Microbial and clay-mediated dolomite formation in a saline playa lake: Implications for biosignatures on Mars

• Microbe–clay–carbonate interplay: Dolomite formation in a saline playa-lake is mediated by microbial activity and closely associated with authigenic Mg-rich clays (palygorskite, vermiculite, Mg-smectites), which act as substrates and catalysts. • Biogenic and geochemical signatures: The coexistence of microbial EPS, specific bacterial taxa, and elevated Sr 2+ , Fe 2+ , and Mn 2+ concentrations provides robust evidence for microbially mediated mineralization. • Mars analog for biosignature detection: This clay-carbonate-microbe system offers a compelling analog for ancient Martian lakebeds, guiding the search for past life on Mars. This study investigates the authigenesis of palygorskite in association with dolomite, as well as the alteration of detrital clays into vermiculite and Mg/Fe smectites, within the upper sediments of the semi-arid saline playa-lake system of Laguna Fuente de Piedra (southern Spain). A combination of analytical techniques, including XRD, SEM–EDS, HRTEM–AEM, ICP-MS/OES, NMR, and 16S rRNA gene sequencing, was used to characterize sediment mineralogy, microtextures, geochemistry, and microbial assemblages. The results reveal an assemblage of authigenic dolomite and clays embedded within an extracellular polymeric substance (EPS) matrix, consistent with microbially influenced mineralization at low temperature. These findings provide new insight into how clay minerals modulate microenvironments (e.g., ion activity, fluid and pore-water composition) and offer reactive surfaces that facilitate dolomite precipitation, while also acting as substrates for microbial colonization and EPS preservation in inland saline lake settings (e.g., playa lakes, continental sabkhas). Because evaporitic to alkaline lacustrine environments and Mg-rich carbonates and clay minerals have been documented or inferred for ancient Martian basins, the co-occurrence of Mg-carbonates with Fe/Mg-rich clays constitutes a testable mineralogical target for biosignature investigations on Mars. In particular, when evaluated together with diagnostic microtextures and complementary geochemical indicators, this assemblage may help distinguish purely abiotic precipitation from microbially influenced mineralization in Martian sedimentary records.