PHOSPHATIZED FILAMENTOUS SULFUR BACTERIA IN MODERN AND ANCIENT PHOSPHORITES

Modern authigenic phosphorites form in close association with accumulations of giant vacuolated sulfur bacteria. Recent studies show that polyphosphate utilization by sulfur bacteria results in the rapid precipitation of apatite – providing a mechanism to explain the relationship between authigenic phosphates and sulfur bacteria. Here we characterize sheaths of filamentous sulfur bacteria that are partially encrusted in fluorapatite collected from modern oxygen-minimum zone sediments along the Pacific margin of Costa Rica. Additionally, we present evidence of phosphatized filamentous microfossils from Miocene phosphorites and compare the ancient structures with modern sulfur bacteria known to mediate phosphogenesis. The filamentous microfossils contain sulfur inclusions that we interpret as the diagenetic remnants of elemental sulfur globules. These results suggest that mineralized colorless sulfur bacteria cells and sheath material are an expected result of microbially-mediated phosphatization, and that similar phosphatized microfossils in older phosphorites mark the antiquity of this process. As organisms that live at the interface between oxic and anoxic/sulfidic conditions, the presence of similar fossil sulfur bacteria and/or associated accumulations of phosphate minerals may provide a clue to benthic oxygenation in ancient sediments.