1. Department of Biological Sciences, Duquesne University, Pittsburgh, PA 15202-1521, USA
2. Department of Microbiology, University of Massachusetts, Amherst, MA 01003, USA
3. Water Resources Division, US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA
4. Nicholas School of the Environment, Duke University, Durham, NC 27708, USA