biodegradation potential of benthic microbial communities. Materials and methods Microcosms containing naphthalene (a model PAH), benthic microbial communities enriched from PAH-impacted sediments, and capping materials were prepared to mimic sediment environments under different redox conditions (nitrate, iron, and sulfate). Microcosms were monitored for 100 days to assess changes in microbial community composition using 16S rRNA gene surveys and naphthalene biodegradation rates. Results and discussion Multivariate analyses showed that microbial community shifts were significant with varying capping materials and redox conditions (PERMANOVA p < 0.05). Microorganisms linked to PAH biodegradation were enriched, and the increased abundance of these genera was positively correlated (PERMANOVA p < 0.05 ), indicating the formation of consortia with naphthalene biodegradation potential. Enrichment of microorganisms associated with PAH biodegradation was most pronounced in microcosms where naphthalene transformation rate ratios were the highest. Conclusion Capping media promote the development of consortia with naphthalene biodegradation potential and affect the extent to which in situ redox conditions impact biodegradation. These findings provide insights on the relationships between capping materials and indigenous microbial communities and suggest that strategic capping material selection and placement can enrich desired communities at contaminated sites under a wide range of electron-accepting conditions.