Ocean acidification (OA) will affect marine biotas from the organism to the ecosystem level. Yet, the consequences for the biological carbon pump and thereby the oceanic sink for atmospheric CO₂ are still unclear. Here we show that OA considerably alters the C/N ratio of organic-matter export (C/N_export), a key factor determining efficiency of the biological pump. By synthesizing sediment-trap data from in situ mesocosm studies in different marine biomes, we find distinct but highly variable impacts of OA on C/N_export, reaching up to a 20% increase/decrease under partial pressure of CO₂effects on a variety of plankton taxa and corresponding shifts in food-web structure. Notably, our findings suggest a pivotal role of heterotrophic processes in controlling the response of C/N_export to OA, thus contradicting the paradigm of primary producers as the principal driver of biogeochemical responses to ocean change.