14C-dead living biomass:: Evidence for microbial assimilation of ancient organic carbon during share weathering

Prokaryotes have been cultured from a modern weathering profile developed on a similar to 365-million-year-old black share that use macromolecular share organic matter as their sole organic carbon source. Using natural-abundance carbon-14 analysis of membrane Lipids, we show that 74 to 94% of Lipid carbon in these cultures derives from assimilation of carbon-14-free organic carbon from the share. These results reveal that microorganisms enriched from share weathering profiles are able to use a macromolecular and putatively refractory pool of ancient organic matter. This activity may facilitate the oxidation of sedimentary organic matter to inorganic carbon when sedimentary rocks are exposed by erosion. Thus, microorganisms may play a more active role in the geochemical carbon cycle than previously recognized, with profound implications for controls on the abundance of oxygen and carbon dioxide in Earth's atmosphere over geologic time.