The effects of global warming and ocean acidification on aquatic ecosystems have been intensely studied in the last decades. However, little attention has been put to the effects of short-term fluctuations, i.e. heat waves as well as CO2 and pH variations, which are also expected to intensify. A range of recent studies have revealed large phenotypic heterogeneity among single cells of cyanobacteria. Postulating that such cell-to-cell heterogeneity may present an advantage under rapidly fluctuating conditions, we propose to systematically investigate the contribution of cell-to-cell heterogeneity as well as physiological acclimation to phenotypic plasticity of cyanobacterial populations. We will investigate cultures grown under different environmentally relevant scenarios of fluctuating temperatures and CO2 levels and validate our findings in mesocosm experiments with natural communities. The analysis will involve detailed physiological characterization, advanced techniques with single-cell resolution (e.g. nanoSIMS) and an innovative combination of FACS and transcriptomics.