Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 108, Issue 33, Pages 13409-13414Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1104160108
Keywords
biomarker; photosynthesis; sterols; hypoxia; ocean
Categories
Funding
- National Aeronautics and Space Administration Astrobiology Institute
- Agouron Institute
- Howard Hughes Medical Institute (HHMI)
- Office of Naval Research
- National Science Foundation
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The power of molecular oxygen to drive many crucial biogeochemical processes, from cellular respiration to rock weathering, makes reconstructing the history of its production and accumulation a first-order question for understanding Earth's evolution. Among the various geochemical proxies for the presence of O-2 in the environment, molecular fossils offer a unique record of O-2 where it was first produced and consumed by biology: in sunlit aquatic habitats. As steroid biosynthesis requires molecular oxygen, fossil steranes have been used to draw inferences about aerobiosis in the early Precambrian. However, better quantitative constraints on the O-2 requirement of this biochemistry would clarify the implications of these molecular fossils for environmental conditions at the time of their production. Here we demonstrate that steroid biosynthesis is a microaerobic process, enabled by dissolved O-2 concentrations in the nanomolar range. We present evidence that microaerobic marine environments (where steroid biosynthesis was possible) could have been widespread and persistent for long periods of time prior to the earliest geologic and isotopic evidence for atmospheric O-2. In the late Archean, molecular oxygen likely cycled as a biogenic trace gas, much as compounds such as dimethylsulfide do today.
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