Selective Inhibition on Growth and Photosynthesis of Harmful Cyanobacteria (Microcystis aeruginosa) by Water Soluble Substances ofDendranthema indicumFlowers

Harmful cyanobacterial blooms become serious environmental issues in various waterbodies, including aquaculture ponds, which inherently need a high biomass and healthy composition of phytoplankton to sustain their high productivity. Indoor bioassays were conducted to investigate the effects of an aqueous extract ofDendranthema indicumflowers on cyanobacteria and green algae species. The thermal stability and polarity of the water-soluble active substances ofD. indicumflowers were also assessed based on the growth and photosynthesis responses ofMicrocystis aeruginosa. There was obvious growth promotion of green algae, includingChlorella vulgaris,Kirchneriellasp. andHaematococcus pluvialisand strong growth inhibition of toxigenic and non-toxigenicM. aeruginosaby aqueous extracts ofD. indicumflowers at concentrations ranging from 0.5 to 2.0 g center dot DW center dot L-1. The cell concentrations ofM. aeruginosaandC.vulgariswere 46.5% and 242.2% of those in the corresponding controls after a 10-day exposure to aqueous extracts ofD. indicumflowers at 1.0 g center dot DW center dot L-1. There must be some water-soluble active allelochemicals released fromD. indicumflowers that are responsible for the selective inhibition ofM. aeruginosarather than green algae. The inhibition ratio of the growth and photosynthesis ofM. aeruginosaby aqueous extracts ofD. indicumflowers at 2.0 g center dot DW center dot L(-1)prepared with water at 25 degrees C and 100 degrees C were above 90% and 80% compared to the cell density and performance index on absorption basis (PIABS) value ofM. aeruginosain the control on day 6, without significant differences. It indicates that the active substances ofD. indicumflowers were thermally stable. The methanol fraction eluted from solid phase extraction (SPE)-enriched aqueous extracts ofD. indicumflowers showed the strongest inhibition of the growth and photosynthesis ofM. aeruginosacompared to the other four fractions. It indicates that the most polar substances ofD. indicumflowers were responsible for the selective inhibition ofM. aeruginosa. More experiments are required to identify the responsible active substances and reveal the underlying mechanisms of aqueous extracts ofD. indicumflowers that selectively inhibit cyanobacteria and regulate the phytoplankton community structure.