[BBG02-P03] Role of extracellular polymeric substances on cyanobacterial calcification
cyanobacteria having different EPS characteristics (Spirulina, Leptolyngbya, Phormidium and Scytonema) were cultivated, and the quantity and quality of acidic groups in their EPS were assessed by acid-base titrations and lectin binding analysis. These cyanobacterial cultures were incubated in an aquarium (pH = about 8.5, [Ca2+] = 2 mM, DIC = 2 mM) with light supply to cause calcification. Microelectrode measurements (pH, O2, and Ca2+) showed that all cyanobacterial cultures induced CaCO3 precipitation by photosynthesis, although the degree of precipitation was different. However, the degree of calcification and the morphology of resulting CaCO3 crystals were significantly different: Spirulina lacking EPS was not calcified and crystals (about 1 mm diameter) were slightly scattered around, Leptolyngbya secreting non-acidic EPS was not calcified and rhombic crystals (about 10 mm diameter) scattered around, Phormidium secreting partially acidic EPS was externally calcified by cubic crystals (about 20 mm diameter), and Scytonema secreting acidic EPS was externally calcified by rhombic crystals (about 20 mm diameter). These results demonstrated that EPS has strong impact on cyanobacterial calcification. For Scytonema, an ultra-thin section was prepared by focused Ion beam processing and observed by transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). Observations indicated that the mineral precipitated upon the EPS of Scytonema was mostly monocrystalline calcite, while features of ACC were recognized at the vicinity of EPS (about 100-200 nm thick). Spotty concentration of carboxylic groups at the outer margin of EPS suggests that such acidic regions would be the starting points of ACC precursor, which would subsequently form calcite nucleation.