The goal of this study was to quantitatively investigate the links between changes in the ocean carbonate system and organism physiology and morphometry (including CaCO3 shells), biogeochemical rates, plankton biodiversity and community structure, food webs and climate-relevant processes. For that purpose we devised and implemented a series of highly replicated (n=8), short term (2-4 days) multi-level (>=4 conditions) carbonate chemistry (E1-E5)/nutrient manipulation (E2b, E3b and E5b) experiments on a range of natural microbial communities sampled in the surface mixed layer (