An Early-Branching Microbialite Cyanobacterium Forms Intracellular Carbonates

Keep It Inside Some cyanobacteria form solid-phase calcium carbonate precipitates as a consequence of fixing CO 2 during photosynthesis. Usually, such carbonates form extracellularly near the surface of the cells, sometimes generating structures called stromatolites. In a biofilm growing on carbonate deposits in Lake Alchichica, Mexico, Couradeau et al. (p. 459; see the Perspective by Riding ) discovered one species of cyanobacteria that also precipitates amorphous carbonate particles internally. Because the structure and chemical composition of these carbonates is distinct from those formed extracellularly, there may be cellular control over the mineralization process. These precipitates may influence physiological processes such as cell buoyancy and the sequestration of excess alkalinity generated during photosynthesis.