At Miette Hot Springs, SO42-/H2S-, Ca2+-, Sr2+-, and CO32--rich waters with a mean temperature of 51.2 °C are ejected from three spring vents and several minor seeps near the floor of Sulphur Creek valley. Runoff channels from the springs are colonized by cyanobacteria (Oscillatoria, Phormidium, Gloeocapsa, Synechococcus, Xenococcus) that grow in resistant mats and as loose filaments within 0.5 m of the spring vents, diatom assemblages (Cymbella, Mastagloia, Brachysira, Sellaphora, Rhopalodia, Nitzschia, Navicula, Pinnularia) that dominate the flow paths 0.5–2.0 m from the vents, and microbial mats with cyanobacteria and diatoms in the distal flow paths. Sulphate-reducing bacteria and green algae are also present. Gypsum, elemental sulphur, and lesser quantities of calcite and strontianite precipitate from the spring waters. Microbial populations influence accumulation of mineral precipitates by (i) forming mats close to the spring vents on which crystals grow, (ii) forming mats alongside the flow paths that trap and bind precipitates, and (iii) providing loose filaments to which microscopic gypsum crystals adhere. The microbes also influence crystal habit by (i) creating pores on the surfaces of gypsum crystals where smaller crystal precipitates form, and (ii) producing intercellular mucus in microbial mats, where suspended crystals can grow in all directions to produce polyterminal calcite crystals. Diatoms also mediate corrosion of the faces of calcite and gypsum crystals. Enriched δ13Cinorganic signatures in the precipitates associated with microbial communities indicate that photosynthesis may promote precipitation of calcite and strontianite.