Slower-growing species promote interspecific cooperation and coexistence under acid stress through cross-feeding.

Acid stress is a central environmental factor shaping the structure and function of microbial communities worldwide. However, there is a lack of predictive understanding of how microbial communities respond physiologically and metabolically to acid stress. Here, we find that higher acid stress favors slower-growing species, promoting population growth and coexistence. Our experiments show that acid stress influences the spatial structure of communities, wherein coexistence is ordered over centimeter-length scales and determined by growth-tolerance trade-offs. We find that interspecific interactions are highly dynamic during acid stress changes, with shifts from competition to cooperation, enhancing resilience under high-stress intensities. Slower-growing species may bolster interspecific coexistence through stress-dependent excretion and cross-feeding of public goods. We construct a resource-consumer-based mathematical model to unravel the processes experienced by species in stress-induced coexistence and their distinct physiological states. Finally, our pairwise bacterial-fungal interaction experiments elucidate universalities in stress-induced coexistence between closely related and phylogenetically distant species with complementary phenotypic profiles. Overall, our work provides insights into how acid stress affects physiological and metabolic responses, as well as overall fitness, resilience, and coexistence.