The distribution of microbial communities in anaerobic and aerobic zones of a shallow coastal plain aquifer

Randomly amplified polymorphic DNA (RAPD) fingerprinting was used to determ ine the genetic similarity of whole-community DNA extracts from unattached microorganisms in several groundwater wells. The study site was a shallow c oastal plain aquifer on the Eastern Shore of Virginia that contains distinc t regions of anaerobic and aerobic groundwater. Several wells in each regio n were sampled, and principal component and cluster analyses showed a clear separation of the microbial communities from the two chemical zones of the aquifer. Within these zones, there was no relationship between the genetic relatedness of a pair of communities and their spatial separation. Two add itional sets of samples were taken at later times, and the same clear separ ation between communities in the different zones of the aquifer was observe d. The specific relationships between wells within each zone changed over t ime, however, and the magnitude and direction of these changes corresponded to concurrent changes in the groundwater chemistry at each well. Together, these results suggest that local variation in groundwater chemistry can su pport genetically distinct microbial communities, and that the composition of the microbial communities can follow seasonal fluctuations in groundwate r chemistry.