Oxygen deficiency (anoxia) is spreading in coastal seas and estuaries as a consequence of anthropogenic loading of nutrients and global warming. This has strong implications for ecosystems functioning including the expansion of “dead zones” and severe alterations of global biogeochemical cycles. The overall aim of this project is to advance the knowledge about the consequences of deoxygenation of coastal systems on the microbial metabolisms (respiration, photosynthesis, mercury transformation) and the subsequent impacts on their functions at an ecosystem-scale.