From the inside out: Elemental compositions and mineral phases provide insights into bacterial calcification

Abstract

Microscale analytical methods were used to investigate calcified bacteria and the surrounding CaCO3 crystallites induced by the ureolytic strain Bacillus cereus LV-1. The macroscale crystal aggregates, often with unusual morphologies (e.g., sunflowers to spheres), were found to have bacterial-shaped pores distributed on the surface and/or inside the crystallites. Focused ion beam (FIB) slices were used to examine the cross sections of isolated bacterial precipitates. Elemental composition via energy dispersive spectroscopy (EDS) of the bacterial sections confirmed the existence of organic matter underlying the calcified layer, and selected area electron diffraction (SAED) across these sections found that different mineral phases were present, with the following distribution from the inside to the outside of the mineral shell: amorphous CaCO3 (ACC) → vaterite→ calcite. We propose that in the early stages, the cell surfaces act as templates that either adsorb or nucleate amorphous CaCO3, which then transforms into the more stable crystalline polymorphs toward the periphery where there is less inhibitory exopolymeric substance (EPS). With prolonged experimental time, the calcium carbonate shells on the cell surfaces continued to grow while also accumulating aggregates of other nanoparticles from solution, to fully encase the calcified bacteria within macroscale crystal aggregates. Thus, we propose that calcified bacteria can serve as a kind of structural unit in microbial induced carbonate precipitates (MICPs).