Water-soluble natural products from seaweed have limited potential in controlling bacterial pathogens in fish aquaculture

Seaweeds producing natural products with in vitro efficacy against bacterial pathogens offer the opportunity for therapeutic services in fish aquaculture, for example inhibiting the growth of pathogens through the direct release of bioactive metabolites into the culture environment. The red alga Asparagopsis taxiformis was used to test this hypothesis because of its effective natural products and mechanisms to release these metabolites into the surrounding environment and because it can be cultivated intensively in land-based systems. Here we quantify the release, accumulation and residence time of the major halogenated metabolites from A. taxiformis in the cultivation medium and subsequently test the in vitro bioactivity of these water-soluble metabolites against multiple strains of the pathogenic bacterium Streptococcus iniae. The two major halogenated metabolites in A. taxiformis (bromoform and dibromoacetic acid) were released into the water and reaching steady-state concentrations of 27.7?±?1.5 μg L^?1 and 4.8?±?1.3 μg L^?1, respectively, under standard cultivation conditions. This water delayed the in vitro growth of the pathogenic bacteria S. iniae but did not prevent it. Increasing the levels of A. taxiformis metabolites in the water by three orders of magnitude with aqueous extracts of biomass inhibited the growth of S. iniae, but it was also toxic to fish. In an immersion challenge experiment, intermediate non-ichthyotoxic levels of A. taxiformis metabolites were ineffective as a treatment for barramundi infected by S. iniae. The evidence suggests that there is little potential for A. taxiformis bioactive metabolites to provide therapy services to fish infected with S. iniae.