Cross-feeding dynamics and metabolic structure explain patterns of diversity in microbial communities

What makes some communities more or less diverse than others? What factors shape diversity in biological communities? Resources play an important role in the composition, diversity, and function of biological communities. Contrary to expectations from classical ecological theory, recent experiments have shown that diversity does not scale additively with the number of limiting resources supplied, instead, in these communities diversity is largely maintained through metabolite exchange (cross-feeding). These communities assemble following a modular pattern of resource consumption along a metabolic cascade. Based on these observations, we predicted that diversity of communities could be predicted by the possition in metabolism of the supplied carbon source. Instead, communities assembled in sugars often have less diversity than those assembled in organic acids (despite sugars being generally upstream of organic acids). Using replicated community assembly experiments we demonstrate that dynamics of resource accumulation and species invasion can explain these results. Overall our experiments highlight how metabolic structure can inform community assembly. These results underscore the importance of different resource axes in shaping diversity, that is quantity, quality, abundance and timing.