The Mg-rich carbonate dolomite is common in Earth’s ancient geological rock record but rarely formed today. The “dolomite problem” has been the subject of heated debate for decades, specifically if mixed cation (Ca-Mg- Fe) carbonates are primary or alteration products. In addition, what is the role of biological control in the formation of primary mixed cation carbonates? This project will investigate if and how microorganisms control the mineralogy and therefore, the geochemistry of carbonate minerals that grow at low temperature (<50° C) to establish if these minerals can act as proxies of the palaeo-conditions of formation. We will examine the hypersaline system in Fuente de Piedra (SW Spain), one of few places of present-day dolomite formation, and perform biotic and abiotic laboratory experiments that mimic natural environments (i.e., vary temperature, pH, salinity etc). The different metabolisms involved in carbonate biomineralization under oxic and anoxic conditions are expected to deliver characteristic nanotextural and geochemical signatures in both mineral and organic phases. Therefore, the characterization of dolomite in the hypersaline and evaporitic environment of Fuente de Piedra, coupled with laboratory experiments, will allow a quantitative interpretation of the processes that cause dolomite formation. Models developed from these data will be applied to examples of early Earth environments where unaltered carbonates are preserved to determine the palaeo-environment and possible role of life. This study is designed to understand mechanisms of carbonate formation in natural systems, which are of fundamental importance not only for understanding modern environments but also as a window into the geologic past of Earth and potentially Mars.