Bioinduced precipitation of smectites and carbonates in photosynthetic diatom-rich microbial mats

The association of MgII clays with dolomite is common in ancient and modern lacustrine environments, although, the role of microbial activity in their precipitation remains a challenge. In this study, microbial culture experiments provide the first evidence of saponite and dolomite precipitation on diatom-bearing photosynthetic microbial mats. Microbial strains were isolated from photosynthetic microbial mats collected from Caballo Alba lake in Segovia, Spain. The microbial communities´ growth and mineral precipitates varied function of the type of media used, semi-solid or liquid, respectively. The prokaryotic composition identified from cultures was dominated by Cyanobacteria and Proteobacteria. Bacteroidetes were the most abundant heterotrophic bacteria. Saponite, aragonite, and dolomite crystals were closely attached to the phytoplanktonic communities grown in liquid cultures. Diatoms are replaced by saponite and dolomite in liquid cultures where diatoms are unprovided of protection of extracellular polymeric substances (EPS). Saponite grew around the diatom frustules, as dissolution of the biogenic silica occurs in a medium depleted in silica, replacing diatoms by saponite pseudomorphs. To a lesser extent, cyanobacterial cell walls were also covered by saponite crystals. In addition, amorphous silica of the frustules also provided a suitable template for the nucleation of dolomite crystals. Alternatively, MgII-rich aragonite crystals with fibrous morphologies precipitated within saponite crystal aggregates, suggesting that clay minerals enhanced the nucleation and precipitation of dolomite. In the semi-solid medium, where calcite precipitated in association with Cyanobacteria, diatoms were capable of developing a sheet of EPS which protected their frustules from dissolution. Here, we suggest that, in alkaline lakes phytoplanktonic diatoms might have contributed to the precipitation of clay minerals and carbonates since the Jurassic.