Dolomite enabled the preservation of fine microbial textures in some Archean and Proterozoic marine microbialites, but has rarely done so during the Phanerozoic. Here, we report precipitation of dolomite in anoxygenic photosynthetic biofilms grown under chemical conditions relevant for Archean seawater. Ordered dolomite nucleates primarily on the surfaces of photosynthetic cells when manganese(II) is present, and nanocrystals of disordered dolomite form on exopolymeric substances in microbial cultures grown either in the dark or without manganese. Dolomite nucleation and maturation on different surfaces in photosynthetically active cultures amended with 0.1–1 mM manganese(II) enables the preservation of biofilm textures at scales larger than individual microbial cells. This provides a new model for the preservation of microbial textures by dolomite before the oxygenation of the oceanic photic zone.