Morphological and microbial diversity of hydromagnesite microbialites in Lake Salda, A Mars Analog Alkaline Lake

AbstractLake Salda, recognized as a terrestrial analog for the paleolake in Jezero Crater on Mars, hosts modern, sub/fossil and fossil hydromagnesite microbialites. A comprehensive study was conducted to reveal the distribution, and morphological-mineralogical, microbial diversity of the microbialites that are currently growing in the shallow (<1 m) and deeper waters (up to 15 m) of the lake. Six major microbialite forming zones were identified (Zone I-VI) comprising previously unknown morphotypes of the microbialites. These newly identified morphologies include; linked columns and microbial pavements, exhibiting distinct surface textures such as bulbous, mini columnar, knobby, cerebroidal and smooth. The shallow microbialites showed well-preserved radially growing stromatolitic layers in cm-scale, producing cauliflower structures while the deeper samples exhibit mm scale layering within the mini columnar structures. 16S rRNA amplicon data reveal that abundance of major bacterial taxa of the thrombolitic microbialites from deeper regions are distinct than those of the shallow-growing microbialites with stromatolitic structures. Cyanobacteria were found to be generally less abundant (0.01% to 2%) in the thrombolitic microbialites compared to the stromatolitic microbialites with smooth surface. The Anaerolineae class of the phylum Chloroflexi was prominently present in the stromatolitic microbialites with smooth surface. Notably, the lake exhibited a high abundance of the genus Exiguobacterium, particularly in the deep thrombolitic microbialites (e.g., 93%). Microscopy analysis of the deep microbial mats showed the presence of abundant nano and microcrystalline platy/flakey hydrated Mg carbonate crystals that co- existed with mineralized filaments and spherulitic aggregates within abundant exopolymeric substances (EPSs). Moreover, palygorskite mineral was exclusively identified within the deep microbialites. Co-existence of aragonite and hydromagnesite minerals in particular within the deep microbialites that grow under non-evaporative conditions and the abundant presence of entombed biomass (e.g., filamentous) collectively suggest the potential preservation of biosignatures within the hydrated Mg carbonate build-ups in Lake Salda.