Selective Inhibition on Growth and Photosynthesis of Harmful Cyanobacteria (Microcystis aeruginosa) by Water Soluble Substances of Dendranthema indicum Flowers

Abstract

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 of Dendranthema indicum flowers on cyanobacteria and green algae species. The thermal stability and polarity of the water-soluble active substances of D. indicum flowers were also assessed based on the growth and photosynthesis responses of Microcystis aeruginosa. There was obvious growth promotion of green algae, including Chlorella vulgaris, Kirchneriella sp. and Haematococcus pluvialis and strong growth inhibition of toxigenic and non-toxigenic M. aeruginosa by aqueous extracts of D. indicum flowers at concentrations ranging from 0.5 to 2.0 g·DW·L−1. The cell concentrations of M. aeruginosa and C. vulgaris were 46.5% and 242.2% of those in the corresponding controls after a 10-day exposure to aqueous extracts of D. indicum flowers at 1.0 g·DW·L−1. There must be some water-soluble active allelochemicals released from D. indicum flowers that are responsible for the selective inhibition of M. aeruginosa rather than green algae. The inhibition ratio of the growth and photosynthesis of M. aeruginosa by aqueous extracts of D. indicum flowers at 2.0 g·DW·L−1 prepared with water at 25 °C and 100 °C were above 90% and 80% compared to the cell density and performance index on absorption basis (PIABS) value of M. aeruginosa in the control on day 6, without significant differences. It indicates that the active substances of D. indicum flowers were thermally stable. The methanol fraction eluted from solid phase extraction (SPE)-enriched aqueous extracts of D. indicum flowers showed the strongest inhibition of the growth and photosynthesis of M. aeruginosa compared to the other four fractions. It indicates that the most polar substances of D. indicum flowers were responsible for the selective inhibition of M. aeruginosa. More experiments are required to identify the responsible active substances and reveal the underlying mechanisms of aqueous extracts of D. indicum flowers that selectively inhibit cyanobacteria and regulate the phytoplankton community structure.