1. Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA.
2. Chevron Petroleum Technology Company, Post Office Box 1627, Richmond, CA 94802-0627, USA.
3. Department of Geology and Geophysics, Post Office Box 208109, Yale University, New Haven, CT 06520-8109, USA.
4. School of Chemistry, University of Bristol, Cantock's Close, Bristol, England BS8 1TS.
5. Department of Biology, Indiana University, S.E., 4201 Grant Line Road, New Albany, IN 47150, USA