Evaluation of antioxidant and antibacterial activities of lipid extracts from Eustigmatos cf. polyphem (Eustigmatophyceae) and preliminary identification of bioactive compound

Eustigmatos cf. polyphem is characterized by the production of large amounts of lipids containing various bioactive compounds; however, few studies have investigated its potential pharmacological properties. Therefore, this study aimed to evaluate the antioxidant and antibacterial activities of an E. cf. polyphem lipid extract and to further isolate and identify its main antimicrobial substances. Orange-red refined algal oils (ORAOs) were successfully extracted and purified from this microalga using a vacuum glass reactor and silica column chromatography to yield 21.19 g/100 g algal powder. ORAOs mainly contained triacylglycerol (84.20%, wt%), palmitoleic acid (38.79%, wt%), and β-carotene (1.96%, wt%). Although ORAOs had a low reducing power, they showed excellent dose-dependent activity for scavenging 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) and inhibiting lipid peroxidation, with a half-maximal inhibitory concentration of 0.56 mg mL−1 and 0.09 mg mL−1, respectively. Moreover, ORAOs exhibited antibacterial activity against Vibrio alginolyticus and V. parahaemolyticus. Palmitoleic acid was preliminarily identified as the dominant anti-Vibrio agent in ORAOs by bioassay-guided fractionation, and it was then isolated and enriched with its content increasing from 59.23% to 78.02%. Meanwhile, the antibacterial activity of the concentrated palmitoleic acid was determined against V. alginolyticus and V. parahaemolyticus, resulting in inhibitory zone diameters (10.23 and 10.49 mm, respectively, at 10 mg mL−1) comparable to those of the palmitoleic acid standard. It suggests that ORAOs containing palmitoleic acids is anti-Vibrio agent. These findings provide new insights into the valorization of functional lipids accumulated by E. cf. polyphem for its potential use in high-value bioproducts instead of low-value biodiesel.