Microbial Community Shifts and Nitrogen Utilization in Peritidal Microbialites: The Role of Salinity and pH in Microbially Induced Carbonate Precipitation.

Microbialites have the potential to record environmental changes and act as biosignatures of past geochemical conditions. As such, they could be used as indicators to decipher ancient rock records. Modern microbialites are primarily found in environments where competitors and destructors are absent or where biogeochemical conditions favor their continuous formation. Many previous studies have essentially focused on the role of photosynthetic microbes in controlling pH and carbonate speciation and potentially overlooked alternative non-photosynthetic pathways of carbonate precipitation. Given that microbial activity induces subtle geochemical changes, microbially induced carbonate precipitation (MICP) can involve several mechanisms, from extracellular polymeric substances (EPS), sulfate reduction, anaerobic oxidation of methane, to nitrogen cycling processes, such as ammonification, ureolysis, and denitrification. Moreover, the peritidal zone where temperate microbialites are mostly found today, is under the influence of both freshwater and seawater, arguing for successive biogeochemical processes leading to mineral saturation, and questioning interpretations of fossil records. This study investigates microbialites in three tide pools from the peritidal zone of Fongchueisha, Hengchun, Taiwan, to address the influence of salinity on microbial community composition and carbonate precipitation mechanisms. Microbial samples were collected across varying salinity gradients at multiple time points and analyzed using next-generation sequencing (NGS) of bacterial 16S and eukaryotic 18S rRNA genes. Our results indicate that dominant bacterial groups, including Cyanobacteria and Alphaproteobacteria, were largely influenced by salinity variations, albeit pH exhibited stronger correlation with community composition. Combining our results on geochemistry and taxonomic diversity over time, we inferred a shift in the trophic mode under high salinity conditions, during which the use of urea and amino acids as a nitrogen source outcompetes diazotrophy, ureolysis and ammonification of amino acids reinforcing carbonate precipitation dynamics by triggering an increase in both pH and dissolved inorganic carbon.