1. Cooperative Institute for Research in Environmental Sciences, UCB 216, University of Colorado, Boulder, Colorado 80309 USA
2. Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado 80309 USA
3. W. K. Kellogg Biological Station, Michigan State University, 3700 East Gull Lake Drive, Hickory Corners, Michigan 49060 USA
4. Department of Microbiology & Molecular Genetics, Michigan State University