2:45 PM - 3:00 PM
[BBC03-05] Calcium carbonate formation processes on cyanobacterial surface
Keywords:CaCO3, Cyanobacteria, EPS
Four cyanobacteria having different EPS characteristics (Spirulina, Phormidium, Scytonema, and Leptolyngbya) were cultivated. Acid-base titrations and lectin binding analysis indicated that all investigated cyanobacteria secreted EPS containing acidic groups. These cyanobacterial cultures were incubated in an aquarium (pH = ca. 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. Observations by transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM) revealed that the characteristics of precipitated CaCO3 were significantly different among the investigated cyanobacteria. Especially, calcite formed around Spirulina, Phormidium, Scytonema exhibited specific features including ACC, sub-micron-sized calcite polycrystals, and superstructures, suggesting that the nucleation occurred on acidic EPS via ACC nanoparticles. In contrast, such features were rarely observed in calcite formed around Leptolyngbya producing only a few amounts of EPS. From these results, CaCO3 formation processes on cyanobacterial surface is suggested as follows: 1) ACC nanoparticles are formed on acidic EPS, 2) crystallization takes place inside of grown ACC particles to form submicron-sized calcite polycrystals, and 3) these polycrystals grow into calcite single crystal while ACC and polycrystals remain partly.