This invention provides co-cultures of photosynthetic microorganisms and biofuel producing microorganisms. In certain embodiments, polysaccharide-producing, photosynthetic microorganisms are microalgae having frustules provide a substrate on which biofuel-producing microorganisms can grow. In other embodiments, the photosynthetic microorganisms produce a lipid and the non-photosynthetic microorganisms produce a solvent in which the lipid is soluble.