Inactivation Mechanism of Algal Chlorophyll by Allelochemical Quercetin

Cyanobacteria harmful algal blooms (CyanoHABs) are a global concern. Application of allelochemicals is a promising solution for cyanobacteria control, due to its high efficiency, low cost and ecological safety. Flavonoids (natural polyphenols produced by aquatic plants) are reported capable of effectively inhibiting the growth of algae; however, the molecular mechanism of algae chlorophyll inactivation is still unclear. In this study, quercetin was used as a typical flavonoid, to investigate the inactivation effect of allelochemical on Microcystis aeruginosa chlorophyll a. The absorption and fluorescence spectra showed that chlorophyll reacted with quercetin to form pheophytin, and the formation rate of pheophytin increased with increasing quercetin concentration (1 × 10 –5 –1 × 10 –2  M). FTIR spectra and DFT calculation showed that Mg 2+ complexed with the 3-OH and 4-C = O groups in the quercetin ring C so that chlorophyll was inactivated due to the loss of Mg 2+ ions. Overall, this study revealed that quercetin inactivated chlorophyll a of cyanobacteria by capturing Mg 2+ ions, providing insights into the molecular mechanisms of algal bloom control by allelochemicals.