Fossil bacteria in Cenomanian–Turonian phosphate nodules and coprolites, Bohemian Cretaceous Basin, Czech Republic

Phosphatized biomorphs, resembling modern and ancient bacteria, were identified for the first time in phosphate nodules, present at the base of the Bílá Hora Formation (uppermost Cenomanian - lower Turonian), and in phosphate coprolites at the base of the Teplice Formation (upper Turonian) in the Bohemian Cretaceous Basin. They are present in colonies as filaments, coccoids, strings, rods and outgrowths, associated with the phosphate as part of the rock constituents and display the characteristics of fossilized bacteria. Two types of bacteria were identified: chemotrophic, sulphur-reducing bacteria in the phosphate nodules and phototrophic cyanobacteria in the phosphate coprolites. Microanalysis of some of the fossil bacteria revealed a fluoride-rich calcium phosphate composition, compatible with the composition of bulk samples, in which carbonate-fluorapatite is the main mineral in the phosphate nodules and coprolites. The environmental indications of these fossil bacteria support the interpretation of an anoxic environment of phosphogenesis in the latest Cenomanian - earliest Turonian and variable redox conditions of coprolite phosphatization in the late Turonian. The potential microbial role in phosphogenesis in the former may have involved the suboxic breakdown of P-rich organic matter by sulphur-reducing bacteria and the release of phosphorus in the pore water, leading to the biochemical precipitation of phosphate. The latter involved initial P-storage by phototrophic bacteria in an oxic environment, followed by P-release below the sediment-water interface under suboxic conditions and subsequent phosphatization of the coprolites.