There are now compelling observations of the ecological impacts of global change on marine ecosystems. A predicted 10% increase in UVB doses at the Earth's surface has been shown to impair indirectly the reproductive output of marine copepods. Wild females of Calanus helgolandicus fed with UVB-irradiated diatoms produced fewer eggs and unhealthy offspring exhibiting a large proportion of high lethal naupliar deformities. By reproducing the same irradiative stress on the pelagic diatom Skeletonema costatum, we looked for potential modifications of the algal properties (cell size, biovolume, chlorophyll content) and biochemical characteristics (fatty acid and amino acid contents). Our results confirmed that the metabolism of S. costatum is adversely affected by enhanced UVB exposure: cell growth was reduced and the biochemical characteristics, and thus the algal nutritional quality, were significantly altered. UVB irradiation reduced cell division, leading to cell elongation and thus increasing the biovolume. Meanwhile, the amino acid and fatty acid contents did not increase concomitantly to the cell enlargement and were thus diluted in the cell. Because of this dilution, the irradiated S. costatum represents a poorer diet for its potential consumers. Moreover, UVB dramatically affects the relative contribution of certain essential fatty acids such as eicosapentanoic acid (EPA), which are essential for the development of marine invertebrates