Enhancing the floating velocity of harmful cyanobacteria by controlling temperature and the extracellular polysaccharides content

ABSTRACT Artificial acceleration of Microcystis buoyancy was attempted by manipulating temperature (15-35 °C) to reduce intracellular polysaccharides and increase gas vesicles, as well as varying the amount of Ca2+ (500 and 1,000 mg/L) and tightly bound extracellular polysaccharides (TB-EPS) (200 mg/L) to expand the colony size in preculture. In preculture, changes in temperature affected the floating velocity of Microcystis, and the floating velocity was 4.9 and 6.4 times higher at 35 °C than that at 15 and 25 °C in dark conditions, respectively. Since the content of intracellular polysaccharides at each temperature condition was not largely different (range: 9.9-11.1 pg/cell, p > 0.05), gas vesicle volume would be concerned with the buoyancy, whereas in dark conditions at 35 °C, the addition of Ca2+ (1,000 mg/L) and TB-EPS (200 mg/L) into the medium containing Microcystis enlarged colony size from 280 μm as the mean for the control to 390 μm, which resulted in 1.9 times faster floating velocity. It was expected that Ca2+ could promote colony size expansion of Microcystis by forming cross-linking structures with the negatively charged functional groups originally present, and externally added TB-EPS. These findings provide new insights into strategies for more efficient removal of Microcystis in lakes.