Kathrin Janssen

This is a very interesting and relevant question. I came across an article indicating that calcite precipitation at low temperatures is a microbially mediated process.

You may find the article at this link https://www.jstor.org/stable/24089018

Hey Victor, thanks for you reply! In the paper they say that at 5 °C no calcification occurs..

Whether or not precipitation will occur has to do with the saturation levels, not only with temperature. In the case of calcite, the lower the temperature is, the higher the solubility of CO2 is, shifting the carbonate system to lower the saturation of calcium carbonates.

At low temperature, the more stable carbonate phase is ikaite, which is calcium carbonate hexahydrate. The temperature of the phase transition to calcite or vaterite varies (about 4-12oC) according to the different authors, and is influenced by a number of factors, including kinetic ones. But that doesn't preclude calcite biomineralization at any temperature. Biomineralization has its own kinetic factors. There is a great deal of literature on ikaite and even more on biomineralisation

I would add that the deciding factor for the calcium carbonate precipitation is the actual ion product, i.e. the product of the activities of Ca+2 * CO3-2. This must be greater than the equilibrium dissociation constant. CO3-2 activities are pH controlled, so alkalisation will lead to crystallisation. Whether calcite, ikait, aragonite or vaterit will arise is another question.