The present research investigates the antibacterial potential of Acanthophora spicifera and Ulva sp. seaweed extracts against Vibrio pathogens affecting Crassostrea gigas and Anadara tuberculosa larvae. The study identified 13 unique Vibrio strains from diseased C. gigas, with strains M4 and M8 significantly affecting the survival of C. gigas larvae (43 ± 13% to 0%) and strain M18 significantly impacting A. tuberculosa larvae (19 ± 5% to 7 ± 2%). Bacterial regrowth tests revealed that Ulva sp. extracts, with higher phenolic content (up to 249.86 ± 22 μg GAE/g), exhibited greater bacteriostatic activity than Acanthophora spicifera extracts (up to 26.18 ± 1.18 μg GAE/g). Ulva sp. extracts at 2 mg/mL inhibited Vibrio spp. M3 and M8, with inhibition rates up to 80%, while Acanthophora spicifera extracts showed moderate inhibition against Vibrio spp. M3 but were less effective against M8 and M18. Both extracts displayed dose-dependent toxicity to bivalve larvae, with 0% survival at 2 mg/mL and reduced survival at 1 mg/mL after 48 hours. This is the first time in Ecuador that Vibrio spp. pathogens are confirmed to be present in Crassostrea gigas and Anadara tuberculosa, two commercially important species. The study highlights the need for optimizing macroalgae extraction methods and employing techniques such as High-Performance Liquid Chromatography (HPLC) to obtain microbicidal and non-toxic compounds for aquaculture.
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