Evaluating the Efficacy and Safety of Limnospira platensis Phycobiliproteins as a Functional Feed Additive for Oxidative Status Modulation in the Pacific Oyster

Despite the recognized antioxidant properties of phycobiliproteins (PBPs) from Limnospira platensis, their potential application in bivalve aquaculture—particularly regarding their modulation of endogenous antioxidant defenses—remains largely unexplored. This study evaluated the effects of an aqueous PBP extract at concentrations of 2, 20, and 80 µg/mL on antioxidant parameters in the Pacific oyster Magallana gigas following 24, 48 and 96 h exposure periods. Activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), levels of lipid peroxidation (TBARS) and protein oxidation, and expression of corresponding genes in gills and hepatopancreas have been evaluated. PBPs elicited significant effects on prooxidant–antioxidant balance in oyster tissues. Low concentrations (2–20 µg/mL) enhanced SOD and CAT activities and reduced protein oxidation, indicating protective effects. The highest concentration (80 µg/mL) suppressed enzyme activities—particularly GPx—while elevating TBARS, suggesting prooxidant effects at excessive doses. Tissue-specific responses were observed, with hepatopancreas more vulnerable to oxidative stress than gills. Notably, GPx transcription was upregulated at 80 µg/mL of PBP extract despite suppressed activity, implying its post-transcriptional regulatory properties. These findings demonstrate that L. platensis PBPs modulate oyster antioxidant status in a biphasic manner, with moderate doses conferring stimulation and high doses inducing oxidative stress. This study provides the first integrative evidence of PBP effects on enzymatic, and transcriptional antioxidant defenses in a commercially important bivalve, underscoring their promise as functional bioactive additives while emphasizing the need for careful dose optimization.