Nutrient availability controls coupling between algae and bacteria

Photosynthetic (phototrophic) microbes are responsible for fixing nearly half of the carbon dioxide on the planet. Mircobial phototrophs perform this feat in the context of communities, where they form tight associations with non-photosynthetic (heterotrophic) microbes. Carbon flow between phototrophs and heterotrophs forms the basis of biomass production in ecosystems across the globe and is critical for the global carbon cycle. Despite the importance of phototroph-heterotroph communities, we do not understand how these consortia assemble and function. In particular, it is unclear how the availability of nutrients influences community assembly. To address this quesion, we grow complex bacterial consortia, obtained from soil, in the presence of the photosynthetic alga, Chlamydomonas reinhardtii. We ask how the frequency of exogenous nutrient supply influences community assembly. We find that when exogenous nutrients are supplied infrequently, the composition of the bacterial community in the presence of the alga converges across replicate communities over time. Our analysis reveals that this convergence arises from the down-regulation of certain bacterial taxa in presence of C. reinhardtii. Our study shows that when the frequency of externally supplied nutrients is low, phototrophs control bacterial community assembly, but when nutrients are supplied frequently, bacterial community assembly is decoupled from the phototroph. We hypothesize that this coupling arises from carbon exchange between the alga and bacteria.