Effects of Nitrate Source (Artificial and Fishpond) and UV Radiation on Physiological, Photosynthetic, and Biochemical Parameters of Porphyra dioica for Sustainable Integrated Multitrophic Aquaculture (IMTA)

The red macroalga Porphyra plays a key role in Integrated Multi-Trophic Aquaculture (IMTA) systems, acting both as a biofilter and as a source of bioactive compounds (BACs) with nutritional and photoprotective value. This study evaluated how nitrogen source and concentration influence its physiological, photosynthetic, and biochemical responses under ultraviolet radiation (UVR). Gametophytes were cultured for four days under two nitrate sources (artificial and fishpond effluents) at 3 and 5 mM concentrations and exposed to PAR (120 µmol·photons·m−2·s−1) and UVR (9 W·m−2 for 6 h·day−1). Morphological responses, photosynthetic performance, and BACs were quantified. Nitrate uptake increased with nitrate concentration, while growth rate remained unaffected. Samples grown with fishpond effluents, particularly at 3 mM, showed darker pigmentation and higher phycoerythrin and mycosporine-like amino acid (MAA) contents, indicating enhanced nitrogen assimilation and photoprotective capacity. Conversely, 3 mM artificial nitrate in the water promoted the highest electron transport rate and lowest non-photochemical quenching, suggesting greater photosynthetic capacity. Polyphenols and antioxidant activity showed no significant differences among treatments, indicating similar stress status. Overall, it is suggested that fishpond effluents acted as a natural biostimulant, enhancing biliprotein and MAA synthesis without compromising physiological stability, reinforcing its potential for sustainable IMTA-based production of high-value photoprotective compounds.