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[BBG02-01] Attempt to form peloids by calcification of coccoidal cyanobacteria
Peloids are commonly found in carbonate rocks, and are observed in rock thin sections as microcrystalline (micritic) and structureless particles. Peloids have various origins, including many that are of microbial origin and often form stromatolites, and are thought to have played an important role in the carbon cycle on the Earth's surface. However, the details of how microorganisms form peloids are still unclear. Several previous studies, based on the examination of microbial carbonates from geological time and the present day, have suggested the possibility that peloids were formed by the embedding of colonies of coccoidal cyanobacteria in calcium carbonate (Adachi et al., 2004; Shiraishi et al., 2017). Therefore, this study experimentally confirmed whether such a phenomenon actually occurs.
The coccoidal cyanobacteria Synechocystis sp. and Stanieria sp. were used in the experiments. The results of acid-base titration and fluorescent staining showed that Synechocystis sp. contained more carboxyl groups on the cell surface and extracellular polymeric substances (EPS). These strains were immersed in experimental water suitable for photosynthesis-induced carbonate precipitation, and the progress was observed under light irradiation. As a result, peloid-like spherical minerals were observed around Synechocystis sp., while such minerals were not observed around Stanieria sp. This suggests that acidic EPS plays an important role in the nucleation of carbonate minerals. Observations using transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM) confirmed that the minerals formed around Synechocystis sp. consisted of single-crystal calcite with twins, and that some contained amorphous calcium carbonate (ACC). However, Synechocystis sp. cells were not incorporated inside the calcite, which was different from those of peloids from geological time and the present day reported in previous studies. This may be due to the low cell density of the cyanobacteria used in the experiment, and resultant insufficient increase in saturation by photosynthesis.
References
Adachi N., Ezaki Y., Liu J. (2004) The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China. Sedimentary Geology 164, 161–178.
Shiraishi F., Hanzawa Y., Okumura T., Tomioka N., Kodama Y., Suga H., Takahashi Y., Kano A. (2017) Cyanobacterial exopolymer properties differentiate microbial carbonate fabrics. Scientific Reports 7, 11085.
The coccoidal cyanobacteria Synechocystis sp. and Stanieria sp. were used in the experiments. The results of acid-base titration and fluorescent staining showed that Synechocystis sp. contained more carboxyl groups on the cell surface and extracellular polymeric substances (EPS). These strains were immersed in experimental water suitable for photosynthesis-induced carbonate precipitation, and the progress was observed under light irradiation. As a result, peloid-like spherical minerals were observed around Synechocystis sp., while such minerals were not observed around Stanieria sp. This suggests that acidic EPS plays an important role in the nucleation of carbonate minerals. Observations using transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM) confirmed that the minerals formed around Synechocystis sp. consisted of single-crystal calcite with twins, and that some contained amorphous calcium carbonate (ACC). However, Synechocystis sp. cells were not incorporated inside the calcite, which was different from those of peloids from geological time and the present day reported in previous studies. This may be due to the low cell density of the cyanobacteria used in the experiment, and resultant insufficient increase in saturation by photosynthesis.
References
Adachi N., Ezaki Y., Liu J. (2004) The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China. Sedimentary Geology 164, 161–178.
Shiraishi F., Hanzawa Y., Okumura T., Tomioka N., Kodama Y., Suga H., Takahashi Y., Kano A. (2017) Cyanobacterial exopolymer properties differentiate microbial carbonate fabrics. Scientific Reports 7, 11085.