Grosso Giordano, F.; Purino, M.; Boon, N.; De Belie, N.; Rodriguez-Navarro, C.

Microbially-induced calcium carbonate precipitation is a widespread natural phenomenon with numerous applications. Recent advances have shown that bacterial calcium carbonates (BCC) form non-classically via amorphous calcium carbonate (ACC) precursors in the presence of organics, but the role of organics in the formation and nanostructural features of BCCs is not fully understood. Here we show that different bacterial strains produce BCCs with diverse textural and structural features at the macroscale but similar at the micro and nanoscale. We show that bacterial organics guide precipitation of calcite, stabilizing ACC to produce nanogranular crystals and these organics are then trapped within the crystal, rather than being released as previously suggested. These organics are N-rich and create regions of low Z-contrast aligned perpendicular to the c-axis of the crystal, yielding a ``Swiss cheese-like`` mesostructure. Moreover, it is these occluded organics that lead to the distinctive biosignatures observed in BCC. Finally, we also observe crystalline 2D-structures made up of self-assembled bacterial proteins templating the oriented crystallization of calcite. The formation mechanism and the resulting distinctive nanostructural biosignatures of BCC are strikingly similar to those of carbonate biominerals formed by eukaryotic organisms in controlled biomineralization, suggesting a common origin for this crucial biomineralization process.