Allelopathic inhibition of harmful algae by mangrove leaf extracts under contrasting habitats and algal densities

Harmful algal blooms (HABs) increase with intensified coastal eutrophication, underscoring the need for environmentally friendly control strategies. Mangrove-derived allelochemicals can regulate HABs, yet variability among plant and algal species remains unclear. This study compared the allelopathic inhibition of aqueous leaf extracts from three mangrove species (Aegiceras corniculatum, Avicennia marina and Kandelia obovata) on two HAB dinoflagellates, Scrippsiella trochoidea and Alexandrium catenella. Leaves collected from two mangrove wetlands in southern China, Futian (FT) and Dapeng (DP), differ markedly in habitat conditions, with significantly higher total nitrogen (TN) and total phosphorus (TP) in both water and sediments in FT than DP. Total polyphenols and condensed tannins in leaves in FT were lower than in DP but FT leaf extracts exhibited stronger inhibition on harmful algae, suggesting phenolic compounds may indicate potential allelopathic capacity rather than directly determine inhibition strength. Inhibitory effects depended on mangrove species, with K. obovata showing the weakest inhibition while A. marina had the strongest inhibitory effects against both HAB dinoflagellates. HAB species also differed in sensitivity, with A. catenella exhibited stronger inhibition intensity than S. trochoidea. The inhibition by leaf extracts substantially reduced or exhibited time-lagged responses under high algal density, and the association between inhibition rates and content of intracellular superoxide dismutase (SOD) activity and malondialdehyde (MDA) depended on mangrove species. These results demonstrated that the allelopathic control of HABs by leaf extracts affecting algal growth and allelochemical-induced cellular stress was modulated by mangrove species, habitat conditions, and algal species and population density.