Temporal variation of crude and refined oil biodegradation rates and microbial community composition in freshwater systems

Freshwater systems are vulnerable to contamination by oil extraction and transportation. Thus, it is critical to understand how large freshwater ecosystems such as the Great Lakes will respond to released oil. In this study, we investigated differences in the microbial response to oil in the Straits of Mackinac at different times throughout the year and if crude (Bakken) and refined (non-highway diesel) oil exposure differentially altered the microbial community composition and hydrocarbon biodegradation rates. We also investigated the impact of temperature on the microbial response to oil by incubating samples collected in October of 2018 at 23 °C and at 4 °C. Ambient microbial communities differed between sample collection times, with significantly enriched microbial groups present between most sample types. We found significantly different microbial communities between control and oil-amended samples, but no significant differences between either oil type. We found that the bacterial family Solimonadaceae were significantly enriched in all oil-amended microcosms compared to the control microcosms across sampling times. We assessed oil biodegradation using CO2 production as a proxy for hydrocarbon metabolism. We observed a general trend of increased respiration rates with oil amendment compared to the control. No statistically significant differences in daily CO2 production rates existed between the two oil types. These findings suggest that microbial community in the Straits of Mackinac shifts over time even without oil amendment, and that the microbial communities in the Straits of Mackinac are compositionally and metabolically responsive to the presence of varying oil types throughout the year