Biomineralization processes in modern calcareous tufa: Possible roles of viruses, vesicles and extracellular polymeric substances (Corvino Valley – Southern Italy)

A modern lithifying biofilm, associated with fluvial calcareous tufa in southern Italy, is composed of a complex community of micro‐organisms including autotrophic and heterotrophic bacteria, together with likely viruses (bacteriophages) and bacterial vesicles. Produced by most of the bacterial forms, extracellular polymeric substances are widespread and are the main sites of mineral precipitation, rather than the microbial cells themselves, but viruses and vesicles also appear to be mineralized. Neoformed biominerals replace these organic substrates starting with an amorphous phase rich in Ca, Si and several other cations followed by two different types of proto‐crystal: a granular−irregular Ca‐rich type and a lamellar/fibrous Si‐Al‐Mg‐rich type. Their growth leads to the formation of Ca‐carbonate and Mg‐silicate crystals that preserve their initial different habits. The coeval precipitation of silicate and carbonate at the initial stage of bio‐mediated mineral formation is a process common within freshwater and saline to hypersaline‐water calcareous microbialites. This is most probably related to the initial formation of poorly‐crystallized Mg‐Si phases that precipitate within the extracellular polymeric substances, related to the rise of pH in the zone of active oxygenic photosynthesis, and the mineralization of viruses, associated with the successive nucleation of Ca‐carbonate in close proximity to bacterial degradation of organic matter.