Microbially mediated calcification can be traced back for at least 2.6 billion years. Although morphological comparison of fossil and recent microbial carbonates suggests that mineralization processes associated with cyanobacteria and their interactions with heterotrophic bacteria have remained similar from the Archaean until today, the metabolic and chemical details remain poorly constrained. Microbial consortia often exhibit an ability to change solution chemistry and control pH at the microscale, passively or actively. This leads to oversaturation of Ca 2 + and ions and to the removal of kinetic inhibitors to carbonate precipitation, like sulphate or phosphate. The kinetic barriers of low carbonate ion activity, ion hydration and ion complexing, especially in saline waters, inhibit spontaneous carbonate mineral precipitation from saturated solutions but oxygenic photosynthesis and sulphate reduction by sulphate-reducing bacteria can overcome these natural barriers. Sulphate in seawater tends to form pairs with Ca 2 + and Mg 2 + ions. The removal of sulphate reduces complexing, raises carbonate alkalinity, and along with pyrite formation, enhances carbonate precipitation. Cyanobacteria can store Ca 2 + and Mg 2 + ions in organic envelopes and precipitate carbonates within their sheaths and extracellular polymeric substances, thus, triggering sedimentary carbonate production. We propose that this interplay of cyanobacteria and heterotrophic bacteria has been the major contributor to the carbonate factory for the last 3 billion years of Earth history.

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