Neoproterozoic Oxygenation of Earth Surface Environments Reflected in the Late Evolution of the O2-Dependent Vitamin B12 Biosynthesis Pathway
Abstract
There are multiple lines of evidence for a significant rise of O2 in the Earth's atmosphere ~2.4 Ga. A second oxygenation event in the Neoproterozoic is not as well constrained. These changes in environmental redox affected the abundances of bioessential elements. Trace elements such as Co, Fe, and Ni were likely favored in the early evolution of metalloenzymes, prior to the first oxidation event. Consistent with this expectation, vitamin B12 is a Co-containing biomolecule whose biosynthesis is thought to have evolved prior to the origin of oxygenic photosynthesis and the first rise in O2. However, biochemical characterization of the many enzymes involved in B12 biosynthesis has revealed two distinct pathways: an O2-independent pathway and an O2-dependant pathway. The major difference between these pathways involves the timing of the insertion of Co. We examined the amino acid sequences of enzymes in the B12 biosynthesis pathway from a set of 100 phylogenetically diverse microbial genomes, focusing on enzymes exclusive to each pathway as well as enzymes shared by both. Molecular clock and phylogenetic analyses were performed on alignments of the sequences obtained from these study genomes. This approach focused on functional genes rather than the phylogeny of microbes in an attempt to understand the evolution of the pathway itself, rather than its presence in individual phylogenetic groups. Clear differences in age are apparent between representatives of each pathway. The O2-independent pathway and enzymes shared in both pathways show the most ancient last common ancestors. In contrast, the enzymes associated exclusively with the O2-dependent pathway diverged from a common ancestor less than a billion years ago. Phylogenetic analysis suggests that these enzymes were recruited from other biochemical pathways. From these results it seems likely that the evolution of the O2-dependent pathway occurred long after the initial evolution of the B12 biosynthesis. This conclusion provides evidence independent of the geological record as to the timing and existence of a second great oxidation event in Earth history.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.B24C..04S
- Keywords:
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- 0406 Astrobiology and extraterrestrial materials;
- 0444 Evolutionary geobiology;
- 0461 Metals;
- 0465 Microbiology: ecology;
- physiology and genomics (4840);
- 0489 Trace element cycling (4875)