LIPID CHARACTERIZATION OF MICROBIAL COMMUNITIES IN GROUNDWATER ADJACENT TO A FRESHWATER LAKE IN NORTH-CENTRAL IOWA

Methane is a greenhouse gas that has a global warming potential of 23 times that of CO₂. Agricultural development has generated increasing nutrient loading (mainly nitrogen and phosphorus) in surface- and groundwaters and has accelerated biological methane cycling (microbial production and metabolism) on a global scale. For this reason, there has been growing interest in studying methane-oxidizing bacteria in soil, groundwater and surface water. In this study, we used a combined approach of lipid biomarkers and stable carbon isotopic composition of individual biomarkers to characterize the microbial community structure of groundwater adjacent to a freshwater lake in north-central Iowa. Clear Lake, the third largest of 34 natural lakes in Iowa, has a surface area of 1,450 ha, a maximum depth of 5.8 m, and an average depth of 2.9 m. Previous and ongoing studies have documented strongly reducing conditions and presence of methane in groundwater entering and exiting the lake. Preliminary sampling of groundwater from till and outwash suggested the presence of methanotrophic bacteria. Phospholipid fatty acids detected include abundant 16:1 with double bond positions at δ8, δ10, and δ11, suggesting the presence of type I methanotrophic bacteria in the ground water. A number of hopenes and hopanes (hop-17(21)-ene, 21-ene, 22(29)-ene or diploptene, and 2-methylhopane), 3β-hopanol, and diplopterol were also identified, further indicating the contribution of methanotrophic bacteria. The stable carbon isotope values of the 16:1 fatty acids and hopanoids ranged from -51.0 to -85.8 and from -76.8 to -93.6‰, respectively, reflecting the utilization of biogenic methane by methanotrophic bacteria. Overall, microbial biomass is high but diversity is low.