The green macroalga, Ulva lactuca, inhibits the growth of seven common harmful algal bloom species via allelopathy

Harmful algal blooms (HABs) are a significant threat to fisheries, public health, and economies around the world, and both HABs and macroalgae are often promoted by nutrient loading. We report on experiments examining the effects of the macroalga, Ulva lactuca, collected from estuaries of NY, USA, on the growth of seven common HAB species: Aureococcus anophagefferens, Chattonella marina, Cochlodinium polykrikoides, Karlodinium veneficum, Karenia brevis, Prorocentrum minimum and Pseudo-Nitzschia multiseries. Fresh thalli of U. lactuca added at environmentally realistic levels (mg L−1) were capable of lysing or strongly inhibiting the growth of all seven HAB species in a dose-dependent manner within controlled laboratory experiments during which high nutrient levels, low bacterial levels, and common pH levels among treatments and controls were maintained. The dramatic allelopathic effects of extracts of dried and powdered U. lactuca with and without post-extraction heat treatment on the HAB species demonstrated that U. lactuca contains heat-stable allelochemicals that play a major role in the observed allelopathic effects. The addition of live U. lactuca thalli in bottle and mesocosm experiments conducted in the field during blooms of A. anophagefferens (‘brown tide’; >105 cells mL−1) consistently yielded a significant (p < 0.05) and often large (>50%) reduction in cell densities in ∼48 h. Our findings combined with the well-known nutrient removal capacity of seaweeds collectively suggest that the use of macroalgae may be a promising mitigation strategy for HABs in coastal ecosystems.

► Ulva lactuca lysed or strongly inhibited seven HAB species.
► HAB inhibition was due to allelochemicals, not changes in bacteria, nutrients, or pH.
► U. lactuca reduced Aureococcus anophagefferens densities during blooms.
► Macroalgae may be a promising mitigation strategy for HABs.