COULD CYANOBACTERIA HAVE PROVIDED THE SOURCE OF OXIDANTS FOR BANDED IRON FORMATION?

It is generally accepted that Cyanobacteria were the source of oxidants for banded iron-formation. However, recently resolved phylogenetic trees based on whole genomic DNA sequences show that Cyanobacteria were one of the last major lineages to diverge off the bacterial tree. This newly resolved tree shows that sulfur-oxidizing bacteria and sulfate-reducing bacteria arose before Cyanobacteria did. This mirrors the changes in the geochemical record, centered around 2.7 billion years ago. At this time, the isotopic fractionation of sulfur compounds becomes large, followed by the sudden increase in oxygen in the atmosphere and surface water environments at about 2.2 or 2.3 billion years ago. The phylogenetic record of life mirrors the changes that are seen in the geochemical record. If these events are truly correlated, this suggests that Cyanobacteria may have originated rather late in geologic history; perhaps as late as 2.3 billion years ago. At this time, this hypothesis is not strongly supported, because some geologic indicators support it, while others do not. More work needs to be done to either support or refute this hypothesis. However, if it is true, then other potential sources of oxidants for the precipitation of banded iron-formations, other than oxygen produced by Cyanobacteria, need to be explored.