Effects of different culture media on growth, composition, quality and palatability of the green algae Ulva sp. cultivated in cylindrical photobioreactors

Ulva spp. are valuable seaweeds with recognized commercial applications, including food, feed, and ecosystem services. Ensuring a sustainable and consistent supply of biomass with desirable profiles aligned with intended uses is fundamental for the successful applications of this seaweed. In this study, the growth rate, morphology, physiology, and composition of Ulva sp. produced by propagation in indoor cylindrical photobioreactors using four different culture media (lagoon water - LW, lagoon water enriched with Guillard medium (LF), with sea urchin wastewater - LU, and cow digestate - LD) was assessed; moreover, the nutrient uptake potential of the species was evaluated. The palatability and attractivity of the produced biomass towards the sea urchin Paracentrotus lividus were investigated. It was found that the media influenced all the parameters examined, the LF biomass weight was double compared to the other treatments and showed a slightly higher absorbance. Colorimetric analyses reported a significant darker color in Ulva sp. grown under enriched media. Ulva sp. showed higher nutrient removal potential in LF. The lipid content did not vary (2-3 % dry weight, DW), while the protein content ranged from 21 % in LF to 6-9 % in the other treatments. Carbohydrates and fiber content were significantly lower in LF (16 % and 30 %) compared to the other treatments, 27-34 %, and 41-48 %, respectively. Pigment content significantly varied, being higher in biomass grown in LF and LU. Sea urchins showed preferences for biomass grown under LU, followed by LD. This study shows how different nutrient sources affect the biochemical composition, growth, quality, and palatability of Ulva sp.. When cultivated under the synthetic enriched media (LF) the species exhibits characteristics better suitable for human consumption, although requiring a higher economic investment for production, while biomass derived from wastewater nutrients (LD, LU) confirms potential applications of the seaweed as valuable feed and for bioremediation services.