Enhanced nutrient uptake is sufficient to drive emergent cross-feeding between bacteria in a synthetic community

Interactive microbial communities are ubiquitous, influencing biogeochemical cycles and host health. One widespread interaction is nutrient exchange, or cross-feeding, wherein metabolites are transferred between microbes. Some cross-fed metabolites, such as vitamins, amino acids, and ammonium (NH4+), are communally valuable and impose a cost on the producer. The mechanisms that enforce cross-feeding of communally valuable metabolites are not fully understood. Previously we engineered a cross-feeding coculture between N-2-fixingRhodopseudomonas palustrisand fermentativeEscherichia coli. EngineeredR. palustrisexcretes essential nitrogen as NH(4)(+)toE. coli, whileE. coliexcretes essential carbon as fermentation products toR. palustris. Here, we sought to determine whether a reciprocal cross-feeding relationship would evolve spontaneously in cocultures with wild-typeR. palustris, which is not known to excrete NH4+. Indeed, we observed the emergence of NH(4)(+)cross-feeding, but driven by adaptation ofE. colialone. A missense mutation inE. coliNtrC, a regulator of nitrogen scavenging, resulted in constitutive activation of an NH(4)(+)transporter. This activity likely allowedE. colito subsist on the small amount of leaked NH(4)(+)and better reciprocate through elevated excretion of fermentation products from a largerE. colipopulation. Our results indicate that enhanced nutrient uptake by recipients, rather than increased excretion by producers, is an underappreciated yet possibly prevalent mechanism by which cross-feeding can emerge.