Allelopathic Inhibition of Raphidiopsis Raciborskii by Vallisneria Natans: a Promising Approach for Cyanobacterial Bloom Control

The poleward expansion of the thermophilic cyanobacterium Raphidiopsis raciborskii, driven by climate change, has intensified harmful blooms in temperate freshwater ecosystems, highlighting the urgent need for effective bloom control strategies. In this study, the allelopathic effects of the submerged macrophyte Vallisneria natans on R. raciborskii were systematically evaluated through three exposure modes: direct cocultivation, exposure to postcultured water, and treatment with aqueous extracts of V. natans. The results revealed concentration-dependent biphasic effects on R. raciborskii growth, with low concentrations promoting algal proliferation and higher concentrations causing significant inhibition. The aqueous extracts exhibited the strongest and fastest suppression, effectively inhibiting algal growth at concentrations of 0.1% (equivalent to compounds extracted from 1 g dry weight per liter) and above. Cocultivation at a V. natans density of 2.5 g/L reduced R. raciborskii growth by 79.3%, whereas conditioned water had weaker effects, with low concentrations even slightly promoting growth. Oxidative stress was identified as a primary mechanism underlying this inhibition, as evidenced by elevated activities of antioxidant enzymes (catalase and superoxide dismutase), and heightened lipid peroxidation (malondialdehyde content). Notably, the epiphytic microorganisms on V. natans enhanced its allelopathic effects. Conversely, exposure to high densities of R. raciborskii negatively affected V. natans growth and induced oxidative damage, indicating a reciprocal stress relationship. However, V. natans displayed robust tolerance to high R. raciborskii densities, maintaining survival at cyanobacterial levels up to 5 × 10⁸ cells/L. This work fills knowledge gaps by integrating multiple exposure modes and exploring reciprocal interactions, providing novel mechanistic insights into submerged macrophyte–cyanobacteria allelopathy. These findings underscore the potential of V. natans as an eco-friendly biological control tool for managing R. raciborskii blooms, with important implications for freshwater ecosystem restoration and sustainable water quality management.