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 H-2 biofuel as a byproduct of its metabolism. However, the extent to which each species contributes to H-2 production and the factors that influence their relative contributions were previously unknown. By comparing H-2 yields in cocultures pairing R. palustris with either wild-type E. coli or a formate hydrogenlyase mutant that is incapable of H-2 production, we determined the relative contribution of each species to total H-2 production. Our results indicate that E. coli contributes between 32 and 86% of the H-2 produced in coculture depending on the level of ammonium excreted by the R. palustris partner. The level of ammonium excretion influenced the time over which E. coli was exposed to formate, the types of E. coli fermentation products available to R. palustris, and the pH of the medium, all of which affected the contribution of each species to H-2 production.