In the present paper, we examine the influence of micrite types, autochthonous or allochthonous, on the dolomitization processes. The recrystallized and dolomitized Carnian samples from Rifugio Vallandro and Alpe di Specie erratic boulders (South Tyrol, Italy) offer a unique example for studying the relationship between microbialites and dolomitization processes. The comparison between the carbonates of the well-preserved erratic boulders of Alpe di Specie and the isochronous, recrystallized, and dolomitized, samples of Rifugio Vallandro, allows for hypothesizing the role of microbialites on dolomitization processes. The Rifugio Vallandro samples represent variously dolomitized boundstone (made of corals, sponges, and peloidal crusts) with a fine texture (aphanodolomite) which contain organic matter relics, suggesting microbial-mediated mineralization. Geomicrobiological characterization of the microbialites from Alpe di Specie indicates that they formed through microbial metabolic activity of sulfate-reducing bacteria, which thrive on organic matter accumulated in the suboxic to anoxic interspaces of the skeletal framework. Similar processes can be hypothesized for the microbialite precursor of Rifugio Vallandro. Extracellular polymeric substance (EPS) and other organic compounds trapped inside the fine crystal matrix can have a role in the dolomitization processes of the microbialites. High pH and high alkalinity, derived from the degradation of organic matter, may be critical in promoting the dolomitization of microbialites because the high pH increases the concentration and activity of the dissolved CO₃²⁻, thereby increasing the dolomite supersaturation and reaction rates. This process produces very fine dolomite (aphanodolomite) that replaces the original organic-rich micrite, while the fine crystalline dolomite forming larger euhedral crystals seems to derive from the allochthonous micrite due to the presence of a large amount of siliciclastics and the absence of organic remains.

Tidally-influenced subterranean settings represent natural geomicrobiological laboratories, relatively unexplored, that facilitate the investigation of new biomineralization processes. The unusual water chemistry of Zinzulùsa Cave and its oligotrophic and aphotic conditions have allowed the development of a unique ecosystem in which complex bacterial activities induce rare biomineralization processes. A diversified microbial community develops on centimeter-thick crusts that form in the submerged part of the cave. The crusts are formed of Ca-phosphate minerals, mostly carbonate-fluoroapatite (francolite), covered by a black crust, few microns in thickness, composed of ferromanganiferous oxides (hematite and vernadite). Diffuse coccoidal and filamentous bacteria and amorphous organic matter are mixed with the minerals. The micromorphologies and comparative 16S rRNA gene-based metabarcoding analyses identify a "core microbiota" also common to other natural environments characterized by FeMn and Ca-phosphate mineralization. The microbiota is characterized by nitrifying, sulfide/sulfur/thiosulfate-oxidizing and sulfate/thiosulfate/sulfur-reducing bacteria. In addition, manganese-oxidizing bacteria include the recently described "Ca. Manganitrophus noduliformans" and an abundance of bacteria belonging to the Planctomycetes-Verrucomicrobia-Chlamydiae (PVC) superphylum, as well as Haliangiales (fruiting body-forming bacteria) and Hyphomicrobiales (stalked and budding bacteria) that are known to produce extracellular polymers that trap iron and manganese oxides. 16S rRNA gene metabarcoding analysis showed the presence of bacteria able to utilize many organic P substrates, including Ramlibacter, and SEM images revealed traces of fossilized microorganisms resembling "cable bacteria", which may play a role in Ca-phosphate biomineralization. Overall, the data indicate biomineralization processes induced by microbial metabolic activities for both ferromanganiferous oxide and francolite components of these crusts.