Our aim is to investigate the indirect effects of ocean acidification on pelagic food webs bydetermining to which extent the variable carbon to nutrient ratio of phytoplanktonpropagates through the food chain, also investigating the bacteria associated with the differentplayers. Until now, we have focused on a model food chain comprising the nanoflagellateRhodomonas salina (prey), the copepod Arcatia tonsa (predator) and their respective bacterialcommunity.We have a closer look at the growth responses and changes in nutrient stoichiometry due toraised CO2-levels. Algae grown under high CO2 have an increased carbon to nutrient ratio intheir tissue, which results in two opposing effects on herbivorous consumers feeding on them:increased food availability comes along with decreased food quality. The extra carbon if notrespired might be metabolized by the bacterial community. Additionally, we are interestedin whether a reduction in sea water pH alone causes shifts in the bacterial communitycomposition.