MICROFOSSIL EVIDENCE FOR 600 MILLION YEARS OF BACTERIALLY-MEDIATED PHOSPHOGENESIS

Phosphorite deposition is relatively uncommon and poorly understood throughout the geologic record. Recent studies indicate that accumulations of Thiomargarita, a giant sulfur-oxidizing bacterium, correlate with increased pore water and mineral phosphorous, thus providing a microbially-mediated mechanism of phosphorite formation. Other sulfur oxidizers (Thioploca, Beggiatoa) are commonly associated with similar environmental conditions, although their potential involvement in phosphogenesis is still under investigation. We have examined two ancient phosphorite deposits, the Neoproterozoic Doushantuo Formation and the Miocene Monterey Formation, and report fossils that are morphologically consistent with members of the colorless sulfur bacteria. Structures from the Monterey Formation phosphorites resemble bundles of filaments studded with small opaque spheres, reminiscent of the sulfur globules characteristically found within sulfur bacteria of the genus Thioploca. The Doushantuo Formation is well known for its diverse assemblage of phosphatized microfossils. Thiomargarita exhibit similar sizes and morphologies to many Doushantuo microfossils, including cell clusters that result from multiple stages of reductive division in three planes, previously interpreted as primitive animal embryos. We propose that Doushantuo phosphogenesis was mediated by these bacteria. In addition to globular fossils, various filamentous forms from the Doushantuo resemble sulfur bacteria (Thioploca/Beggiatoa). We do not suggest that the Doushantuo microbiota is composed entirely of sulfur bacterial remains and the hypothesis posed here does not invalidate the possibility that some Doushantuo globular microfossils are indeed animal embryos. Rather, it provides a viable mechanism for phosphorite deposition. Colorless sulfur bacteria live at benthic interfaces between oxidizing and sulfidic conditions: penetration of oxygen to the sediment-water interface during Neoproterozoic times would have allowed not only for the evolution of benthic eumetazoans, but also for these sulfur bacteria. The Doushantuo and Monterey Formation assemblages suggest that colorless sulfur bacteria evolved during the Neoproterozoic and remained important in phosphorite deposition for the last 600 million years.