Impacts of Eichhornia crassipes (Mart.) Solms stress on the physiological characteristics, microcystin production and release of Microcystis aeruginosa

Eichhornia crassipes (Mart.) Solms is effective in assimilating nutrients from eutrophic waters. However, it is not clear whether E. crassipes has an adverse impact on the waters in which heavy blooms of Microcystis aeruginosa occur. The objective of this study was to understand the interactions of E. crassipes with toxigenic M. aeruginosa and the consequences on environmental safety. Thus, the growth, physiological characteristics, microcystin production and release of M. aeruginosa influenced by E. crassipes were investigated using a co-existence experiment. The risk of microcystin-LR (MC-LR) accumulation in E. crassipes was also evaluated. Our results indicated that the cell death of M. aeruginosa occurred at a quicker pace due to the presence of E. crassipe. But the caspase-3 activity of M. aeruginosa, as the proxy for programmed cell death, was suppressed significantly by E. crassipes. Photosystem (PS) II-Hill reaction in M. aeruginosa was not significantly interrupted by E. crassipes, but a direct positive relationship between phycocyanin (PC) and algae biomass (R2 = 0.661, P < 0.01) and a consistent relationship between phycocyanin/allophycocyanin (PC/APC) ratio in M. aeruginosa and algae biomass (R2 = 0.598, P < 0.01) were found to be statistically significant. These results suggested that the energy harvest and electron transfer processes in the photosystem of M. aeruginosa might be disturbed by E. crassipes due to its damage of PC and a change in the PC/APC ratio. After this 12-day experiment, the level of extracellular MC-LR was significantly eliminated from 212.68 ± 25.05 μg L−1 to 18.98 ± 0.35 μg L−1 and the MC-LR production in M. aeruginosa was not stimulated by the influence of E. crassipes. The MC-LR level in the whole plants of E. crassipes was 3.88 ± 0.49 ng g−1 FW.