[U-05_30PO1] Interrelation between Life, Water, Mineral, and Atmosphere

[U05-P11] Role(s) of extracellular polymeric substance in microbial mineralization

*Yuki NAKAMURA¹, Fumito SHIRAISHI¹ (1.Hiroshima University)

Microbialites are defined as organosedimentary deposits that have accreted as a result of benthic microbial community binding detrital sediment or forming the locus of mineral precipitation. Most of microbialites are consisted of carbonate minerals, and considered to be formed by microorganism, such as cyanobacteria and sulfate reducing bacteria. Microbialite records the history of interaction between life and Earth environment, and therefore, it is important to understand their formation. Microbialites are formed mainly by three processes, including grain-trapping, mineral precipitation by metabolism and mineral nucleation by extracellular polymeric substances (EPS). Grain-trapping is locally important, but key processes are precipitation and nucleation. The knowledge of precipitation process by bacterial metabolism has increased, while that of EPS is still limited. Therefore, this study aims to investigate the influences of EPS on microbialite formation. We examined carbonate deposit developed at Kibedani hot spring, Shimane Prefecture. Calcite was despite of undersaturation in bulk water. Microerectrode measurement revealed that this deposit is formed as a result of photosynthesis-induced CaCO₃ precipitation. The result of EPS staing observation by Confocal Laser Scanning Microscope revealed that this deposit contains abundant acidic EPS, which is generally considered to have important roles in mineral nucleation. This deposit composed of two layers: the upper layer is consisted of empty EPS sheaths and the lower layer is of cyanobacteria with EPS sheaths. Both layers contain acidic EPS, while only lower layer was mineralized. This observation implies that acidic EPS cannot solely cause nucleation, and requires high mineral saturation state induced e.g.by photosynthesis.