1. Marine and Environmental Biology, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089, United States
2. Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, United States
3. Blue Marble Space Institute of Science, Seattle, Washington 98104, United States
4. Exobiology Branch, NASA Ames Research Center, Moffett Field, California 94035, United States