Heterotrophic and chemoautotrophic prokaryotes are probably differently affected by increasing CO₂ concentrations. Whereas the former are mainly indirectly impacted via phytoplankton and dissolved organic matter production (DOC, supply and composition), the activity of the latter group could be directly influenced by changing pH and DIC. During BIOACID phase 2 our goal will be to examine the response and adaptation of selected prokaryotic key players, which are known to be relevant in the Baltic Sea pelagial, by a combination of laboratory and mesocosm experiments. Currently, existing experimental evolution experiments with microbes have used typical laboratory microbes (e.g., E. coli) but generally not those taxa which are environmentally relevant (Elena & Lenski 2003).