Substrate toxicity drives successive range expansions opposing spatial intermixing in cross-feeding consortia

Spatial organization plays a critical role in shaping microbial community structure and function, influencing ecological stability, resource utilization, and evolutionary dynamics. Microbial interactions such as competition and cooperation are key drivers of spatial patterning, yet the environmental factors modulating these interactions remain incompletely understood. Here, we investigated how toxic substrates influence the spatial organization of synthetic microbial communities engaged in metabolic cross-feeding. Using a synthetic Pseudomonas stutzeri consortium consisting of the detoxifier and consumer that cooperatively degrade the toxic compound salicylate, we found that increasing the substrate concentration leads to a distinct shift in spatial organization: the detoxifier increasingly dominates the outer periphery of the expanding colony, forming a "detoxifier-first" succession pattern. Mathematical modeling further revealed that this spatial arrangement emerges from substrate toxicity, which selectively favors the detoxifier. Substrate toxicity inhibits consumer proliferation. However, the detoxifier, capable of degrading the substrate, locally reduces toxicity and creates a protective microenvironment that enables nearby consumer cells to survive and grow. In return, the consumer provides essential final products that support the growth and expansion of the detoxifier. This reciprocal interaction establishes a directional dynamic in which the detoxifier, favored by its detoxification capability, colonizes first, paving the way for subsequent consumer proliferation. Our findings demonstrate that substrate toxicity is a crucial environmental factor shaping spatial organization and diversity in microbial communities. This study highlights the importance of considering both metabolic interactions and substrate properties in understanding microbial ecology.