Acid stress and cross-feeding provide a dynamic mechanism of microbial coexistence*

Despite the ubiquity of microbial diversity observed across environments, mechanisms of cooperativity that enable species coexistence beyond the classical limit of one-species-per-niche have been elusive. Here we report the observation of a transient but substantial cross-feeding of internal metabolites between two marine bacterial species under acid stress, and further establish through quantitative physiological characterization of the individual strains that this cross-feeding is central to the coexistence of these species in growth-dilution cycles. The coculture self-organizes into a limit cycle in which acid-stressed producers excrete various internal metabolites upon entering growth arrest, enabling the cross-feeders to grow, restore medium pH, and protect the producers from death. These results establish a mechanism in which functional niches are dynamically emergent from self-inhibited growth of a fast-growing species and subsequent resource sharing that allows a slower-growing species to sufficiently grow to enable coexistence.