The objective of this study was to investigate the effect of Mg:Ca ratio in the medium on the formation of low- and high-Mg calcite by aerobic microorganisms enriched from rhodoliths (mainly Proteus mirabilis, Wu Do-1). XRD analyses showed that both low- and high-Mg calcites were formed depending on the Mg:Ca ratio in the medium. Calcite was formed at Ca:Mg ratios of 6:0 and 3:1 and high-Mg calcite was formed at Ca:Mg ratios of 1:1 and 1:3 in the medium. Huntite was formed with a Ca:Mg ratio of 0:6. SEM-EDS analyses showed that the low- and high-Mg calcite crystals had a rhombohedron shape and consisted of Ca, Si and Mg with extracellular polymeric substances (EPS). These results indicate that Wu Do-1 induced precipitation of low- and high-Mg calcite crystals depending on the Ca:Mg ratio in the medium. The carbonate minerals were precipitated on the cell walls and EPS via the accumulation of Ca and/or Mg ions. Therefore, microbial formation of carbonate minerals may play an important role in Ca, Mg, and carbon biogeochemistry as well as CO2 fixation in the natural environments.
Keywords: Calcium; Magnesium; Biomineralization; Calcite; Mg-Rich Calcite.