Calcium carbonate precipitation by Synechocystis sp PCC6803 at different Mg/Ca molar ratios under the laboratory condition

The formation of carbonates is not yet a well-understood process that is biogenic or abiotic origin remains controversial. To obtain further insights on the controls of Mg incorporation in carbonates formed via biogenic process, the impacts of cyanobacteria Synechocystis sp. PCC6803 on carbonates precipitation have been investigated in BG11 medium at different Mg/Ca molar ratios in the laboratory. Initial concentration of Ca2+ was 0.01 M and Mg/Ca molar ratio was 0, 2, 4, and 6, respectively. The results showed that pH values in the experimental group increased rapidly, greater than those in the control group. Ca2+ concentration decreased greatly while Mg2+ concentration was almost unchanged in these two groups. There were significant differences in the sizes, shapes, elemental compositions, and surface morphologies of the mineral precipitates between control group and experimental group analyzed by polarizing microscopy, scanning electron microscopy and energy dispersive X-ray detector. X-ray diffraction result showed that calcite was induced at Mg/Ca molar ratio of 0 and aragonite was induced at Mg/Ca molar ratio of 4 and 6. X-ray diffraction and selected-area electron diffraction results showed the minerals at Mg/Ca molar ratio of 2 were calcite and aragonite. The cracks on the surface of aragonite crystals may be a characteristic of biogenetic aragonite. Thermogravimetric analysis of the carbonate minerals showed that the thermal stability decreased gradually with increasing Mg/Ca molar ratios. These results are useful for further understanding of the formation mechanism of biogenic carbonates in natural condition.