MICROBIAL COMMUNITY STRUCTURE AND MAGNETIC SUSCEPTIBILITY AT THE NATIONAL CRUDE OIL SPILL FATE AND NATURAL ATTENUATION RESEARCH SITE NEAR BEMIDJI, MINNESOTA

The National Crude Oil Spill Fate and Natural Attenuation Research Site near Bemidji, Minnesota has been the focus of research on the bioremediation of crude oil over the last 40 years. The research at this site has resulted in numerous studies on the microbiology, geochemistry, and geophysics of hydrocarbon spills. Nevertheless, no studies at this site have aimed at the long-term monitoring of the microbial community structure associated with the site’s hydrocarbon spill. During the years 2011-2015, we characterized the microbial community in samples from sediment cores acquired from the surface to the oil plume with 16S rRNA gene amplicon next generation sequencing. In addition, we measured the magnetic susceptibility (MS) in situ and in core samples to determine if iron-reducing and hydrocarbon-degrading bacteria were associated with elevated MS in the sediments. We found that the microbial populations grouped together in four zones: the surface/upper vadose, the lower vadose, the smear zone, and the oil plume. The surface/upper vadose zone contained mostly aerobic bacteria. The lower vadose consisted of overlapping methylotrophic, iron reduction, and iron oxidation zones containing bacteria of the genera Methylocystis, Geobacter, Thermincola, and Rhodopseudomonas. The smear zone was a transitional zone that contained microorganisms from both the lower vadose zone and the oil plume. The oil plume itself was dominated by the syntrophic fermenter Smithella, together with the methanogen Methanoregula. Interestingly, the composition of the microbial communities stayed constant over the years, while the MS measured in situ was decreasing. Our microbial analysis will tease apart which microbial groups might be connected with the decreasing magnetic susceptibility values.