Bacterial communities contain hundreds of species, so uncovering how each species contributes to ecosystem functioning can be challenging. We separated hundreds of aquatic bacterial communities from their environmental matrix and assayed each community in a common environment. The method allowed us to infer how species impacted a range of functional measurements, including measurements that were broad (e.g. metabolic activity) and narrow (e.g. degradation of specific substrates). The method allowed us to infer 'functional interactions': whether changes in ecosystem functioning could be attributed to the combined effects of pairs of species.
The results show sharp differences in the impact of bacterial species on broad vs. narrow functions. While broad functions were mediated by interactions among common species, narrow functions were dominated by positive interactions among rare species. The results illustrate the capacity for common garden experiments to unpick species contributions to functioning, and show contrasting roles of common and rare species in bacterial communities.