Description: Since the beginning of the industrial revolution the atmospheric partial pressure of CO2 (pCO2) has increased exponentially, reaching 380 μatm nowadays, and is expected to rise to values up to 700 μatm by the end of this century. These changes affect marine plankton in various ways, positively as for cyanobacteria, or in most cases, negatively as for coccolithophores. However there is a lack in the understanding of the effect of this increase in carbon for some important organisms as diatoms, an important primary producer in the ocean. Diatoms have not been reported as affected by ocean acidification, although several studies have reported a change of the total lipid content in some diatoms when cultured at high CO2 conditions. With this perspective, a set of two experiments were designed; the first was intended to determine if the amount of different fatty acids (the building blocks of lipids) of the diatom Thalassiosira pseudonana is altered when cultures under diverse CO2 conditions; while the second experiment was intended to determine the possible effects of the change in the fatty acids of T. pseudonana on the life cycle of the copepod Acartia tonsa when feed with this diatom. The first experiment showed that the fatty acid content of T. pseudonana change toward high CO2 levels, with an increase in the amount of saturated fatty acids and a decrease of unsaturated fatty acids content. The second experiment showed that the growth rate, amount of egg produced per female, and fatty acid content per female are reduced when feed with T. pseudonana cultured at high CO2 conditions. Our results show that CO2 actually affects the fatty acid composition of T. pseudonana and that this fatty acid alteration in the diatom have a significant influence on the life cycle of A. tonsa. However, further studies are required to determine if the effects observed in this study also take place in the environment.