To achieve our research objectives we studied various lakes and their associated gypsum microbialites, examining their hydrochemical, isotopic, and mineralogical features to comprehend the diverse conditions influencing their formation. Distinct patterns emerged: moderate salinity lakes featured mineralizing microbial mats transitioning into stromatolites, while hypersaline lakes hosted endolithic communities within gypsum formations.
Sulfur isotope measurements unveiled insights into the origin of sulfur in these formations, pointing to abiotic processes involving seawater mixing with hydrothermal brines. Geochemical analysis indicated an enrichment of gypsum formations in specific elements and revealed a diverse array of mineral phases associated with these stromatolites, encompassing gypsum, anhydrite, smectites, brushite, pyrite, iron-monosulfides, halite, sylvite, and amorphous silica.
Microbial diversity and structure analysis showed different dominant phyla in the microbialites of moderate salinity and hypersaline lakes. Taxon-based metabolic predictions indicated the presence of sulfur metabolizing microorganisms, such as anoxygenic photosynthetic bacteria in the moderate salinity formations and sulfate reducing bacteria in the hypersaline endolithic communities. Metagenomic analysis confirmed these results identifying the principal clades that harbour genes for the oxidation of sulfur compounds and for sulfate reduction. These results are indicative of the active sulfur cycling within these microbialites, affecting gypsum mineral precipitation and dissolution.
Microscopic investigations indicated detailed associations between microbial communities and minerals within the stromatolites, showing abundant microbial presence among gypsum crystals and their association with specific mineral phases. Synchrotron-based techniques provided insight into nanoscale mineral phases linked to microbial relics, confirming the diverse sulfur species present in the stromatolites and corroborating the active biologic sulfur cycling in these environments. Finally, statistical analyses revealed correlations between microbial community composition, metabolic functions related to sulfur, and environmental factors like salinity and sulfur isotopic composition. These results have already been presented in three international conferences, two workshops and three invited talks while they are included in at least two manuscripts in preparation. In addition, the overarching goal of this MSCA project and its broader implications have been presented in science communication talks in secondary schools and in science festivals.
