<p>Microbialites are lithifying microbial mats that form multi-layered structures via biological carbon (C) uptake and carbonate precipitation. Here we relate C uptake and precipitation rates to taxonomic diversity and functional capacity of bacterial communities in supratidal freshwater microbialites. Diel assays and analysis of functional gene capacity reveal that photosynthesis is bolstered by light-independent, biological C uptake mechanisms, including biomineralization and chemoautotrophy. Through integration of these mechanisms, microbialites can capture inorganic C over a 24-hour cycle at a rate of 7-12 g C m^-2 24 h^-1. Notably, up to 87 % of the C taken up is precipitated as inorganic carbon, capturing 2.4 − 4.3 kg C m^-2 year^-1. Based on observed porosity and laboratory-based accretion rates, this equates to 13-23 mm of vertical calcium carbonate accumulation per year. Hence, contemporary microbialites provide a highly effective biological mechanism to precipitate dissolved CO₂ as geologically stable carbonate mineral deposits.</p>