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[BCG09-02] Cyanobacterial extracellular polymeric substances (EPS) control stromatolite and thrombolite formations
Water chemistry at the depositional sites of stromatolite and thrombolite are similar, and microelectrode measurement revealed that they both are mainly formed by photosynthesis-induced CaCO3 precipitation. Confocal laser scanning microscopy observation and DNA analysis indicated that cyanobacteria colonizing the stromatolite are mostly Phormidium sp., and their extracellular polymeric substances possess acidic group to provide mineral nucleation sites, and resulted in the stromatolite formation. On the other hand, cyanobacteria colonizing the thrombolite are mostly Leptolyngbya sp., and their extracellular polymeric substances does not possess acidic group to be unsuitable for the mineral nucleation sites, and resulted in the thrombolite formation. These results indicate that chemical characteristics of extracellular polymeric substances are crucial for the formation of microbialite fabrics.
By simply applying the results above, we can hypothesize that the evolution of cyanobacteria lacking acidic extracellular polymeric substances caused the microbialite transition in the early Paleozoic. Nonetheless, further investigation is necessary for validating this hypothesis.