Fermentative Escherichia coli makes a substantial contribution to H2 production in coculture with phototrophic Rhodopseudomonas palustris
Individual species within microbial communities can combine their attributes to produce services that benefit society, such as the transformation of renewable resources into valuable chemicals. Under defined genetic and environmental conditions, fermentative Escherichia coli and phototrophic Rhodopseudomonas palustris exchange essential carbon and nitrogen, respectively, to establish a mutualistic relationship. In this relationship, each species produces H2 biofuel as a byproduct of their metabolism. However, the extent to which each species contributes to H2 production and the factors that influence their relative contributions were previously unknown. By comparing H2 yields in cocultures pairing R. palustris with either wild-type E. coli or a formate hydrogenlyase mutant that is incapable of H2 production, we determined the relative contribution of each species to total H2 production. Our results indicate that E. coli contributes between 32% and 86% of the H2 produced in coculture depending on the level of ammonium excreted by the R. palustris partner. An R. palustris strain that stimulated rapid E. coli growth through a high level of ammonium excretion resulted in earlier accumulation of formate and acidic conditions that allowed E. coli to be the major contributor to H2 production.