Cyanobacteria can allelopathically inhibit submerged macrophytes: Effects of Microcystis aeruginosa extracts and exudates on Potamogeton malaianus

Submerged macrophytes are known for their positive effects on water clarity, especially in shallow lake ecosystems. Eutrophication of these systems, however, has often resulted in a decline of submerged vegetation and a dominance of cyanobacteria, producing and releasing allelopathically active substances. Whether and how these allelochemicals affect submerged macrophytes is, however, largely unknown. We tested the effect of exudates and extracts of one of the most common cyanobacteria species, Microcystis aeruginosa, on seed germination and seedling growth and photosynthesis of the submerged macrophyte Potamogeton malaianus, a common species in Chinese shallow lakes. We found that: (1) low and high exudate concentrations significantly promoted or inhibited P. malaianus seed germination, respectively, while extracts had no effect; (2) low exudate and medium extract concentrations significantly increased, whereas high concentrations decreased the biomass of P. malaianus seedlings; (3) chlorophyll content and maximum quantum yield of PS II (Fv/Fm) of the seedlings were decreased by all levels of exudates; (4) of the various responses tested, seedling leaf Fv/Fm showed the greatest sensitivity to M. aeruginosa allelochemicals; (5) the allelopathic effects of M. aeruginosa exudates were stronger than those of extracts. Our results indicate that cyanobacteria exudates can allelopathically inhibit submerged macrophytes. The early growth of seedlings rather than seed germination seems to be a sensitive phase for the impact of cyanobacterial allelochemicals.