A microbial seedbank from a natural oil seep accelerates hydrocarbon degradation in freshwater oil spills

Anthropogenic oil spills are a major source of hydrocarbon pollution, yet freshwater oil spills are studied less than marine once. We hypothesize that microorganisms from freshwater natural oil seeps are adapted to use oil as carbon source and could serve as a seedbank to accelerate degradation of anthropogenic oil spills. To test this, microcosms were set up using water and oil from a natural oil seep in Germany, while light oil and oil naïve river water simulated an anthropogenic spill. Seven conditions combined different water types (adapted, naïve) and oil types (heavy, light, mixed). Oil degradation was monitored over 81 days via reverse stable isotope labeling, and pH and microbial communities were analyzed by 16S rRNA gene amplicon sequencing at the start and end. Microcosms with adapted microbes exhibited higher oil mineralization rates. In mixed-oil incubations, simulating the application of a microbial seedbank, degradation rates were 0.26 mM/day with adapted microbes versus 0.15 mM/day in naïve water. Applying seep-derived microbes to naïve light-oil microcosms increased hydrocarbon mineralization by 32%. These results suggest that leveraging natural microbial seedbanks could accelerate oil weathering and reduce the environmental impact of freshwater oil spills, offering a promising strategy for spill mitigation if applied safely. Importance Freshwater oil spills remain poorly studied despite their substantial contribution to global hydrocarbon pollution. This study demonstrates that microbial communities from freshwater natural oil seeps are pre-adapted to degrade crude oil and can significantly enhance the breakdown of anthropogenic oil spills. Applying such microbial seedbanks increased hydrocarbon mineralization by over 30%, highlighting their potential as an effective and environmentally relevant tool for oil spill mitigation. These findings support the development of nature-based strategies to accelerate oil weathering and reduce the environmental impact of freshwater oil spills.