This work intended to optimise the antioxidant activity (AA) of two strains of Ulva lacinulata (Portuguese and Greek) used for food packaging. The enhancement of AA was attempted through the application of hyposaline stress on Ulva for 10 days in one experiment and stress caused by a high light intensity for 5 days in another. The AA was measured using the ABTS radical decolourisation assay. During the light experiment rapid light curves were conducted with help of a Pulse Amplitude Modulated (PAM) fluorometer (Imaging PAM, Heinz Walz GmbH, Germany) to assess the seaweed’s adaptation to a higher light intensity. The Fv/Fm values, initial slope, maximum electron transport rate and light saturation point of the samples were established and compared with the control group. Hyposaline conditions did not enhance AA of the Greek seaweed significantly. The Greek strain was found to have a significantly higher AA regardless of treatment after five days. An exposure to hyposaline conditions of 10 PSU for ten days resulted in a significant increase in AA of the Portuguese algae. A high light intensity resulted in significantly higher AA after five days of exposure in the Portuguese strain, whereas the seaweed of the Greek strain reached maximum AA after 48 hours regardless of treatment. The analysis of the rapid light curves and Fv/Fm values concluded, that the photosynthetic apparatus was not damaged by the high light intensity.