Phenolics in microalgae: a forgotten fraction with high antioxidant potential Goiris, K., Muylaert, K., De Cooman, L. Over the last decades, the antioxidant potential of microalgae has been acknowledged by many researchers. Most of the research on antioxidant properties of microalgae focused on the wellcharacterised carotenoid fraction and on the enzymatic antioxidant response. However, much less is known about the composition and antioxidant properties of the (poly)phenolic fraction of microalgae. The objective of our work was to evaluate the role phenolics play in the antioxidant action of microalgae. Therefore, in 32 microalgal samples of varying species, both carotenoid and phenolic contents were measured and antioxidant activities were assessed using 4 different assays (TEAC assay, FRAP assay, AAPH induced oxidation of linoleic acid, and square wave voltammetry). Additionally, the effects of nitrogen or phosphorus limitation on phenolic content, antioxidant activity and resistance towards UV-stress were studied in batch culture with three species (Tetraselmis suecica, Chlorella vulgaris and Phaeodactylum tricornutum). The results showed that antioxidant activity strongly depends on the assay used, varies amongst algal species and that, next to carotenoids, phenolics are also important contributors to the antioxidant power of microalgae. It was further found that antioxidant activity of microalgae is lower under nitrogen or phosphorus limitation as is phenolic content. Furthermore, nutrient limited cultures were also more susceptible to UV-radiation, despite of the higher carotenoid/chlorophyll ratio in these cultures. This indicates that not only xanthophyll cycling is responsible for the protection against UV but also other antioxidants that are less formed under phosphorus or nitrogen limitation play a significant role.