Translating the diatom-grazer defense mechanism to antiparasitic treatment for monogenean infection in guppies

Monogenean parasites impose a major constraint on sustainable aquacultural production and trade due to fish morbidity and mortality. Traditional chemical treatments carry environmental and human health risks and are frequently banned in aquaculture; associated development of pathogen resistance is also common. These highlight the need for alternative. The pennate diatom Phaeodactylum tricornutum is a potent source for food additives and pharmaceuticals due to its robustness and content of high-value compounds. Its extracts also have antibacterial properties. The use of P. tricornutum to develop a treatment against the monogenean parasite Gyrodactylus turnbulli affecting Poecilia reticulate (guppies) was investigated. Ethanolic extracts of freshly broken P. tricornutum biomass were effective against the parasite in vitro and in vivo. Sonication and Kolliphor EL (food-grade nonionic solubilizer and emulsifier) increased the efficacy of the ethanolic extract in vitro; complete detachment and 97% mortality of the parasite were achieved within 240 min at a concentration of 5 μL mL⁻¹. Complete infection clearance from fish was achieved following a bath treatment with 2.5 μL mL⁻¹ ethanolic extract for 24 h. The diatom extract's antibacterial properties, tested against the Gram-positive Streptococcus iniae and Gram-negative Vibrio harveyi, were used to calibrate P. tricornutum culture conditions and extract preparation. Maximal antibacterial effect was obtained with a two-week-old culture, following cell breakage of the wet biomass and extraction in 100% ethanol. Fatty acid composition analysis of thin-layer chromatography fractions of the ethanolic extracts revealed that the free fatty acids liberated during cell breakage, enriched in 16:0, 16:1n7, and eicosapentaenoic acid (20:5n3), are the major active compounds. In-vitro antiparasitic assays and gas chromatography–mass spectrometry analysis confirmed that free 16:1n7 and 20:5n3 are responsible for most of the antiparasitic activity. Results support the use of broken P. tricornutum ethanolic extracts as a potential treatment against monogenean infection in cultured fish.