Evidence for priming-enhanced microbial degradation of polycyclic aromatic hydrocarbons in marine sediments

Priming effect is the enhanced microbial degradation of refractory organic carbon in the presence of labile organic matter. It has been proposed to explain the disappearance of supposedly recalcitrant terrestrial organic matter that reaches the coastal ocean. Here, we experimentally evaluated whether labile organics boosts the degradation of persistent pollutants, phenanthrene and pyrene (polycyclic aromatic hydrocarbons, PAHs). Microcosm incubations were conducted during 11 and 50 days using marine surface sediments from Concepción Bay, a PAH-impacted upwelling ecosystem in central Chile, and the Almirante Montt Gulf, a rather pristine Patagonian Fjord. Addition of yeast extract enhanced the degradation of phenanthrene and pyrene by 14-170 %, while microalgal extract increased the decay rate constant of phenanthrene by 67 %. PAH degradation was accompanied by a pronounced enrichment of hydrocarbonoclastic bacteria, which increased approximately 840-fold (average Log₂FC = 9.7) with rather pristine Patagonian sediments but only 12-fold increase (average Log₂FC = 3.6) in PAH-impacted Concepción Bay sediments. The broader shift in microbial community structure detected in Patagonian fjords coincides with low ambient PAH contents, as expected in unpopulated areas. Considering the long record and intensity of industrial activity off Concepción Bay and the surrounding area, we suggest that those sediments contain a core microbiome with a toolkit of potential enzymes involved in PAH-degradation pathways capable of processing high inputs of PAHs. This finding may imply that coastal marine sediments store microbial capacity for degradation of contaminants induced by a long history of PAH emissions, and this ability can be further enhanced by exposure to primers.