Using byproduct of fucoxanthin production by Phaeodactylum tricornutum for the development of a treatment against monogenean infection in fish

Residual material from commercial production of the marine diatom Phaeodactylum tricornutum was used to develop a cost-effective treatment against monogenean infection, specifically Gyrodactylus turnbulli affecting ornamental fish guppies ( Poecilia reticulata ). The transparent and highly parasitized tail fin enabled direct observation of the parasites’ detachment and mortality. The residual biomass after fucoxanthin extraction was obtained as dry powder from a commercial producer and used for the antiparasitic preparations. Different preparations were normalized to the amount of biomass, using a constant ratio of 100 mg of residue powder per 2 mL, and included: (1) ethanolic residue extract (RE); (2) free fatty acids (FFAs), and (3) fatty acid ethyl esters (FAEEs). These preparations were tested in situ, and in vivo, using infected tail clips and fish, respectively. FAEE preparation, obtained by direct transesterification of residue, was the most effective, inducing 40% and 100% parasite mortality within 240 min of exposure to 2.5 and 5 µL mL –1 , respectively. In vivo, the 24-h immersion treatment of infected fish decreased infection prevalence from 100% to complete parasite clearance following exposure to 2.5 µL L –1 of all the preparations. The total fatty acid content of the FAEE preparation was comparable to that of the original residue, while extraction (RE) and FFA preparation from RE reduced the fatty acid yield to about 50%. Commercial FAEEs, representing the four major fatty acids (14:0, 16:0, 16:1, and 20:5 n 3) in all preparations, demonstrated antiparasitic activity in situ, of which EPA was the most active and showed a dose response.