Actively forming microbial mats provide insight into the development of microdigitate stromatolites.

Stromatolites can be traced back to ∼3.5 billion years. They were widespread in the shorelines of ancient oceans and seas. However, they are uncommon nowadays, and basic information is lacking about how these unique carbonate structures developed. Here we study the unusually thick (3-5 cm) biofilms of the 79.2 °C outflow from Köröm thermal well (Hungary) and demonstrate that its microbial mat - carbonate architecture is similar to fossilized microdigitate stromatolites. Our observations reveal vertically oriented fibrous mineral fabrics, typical of stromatolites, in the red biofilm and clotted mesostructures, typical of thrombolites, in the green biofilm. These layers contain carbonate peloids and show network structures, formed by filamentous microbes. The 16S rRNA gene-based amplicon sequencing implies that numerous undescribed taxa may contribute to the carbonate mineralisation. The biofilms abundantly contain the phyla Bacteroidota, Pseudomonadota and Cyanobacteria. Geitlerinema PCC-8501 and Raineya are characteristic for the green biofilm, whereas uncultured Oxyphotobacteria, unc. Saprospiraceae and unc. Cytophagales are abundant in the red biofilm. A hydrogen-oxidizing Hydrogenobacter within the phylum Aquificota and unclassified Bacteria together with the phylum Deinococcota dominate the water and carbonate samples. The morphological structure and taxonomic composition of Köröm biofilm is a unique representation of the development processes of microbialite formations.