Our understanding of the role of microbe-host associations in driving biogeochemical cycling in coastal benthic ecosystems is limited at best, and so is our ability to predict the consequences of human impacts on the functioning of these ecosystems. One of the most serious impacts of anthropogenic emissions is ocean acidification (OA). However, very little is known on the effects of OA on biogeochemical cycling driven by benthic microbe-host associations. Therefore, this project will aim to answer the overarching question of whether microbe-host associations act as drivers of biogeochemical cycling in benthic coastal ecosystems, and whether and how OA will affect fluxes of C and nitrogen (N) at the (meta)organism and at the ecosystem scale.