Dissolved silicon as a beneficial factor for biomineralization of disordered dolomite by a halophilic cyanobacterium

Abstract

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 CaMg 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 CaMg 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 CaMg 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 CaMg carbonates during the biomineralization process, intimately associated with the grains of CaMg carbonates. Subsequent abiotic carbonation experiments demonstrated its catalytic role in the nucleation of CaMg 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.