Abstract

Triassic stromatolitic dolomite from Italy preserves mineralized bacterial remains, one of the first unequivocal identifications of such structures in the geological record. They consist of empty spheroids ∼1.0 µm diameter resembling coccoid bacteria, and smaller, 150–400 nm, objects interpreted as dwarf bacterial forms, occurring within and between syn-sedimentary dolomite crystals. Moreover, gently folded sheets, 100–200 nm thick and several micrometers long, form a sub-polygonal network reminiscent of EPS (extracellular polymeric substance). Their granular-textured surfaces suggest bacterial degradation of original organic matter. These features confirm a biological origin for the stromatolites, as in modern microbial mats, and the preserved original geochemical signatures indicate early precipitation of Mg-carbonates induced through microbial sulfate-reducing metabolic activities.

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