1. a Center for Biomarker Analysis , University of Tennessee , Knoxville Tennessee , USA
2. b Environmental Microbiology Group, Pacific Northwest National Laboratory , Richland , Washington , USA
3. c Department of Geosciences , Princeton University , Princeton , New Jersey , USA
4. d Department of Microbial, Biochemical and Food Biotechnology , University of the Free State , Bloemfontein , Free State , Republic of South Africa