The extract of aquatic macrophyte Carex cinerascens induced colony formation in bloom-forming cyanobacterium Microcystis aeruginosa

Colony formation in Microcystis aeruginosa played important roles in blooms formation. To study the effects of plant allelopathy on colony formation in M. aruginosa, unicellular M. aeruginosa was cultivated under laboratory conditions treated with various extract concentration of Carex cinerascens. The growth of M. aeruginosa in the treatments with 0.05 and 0.1 mg L-1 extract of C. cinerascens was promoted but the growth in the treatments with 1.5, 3.0, and 6.0 mg L-1C. cinerascens extract was inhibited. Obvious colony formation in M. aeruginosa was observed in all treatments while no colony formation was detected in the control. The cell number per colony at the first day was the largest and decreased along with culture time. The cell number per colony in the control ranged from 3.0 to 4.0 during the experiment. However, the values in the five treatments at the first day were 33, 80, 58, 41, and 30, respectively. A positive exponential relationship between cell number per colony of M. aeruginosa and extracellular polysaccharides (EPS) content was found as well. Compared the fold-increase in cell number per colony and the fold-increase in total biomass of M. aeruginosa at various day, it was found that colony formation induced by extract of C. cinerascens was primarily dependent on promotion of cell adhesion during the first 2 days. The cell number per colony decreased afterward was due to the increasing proportion of single cells in the culture because single cells had a great higher growth rate than M. aeruginosa colonies under culture condition. Our results suggested that plant allelopathy be one of the major factor contributing to colony formation in M. aeruginosa.