The increasing CO2 levels in the atmosphere and its high uptake by the oceans are lowering the pH of the oceans. Dramatic changes in ocean pH have been shown to negatively affect ocean biota such as phytoplankton and coral reefs. However, there is little known regarding the impact of decreasing ocean pH on marine bacteria. This project seeks to evaluate the effects of the ocean acidification on the structure and function of marine bacterioplankton communities. Proteorhodopsin (PR) phototrophy are widespread among marine bacterioplankton species and its functioning has been predicted to be seriously affected by future pH reductions. This proposal will examine the performance of PR-containing bacteria in this global change scenario. Bacterial responses to pH will be analyzed in: 1) highly controlled systems (physiology of bacteria in pure cultures) 2) relatively controlled systems (physiology and diversity of bacteria in mesocosm experiments), and 3) field studies (in situ physiology and diversity analyses of native bacterioplankton communities). These three degrees of control/manipulation will give this project a unique perspective that will improve our capacity to predict the consequences of the projected pH alteration. The proposed research will follow the European Science Foundation recommendations by addressing the consequences of the important climate change problem of ocean acidification from the organismal and ecosystem perspective in a national and international level. Moreover, this project will contribute to the objectives of the Work Programme by: 1) allowing me to gain new skills in oceanographic fieldwork and experimental manipulations; and to become an EU leading specialist in a rapidly growing research field 2) transferring the acquired knowledge to the EU; 3) establish a long-term collaboration between two centers of excellence like the USC (USA) and the ICM-CSIC (Spain) that will enhance the quality of research on Europe.