Liquid polymer enhances methanogenesis and restructures prokaryotic communities in freshwater sediments

The widespread use of synthetic hydrophilic polymers, such as polyvinylpyrrolidone (PVP), has raised concerns about their potential effects on environmental biogeochemical processes, yet their impact on sediment ecosystems remains largely unexplored. We investigated how PVP influences methane (CH ₄ ) production and prokaryotic community composition in freshwater sediments over a 56 d anoxic incubation. PVP exposure accelerated the onset of methanogenesis, increased maximum CH ₄ production rates, and elevated maximum CH ₄ concentrations. Notably, a substantial proportion of the response occurred already at environmentally relevant concentrations. These functional changes were accompanied by shifts in bacterial communities, particularly among fermentative taxa known to produce key methanogenic substrates. In contrast, archaeal communities, including methanogens, exhibited comparatively weak and transient responses. The concomitant increase in CO ₂ and CH ₄ production and the observed microbial community shifts suggest that PVP altered microbial carbon-processing pathways, although the mechanisms responsible for these effects remain unresolved. As a proof-of-concept study, our results provide initial evidence that a widespread synthetic hydrophilic polymer can modify sediment microbial communities and stimulate methane production. These findings highlight the need for future studies addressing the mechanistic basis of these responses and their relevance under environmentally realistic conditions.