FORMATION OF MODERN CYANOBACTERIA CALCIFIED-MICROFOSSILS, BAHAMAS

Calcified cyanobacteria fossils are a common component of marine carbonate reefs and sediments from 540 to 300 million ago, but appear to be absent from normal marine settings since the late Cretaceous. Here we report the finding of calcified cyanobacterial filaments in a modern marine setting. Carbonate precipitation takes place in photosynthetically active cells without any signs of filament or mucilaginous sheath degradation. Specifically, the precipitation occurs in the interspace between the mucilaginous sheath and filaments and within the mucilaginous sheath, areas which experience significant increases in carbonate saturation during photosynthesis due to carbon dioxide uptake. The presence of more positive 13δ carbon isotope values in the filament precipitates than in bulk samples or associated micritic precipitates provides further evidence for carbonate precipitation in a microenvironment influenced by photosynthetic carbon dioxide uptake. A signal variety of cyanobacteria, identified as Dichotrix, is producing the calcified filaments. The absence of calcification in other sheathed cyanobacteria at Highborne Cay - which presumably experience similar levels of photosynthetically driven supersaturation – suggests that the variation in availability of nucleation sites also influences calcification. The distinct calcification styles in different cyanobacteria at Highborne Cay indicates a strong physiological influence on cyanobacteria calcification. The Highborne Cay model-microbialite system supports that the variability in microbial communities and the evolutionary history of cyanobacteria are one of the factors causing spatially and temporally disparate microbialite fabrics.