Process Understanding – The determination of an empirical relationship between stressors and the respective responses in growth and elemental composition of a phytoplankton species is the necessary first step to estimate the impact on organisms, ecosystems and the cycling of elements. Nevertheless, this commonly used approach cannot explain the observed responses, as it does not provide any information about the underlying processes. Moreover, the sensitivity of a phytoplankton species towards stressors may differ depending on the investigated ‘cellular level’. Only by extending the observations to the level of subcellular processes, we are, therefore, able to make sound statements on what is actually happening in the cells and more importantly why. To achieve this goal, we apply several approaches that target different levels of physiology by using a combination of mass-spectrometric (membrane-inlet mass spectrometry, MIMS), fluorometric (fast repetition rate fluorometry, FRRF) and isotope tracer techniques (14C-/18O-labelled substrates). These in vivo measurements are supported by ‘functional transcriptomics’ to identify the underlying molecular machinery and regulation patterns therein. The deduced patterns are confirmed with basic ‘targeted metabolomics’ approaches, in which quotas of key metabolites are assessed. Measurements on these different levels of cell biology then allow us to explain changes in cellular fluxes of elements (e.g. C, N, P) and energy (e.g. ATP, [e-]) under different future scenarios, and to identify bottlenecks and trade-offs in metabolic pathways that explain different sensitivities of species and groups.