Effects of culture conditions on the expression level of lectin in Microcystis aeruginosa (freshwater cyanobacterium)

Effects of culture conditions on the lectin level by a unicellular laboratory culture of Microcystis aeruginosa M228 (freshwater cyanobacterium) were investigated. The lectin level per cell at stationary phase increased two- or fourfold by lowering the light intensity from 45 or 90μE/m² per s to 12μE/m² per s, respectively, at 25°C as measured by the enzyme-linked immunosorbent assay method. The hemagglutinating activity against rabbit erythrocytes increased in response to the lectin level. The lectin level was threefold greater at 15°C than at 25°C, and nearly ninefold greater than at 30°C. Thus, although the specific growth rate of the cyanobacterium decreased with lowering light intensity and temperature, the lectin level per cell increased significantly. When the cyanobacterium was cultured at 15°C with a light intensity of 12μE/m² per s, the lectin level reached the maximum near the end of exponential phase, and then decreased toward stationary phase. The level near the end of exponential phase was nearly threefold greater than that of the culture incubated at 25°C with a light intensity of 45μE/m² per s. These findings show that the lectin production in M. aeruginosa M228 was induced by unfavorable conditions for the cell growth.