Earthworm granules: A model of non-classical biogenic calcium carbonate phase transformations

Different non-classical crystallization mechanisms have been invoked to explain structural and compositional properties of biocrystals. The identification of precursor amorphous nanoparticle aggregation as an onset process in the formation of numerous biominerals (crystallization via particle attachment) constituted a most important breakthrough for understanding biologically mediated mineralization. A comprehensive understanding about how the attached amorphous particles transform into more stable, crystalline grains has yet to be elucidated. Here, we document structural, biogeochemical, and crystallographic aspects of the formation as well as the further phase transformations of the amorphous calcium carbonate particles formed by cultured specimens of earthworm Lumbricus terrestris. In-situ observations evidence the formation of proto-vaterite after dehydration of earthworm-produced ACC, which is subsequently followed by proto-vaterite transformation into calcite through nanoparticle attachment within the organic framework. In culture medium spiked with trace amounts of Mn²⁺, the cauliflower-like proto-vaterite structures become longer-lived than in the absence of Mn²⁺. We propose that the formation of calcite crystals takes place through a non-classical recrystallization path that involves migration of proto-vaterite nanoparticles to the crystallization site, and then, their transformation into calcite via a dissolution-recrystallization reaction. The latter is complemented by ion-by-ion crystal growth and associated with impurity release. These observations are integrated into a new model of the biocrystallization of earthworm-produced carbonate granules which highlights the sensibility of this process to environmental chemical changes, its potential impact on the bioavailability of contaminants as well as the threat that chemical pollution poses to the normal development of its early stages. STATEMENT OF SIGNIFICANCE: Understanding the mechanisms of nucleation, stabilization and aggregation of amorphous calcium carbonate (ACC) and factors controlling its further transformation into crystalline phases is fundamental for elucidation of biogenic mineralization. Some species of earthworms are natural workbench to understand the biogenic ACC, stabilization and the transformation mechanisms, because they create millimeter-sized calcareous granules from amorphous calcium carbonate, which crystallize to a more stable mineral phase (mostly calcite). This study undergoes into the mechanisms of ACC stabilization by the incorporation of trace elements, as manganese, and the ulterior precipitation of calcareous granules by a coupled process of amorphous particle attachment and ion-by-ion growth. The study points to sensibility of this process to environmental chemical changes.