CHARACTERIZING BACTERIAL COMMUNITIES IN BEAVER PONDS FROM THE AVERY BROOK WATERSHED, WHATELY, MASSACHUSETTS

Mercury is a global pollutant toxic to organisms. Compared to elemental mercury, which cycles through systems, methylmercury bioaccumulates and increases in concentration at higher trophic levels. Sulfate-reducing bacteria (SRB) are principal mercury methylators. Ponds created by North American beaver, Castor canadensis, have a major environmental impact on watersheds and provide the anaerobic sediment-water interface required for the growth of SRB. Avery Brook, a 756 hectare watershed in West Whately, Massachusetts, is one such setting. In this study, we characterize the sulfate-reducing communities in several Avery Brook beaver ponds, investigate species variation between the inlets and outlets of each pond, and correlate different species to sediment-water interface mercury levels. The dissimilatory sulfite reductase (dsr) gene, responsible for sulfur reduction (unique to sulfate-reducing species) is diagnostic and can therefore be used to distinguish SRB from the greater bacterial community. Bacterial DNA was extracted from sediment samples of various beaver ponds and analyzed using polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE), a technique that denatures PCR products of different species and separates them according to their AT/GC ratio. This produces bands along an acrylamide gel, which are then sequenced to identify species and find commonalities between sites. Bacterial communities from each site were compared, as previous studies have found that sites within the same pond share more species than sites between ponds. Previous studies of beaver pond sediments have found high diversity in SRB, with 85.2% identified as novel species. Similarly, in our study, a variety of previously unknown SRB were found. In contrast to the prior studies, which collected samples from different micro-environments, here bacteria were obtained from pond inlets and outlets, contributing to the diversity of species found and facilitating a comparison with mercury levels.