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Assessment of abiotic impact on seaweed chemical composition is required for their use as crops and feedstock for biorefinery. This study investigated how seasonality and the culture age of Ulva lactuca grown in plastic sleeves in a marine photobioreactor with flow-through seawater in Mikhmoret onshore (East Mediterranean, Israel), affected the composition of the amino acids and monosaccharides. Fresh samples of young (14 days) and old (21 days) U. lactuca were sampled from the photobioreactor cultivation system from January to December 2020. DNA sequences of all studied seaweed samples, which corresponded to the U. lactuca sequence, were identified using the rbcL and ITS markers as well as phylogenetic analysis. In addition, the protein and carbohydrate contents as well as the amino acid and monosaccharide profiles were determined and quantified by thermal acid hydrolysis, silyl-based derivatization, and gas chromatography-mass spectroscopy (GC–MS) analysis. The biomass components of U. lactuca were more influenced by season than by their culture age. The total protein in the dry weight of U. lactuca was significantly higher in winter (170–237 mg g−1) than in summer (64–123 mg g−1), whereas the monosaccharide content was significantly higher in summer (117–182 mg g−1) than in winter (88–98 mg g−1). These findings indicated that seasonality is an important parameter that must be considered to predict the best time to harvest U. lactuca biomass for biorefinery and food processing. Finally, an in-vitro digestibility study indicated that U. lactuca has the potential to compete with plants as a suitable protein source. [ABSTRACT FROM AUTHOR]