Substrate complexity and species number jointly shift the balance between competition and facilitation in a chitin-degrading bacterial consortium.

Biopolymer-degrading microbial communities exhibit complex interactions like division of labor, sharing of public goods, and resource-competition, resulting in intricate social dynamics between their diverse members that shape the functions they perform. How these interactions shift with substrate complexity as species number increases remains largely unresolved, especially as substrate type can fluctuate. Here, we investigated how these factors modulate microbial interactions and community functioning in a synthetic three-species chitin-degrading soil bacterial consortium. All mono-, duo-, and tri-cultures were grown under well-mixed conditions on a labile substrate (N-acetylglucosamine, NAG), a complex polymer (chitin), or a combination of both. We quantified respiration, final biomass, and community structure, chitinase activities, degradation products and cross-feeding responses, and integrated these data with consumer-resource modeling. Substrate complexity altered interaction patterns. On NAG, we frequently observed competitive exclusion; while on chitin less competitive species were still detectable at the end. This suggests a shift from competition toward facilitation. The tri-culture showed significant final biomass increases on NAG, likely related to cross-feeding effects and enhanced polymer degradation and respiration on chitin, indicating multi-species facilitation. Model-experiment discrepancies indicate that competition for primary resources and decomposition product-sharing alone cannot explain the observed community dynamics, suggesting additional metabolic and behavioral interactions. While it remains to be tested whether our findings translate beyond the well-mixed conditions studied here, they show how the balance between positive and negative interactions in this consortium shifts with substrate complexity and species number, shaping community structure and functioning.