Microorganisms on siliceous-speleothems: example from sandstone caves, Brazil

This study investigates siliceous speleothems by characterizing their macro- and microscopic features in caves formed within the Botucatu Formation, Paraná basin, Brazil. The analyzed speleothems consist of aerial coralloids composed of amorphous opal and detrital quartz grains, with biological components such as microbial-like filaments and diatoms. Two centric diatoms morphotypes are reported: Orthoseira sp. and Gen. 1 sp.1 and also a possible pennate species. Whereas diatom assemblage is confined to the photic and twilight zones, microbial filaments are reported both within these zones in addition to environments with limited sunlight exposure. The presence of biofilms and diatoms may increase local pH, facilitating silica dissolution and reprecipitation processes that contribute to speleothem formation. These caves are cataloged as geosites within the Corumbataí Geopark project, a pioneering initiative aimed at becoming the first geopark in the São Paulo State. This research enhances the first report of microbial assemblage in association with siliceous speleothems from the Serra de Itaqueri caves in the São Paulo State. In addition, understanding the biochemical processes that influence the development of siliceous speleothems in quartzose lithologies is crucial for deciphering their genesis mechanisms and potential environmental implications. All these dynamics involved in these caves highlight its relevance as a strategic geosite for paleoenvironmental, ecological, and conservation studies.