Insights on the biomineralisation processes and related diversity of cyanobacterial microflora in thermogenic travertine deposits in Greek hot springs (North-West Euboea Island)

The aim of this study is to identify the biomineralisation processes in hot springs of North-West Euboea Island by assessing the physico-chemical parameters of the hot water, the travertine mineralogical composition and facies, and the cyanobacterial microflora. In the studied area, the main mineral phases are calcite and aragonite, creating laminated and shrub facies of travertine deposits in close association with the cyanobacterial microflora. Microscopic analysis of fresh and cultured field samples shows the presence of 81 taxa of Cyanobacteria belonging to six orders, that is, Oscillatoriales, Synechococcales, Spirulinales, Chroococcales, Nostocales and Chroococcidiopsidales with the main factors controlling biodiversity being temperature, salinity and access to sunlight. No Cyanobacteria species were identified in areas with temperatures over 65^oC. In areas with high salinity (27–37‰), the order Oscillatoriales predominates. On the other hand, in areas with high temperatures (63^oC), fewer orders were observed, usually only Synechococcales and Spirulinales. In areas with lower temperatures (37^oC), larger numbers of Cyanobacteria orders were identified. Additionally, salinity seems to regulate the presence of the Nostocales order. The combined geobiological study revealed the presence of four biomineralisation processes involving calcium carbonate minerals, that is, (i) filamentous Cyanobacteria and extracellular polymeric substances trapping calcium carbonate crystals, (ii) extracellular polymeric substances acting as a template favouring mineral precipitation for crystal nucleation, (iii) formation of calcified Cyanobacteria sheaths and (iv) alteration of calcium carbonate crystals by endolithic Cyanobacteria. The identified biomineralisation processes suggest that the formation of calcium carbonate crystals is due to the metabolic activity of Cyanobacteria, or that the Cyanobacteria favour the deposition or the alteration of already existing crystals. The combination of these processes and the non-biotic (abiotic) mineralisation result in the formation of hybrid carbonates in the study area.