The genesis of sedimentary dolomite has long been a mystery in Earth science. It has been shown that the formation of low-temperature disordered dolomite, a crucial precursor of sedimentary dolomite, can be catalyzed by various heterotrophic microorganisms through organic mineralization. However, the potential role of cyanobacteria, which are common and cosmopolitan photoautotrophs in sedimentary environments, in the precipitation process of disordered dolomite has been largely underestimated. Moreover, recent emphasis on the beneficial influence of dissolved silicon (Si) in directly enhancing the Mg content in Ca-Mg carbonates under oversaturated conditions has emerged. Nevertheless, the effect of dissolved Si on the formation of microbially-induced disordered dolomite remains poorly understood. To examine the influence and mechanisms of dissolved Si on the biomineralization of Ca-Mg carbonates by cyanobacteria, biomineralization experiments were conducted using a halophilic cyanobacterium (Synechococcus elongatus FACHB-410) in solutions with varying concentrations of dissolved Si (ranging from 0 to 2 mM). Our findings revealed that the presence of dissolved Si resulted in higher Mg levels in the bio-precipitated Ca-Mg carbonates compared to Si-free systems, with disordered dolomite formation observed at 2 mM Si. Importantly, an amorphous Mg-silicate phase formed prior to the crystallization of Ca-Mg carbonates during the biomineralization process, intimately associated with the grains of Ca-Mg carbonates.

Subsequent abiotic carbonation experiments demonstrated its catalytic role in the nucleation of Ca-Mg carbonates. These observations suggest that, in addition to the direct catalytic effect of dissolved Si, the positive role of Si in the biomineralization of disordered dolomite by cyanobacteria may also be attributed to the templating role of the neoformed amorphous Mg-silicate. These results have important implications for understanding the co-occurrence of Mg-silicate and dolomite observed in geological records.