The aim of this proposal is to investigate the role of algae-bacteria interactions on the carbon and nutrient sequestration potential of phototrophic biofilms. This will be done using a combination of field and laboratory studies to obtain a better insight into the nature of the associations between microalgae and prokaryotes in biofilm communities along a nutrient loading gradient, and how the microalgae-microalgae and microalgae-prokaryote interactions affect the productivity and nutrient retention by biofilms communities.