Thiamine cross-feeding sustains diversity and promotes carbon mineralization during soil microbial community coalescence

Environmental disturbances often drive the mixing of previously isolated microbial communities, a process termed coalescence, which reshapes community structure and interactions. While coalescence is recognized as a major driver of microbial diversity and ecosystem functioning, the specific metabolic mechanisms that stabilize coexistence remain poorly understood. In this study, we established controlled soil microcosms with single-source (derived from individual sites) and mixed-source inocula (coalesced from multiple sites) and utilized an integrated approach of metagenomics, genome-scale metabolic modeling, and vitamin supplementation to investigate the mechanisms that sustain diversity during coalescence. Coalescence increased α‑diversity but reduced β‑diversity, with most taxa originating from their single-source communities. At the genomic level, coalescence increased both metagenome-assembled genome (MAG) richness and functional trait breadth, with Bacteroidota and Pseudomonadota remaining the dominant lineages across all treatments. Coalesced communities exhibited extensive chimeric associations (newly formed associations), which were accompanied by enhanced metabolic complementarity and more frequent positive interactions among dominant MAGs affiliated with Bacteroidota and Pseudomonadota. We identified thiamine metabolism as a key mechanism stabilizing this co-dominance. Genomically streamlined Pseudomonadota prototrophs harbored the complete thiamine biosynthesis pathway, while most Bacteroidota MAGs (82%) were thiamine auxotrophs, forming the genetic basis for potential thiamine cross-feeding interactions that may alleviate widespread thiamine auxotrophy. The relative abundance of thiamine biosynthesis genes positively correlated with the abundance of cellulose and lignin derivative degradation genes (e.g., biphenyl, phenylacetate, and protocatechuic acid). In parallel, coalesced communities exhibited significantly higher soil respiration rates, indicating enhanced carbon mineralization potential. Exogenous thiamine supplementation further supported this mechanism by increasing community richness across dilution gradients. Together, our results indicate that vitamin-mediated metabolic dependencies support microbial coexistence during microbial community coalescence and are positively associated with enhanced soil carbon mineralization, with implications for understanding soil carbon processing under global change.