It is thought that morphologies of bacterial carbonates can be used to identify microbial fossils and/or precipitates in sediments and rocks. This study shows that calcite and vaterite formed in a gel medium in the presence of Myxococcus xanthus display a range of morphologies that depend on whether the bacteria are live or dead. Metabolic activity of the bacteria induced: (1) aggregates of calcified bacteria formed at maximum supersaturation; (2) vaterite spheres (final growth stage of dumbbell fibrous-radiated aggregates); and (3) dipyramid- and disphenoid-like calcite crystals (combination of {01̄11} and {0001} forms). Morphologies (2) and (3) developed at a lower supersaturation and are typically found in gel-like media. Dipyrimidal-like calcite crystals were also obtained abiotically in gel medium. Dead M. xanthus cells induced heterogeneous precipitation of calcite with rhombohedral morphologies at low supersaturation. A growth mechanism resulting from self-assembly of calcium carbonate nanocrystals may account for the observed morphologies, crystal microstructure, and crystallite size measurements.

All of the above-mentioned morphologies of bacterial carbonate have been observed in other laboratory experiments and in continental and marine environments. However, all of them have also been produced abiotically, with the exception of calcified bacterial cells. This may make it more difficult to identify bacterial activity in the rock record. Nonetheless, bacterially induced alkalinization appears to be a prerequisite for the development of spherulitic and dipyramid- or disphenoid-like forms in natural mucilaginous biofilms and microbial mats. The morphologies reported here may facilitate the recognition of early and recent marine and continental microcrystalline bacterial carbonates and cements.

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