Toxicity of allelochemicals released by submerged macrophytes on phytoplankton

The allelopathic mechanism of submerged macrophytes was investigated on the cyanobacterium Microcystis aeruginosa and the green alga Pseudokirchneriella subcapitata. The effect of the allelochemicals pyrogallic acid (PA), gallic acid (GA), (+)-catechin (CA), caffeic acid (CFA), protocatechuic acid (PCA), nonanoic acid (NA) and cis-9-octadecenoic acid (COA) was evaluated by flow cytometry on esterase activity, membrane integrity, chlorophyll a fluorescence, cell volume and internal cell complexity. The highest sensitivity was observed for esterase activity in both test species. NA damaged the cell membrane and severely altered the internal cell complexity of both species. PA changed the esterase activity of both species and significantly decreased the fluorescence of chlorophyll a in M. aeruginosa cells (P < 0.05). Both GA and PCA inhibited the fluorescence of chlorophyll a and caused morphological changes in M. aeruginosa (P < 0.05). CFA inhibited the esterase activity, fluorescence of chlorophyll a and cell volume in M. aeruginosa at 50 and 100 mg L-1 concentrations; In P. subcapitata, the fluorescence of chlorophyll a and cell volume was inhibited at 50 mg L-1 but was stimulated at 100 mg L-1. COA changed the esterase activity. CA at 100 mg L-1 slightly inhibited the fluorescence of chlorophyll a in M. aeruginosa and stimulated the esterase activity in P. subcapitata. The responses observed for each parameter investigated were highly dependent on the specific inhibitory mechanism of each allelochemical. These allelochemicals act on multiple target sites.