Bacterial fossils and microbial dolomite in Triassic stromatolites

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.