Documenting the occurrence of microbial carbonate deposits in Puerto Rico

Microbial carbonate deposits, including microbial mats and microbialites, record coupled biological, geochemical, and sedimentary processes, providing insight into biosedimentary dynamics through Earth’s history and serving as analogs for ancient and potentially extraterrestrial environments. This study presents the first documentation of lithifying microbial systems in Puerto Rico, identified within three proximal coastal lagoons (Salineta, Vernales, and Providencia) along the southwestern coast of the Guánica municipality. These lagoons define a continuum of lithification, ranging from non-lithifying hypersaline mats to fully lithified microbialite deposits. An integrated field, geochemical, petrographic, and microbial ecological approach was used to characterize deposit morphology, mineralogy, and microbial community structure. At Salineta, elevated salinity, low pH, and evaporitic conditions correspond to halite- and gypsum-dominated mats hosting halophilic Archaea and anoxygenic phototrophs, with minimal carbonate precipitation. In Vernales, carbonate precipitates closely associated with microbial filaments and EPS indicate enhanced organomineralization. Providencia hosts extensive lithified deposits displaying composite stromatolitic and thrombolitic fabrics, localized occurrences of high-Mg carbonate phases, and microbial communities enriched in sulfate-reducing bacteria relative to the other lagoons. Collectively, variations in hydrology, salinity, substrate, and microbial community composition correspond to differing degrees of lithification across a small coastal region, establishing southwestern Puerto Rico as a natural laboratory for investigating microbialite formation and biosignature preservation in coastal carbonate systems.