Associations between microalgae and bacteria represent one of the most important ecological relationships in aquatic microbial ecosystems In this proposal, we focus on the largely unknown diatom-bacteria interactions in marine biofilms We forward the general hypothesis that the high diversity observed in diatom-bacteria consortia in these systems is to a great extent mediated by specialized and species-specific interactions related to the production and consumption of diatom exudates, mainly extracellular polymeric substances (EPS) More specifically, we will test the following hypotheses: (1) different diatoms secrete different EPS mixtures, and these select for and enhance the growth of different bacterial taxa specialized in the use of specific EPS components; (2) bacteria stimulate diatom growth and EPS production, and affect its composition, in a species-specific manner; (3) these mutualistic, substrate-mediated interactions enhance the fitness of both diatoms and bacteria and facilitate coexistence and hence diversity in marine biofilms; (4) this EPS-based niche segregation is micro-spatially structured, further facilitating coexistence To unravel these substrate-related interactions in microbial biofilms, we will use a combination of advanced analytical, molecular, stable isotope probing and imaging techniques