Evaluating the influence of capping materials on composition and biodegradation activity of benthic microbial communities: Implications for designing bioreactive sediment caps

Sediment contamination by polycyclic aromatic hydrocarbons represents a chronic environmental issue and challenges remediation practices. Capping placement contains contamination and minimizes risk of exposure to the benthic community. Biotransformation within sediment caps has the potential to degrade toxic compounds improving efficiency of the remedy. However, little is known about biodegradation within sediment caps. The goal of this work was to elucidate the impact of capping materials on composition and activity of benthic microbial communities using naphthalene as a model contaminant. Specific aims were to: i) summarize current literature and identify gaps of knowledge; ii) examine how biogeochemistry influences the biodegradation of naphthalene in different capping amendments; iii) describe the role of adsorption/desorption and biodegradation processes; iv) ultimately identify capping materials that can promote biodegradation. Microcosms were prepared containing capping materials (sand, organoclay, and activated carbon), sediment enrichments, naphthalene, and terminal electron acceptors. Results and multivariate analyses showed that microbial community shifts were significant with respect to both capping amendments and biogeochemistry for all geochemical conditions examined. Microorganisms linked to PAH biodegradation were enriched under appropriate electron accepting conditions, and the increased abundance positively correlated between genera, suggesting the formation of biodegrading consortia. Under oxic conditions, biofilm formation, naphthalene decay and biomarker levels increased with activated carbon, and naphthalene exerted a strong selective pressure in all systems except clay, where attached growth was not enhanced. Activated carbon stimulated naphthalene biodegradation that would otherwise not occur under strictly sulfate reducing conditions, and mineralization corresponded with enrichment of genera that biodegrade naphthalene (Geobacter and Desulfovirga spp.). Results of this study showed that capping materials influence indigenous microorganisms and can stimulate biodegradation under highly reducing conditions, ultimately providing recommendations for designing bioactive caps.