Cultivable seaweeds eliminate the lethal effects of the harmful alga, Margalefidinium polykrikoides, on early life stage fish

Margalefidinium polykrikoides (aka Cochlodinium polykrikoides) is a harmful algal bloom (HAB)-forming dinoflagellate that is well-known for its ichthyotoxic properties, causing economically significant fish kills globally during bloom events. In this study, we evaluate the ability of seaweeds, including Ulva spp., Gracilaria sp., Chondrus crispus, and Grateloupia turuturu to mitigate the ichthyotoxicity of cultured and bloom populations of M. polykrikoides towards early life stage forage fish (Menidia menidia, Menidia beryllina, and Cyprinodon variegatus). In assays with both cultured and wild M. polykrikoides, live seaweed thalli – particularly Ulva spp. – significantly extended fish survival in all cases, with higher concentrations of seaweed resulting in a greater protective effect. In some cases, seaweeds completely mitigated fish mortality, resulting in survival not statistically different from the control. DMSP, an antioxidant compound produced by seaweeds, significantly reduced the ichthyotoxic effects of M. polykrikoides (p < 0.005) with no Ulva present, suggesting that the antioxidant properties of seaweeds and seaweed metabolites may be partly responsible for the protective effects. Partition chamber survival assays demonstrated that close physical proximity between the fish and Ulva was necessary for a protective effect to occur. Finally, we show that over longer timescales, very low concentrations of Ulva (0.2 g L-1) can significantly reduce (68–92% reduction) bloom populations of wild M. polykrikoides compared to control (p < 0.05). Collectively, the results of this study demonstrate that (1) aquaculture of Ulva spp. may be a viable strategy to protect caged fish farms from M. polykrikoides blooms, (2) existing beds of Ulva and other seaweeds may provide a refuge for vulnerable fish populations during M. polykrikoides blooms, and (3) the protective effects of seaweeds from M. polykrikoides ichthyotoxicity is due to a localized antioxidant-rich microenvironment around the surface of the seaweed, neutralizing short-lived reactive oxygen species produced by M. polykrikoides.