Diverse microbial communities assemble on both recalcitrant and labile carbon sources

Microbial community assembly is shaped by the nature of available resources, with labile carbon sources such as glucose often expected to support low diversity due to rapid growth and competitive exclusion. In contrast, recalcitrant substrates like cellulose are thought to support higher diversity through slower growth and increased niche partitioning. In previous work, we showed that compost-derived microbial communities propagated on cellulose maintained high diversity over nearly a year. To determine whether such diversity is specific to recalcitrant substrates or reflects more general features of assembly, we tracked community dynamics in three environments - cellulose paper, cellulose broth, and glucose - using daily 16S rRNA profiling over eight weeks. Communities were maintained through bi-weekly serial transfers, with five replicates per treatment, yielding a high-resolution dataset of over 800 samples. Despite originating from the same inoculum, communities diverged sharply in both taxonomic and functional composition. Cellulose environments yielded stable communities enriched in specialists, while glucose environments exhibited rapid succession and dominance by generalists. Surprisingly, all environments sustained comparably high levels of taxonomic diversity. Functional inference suggested extensive cross-feeding and resource salvaging in both cases. While the mechanisms maintaining diversity on glucose remain unresolved, our results reveal distinct assembly trajectories under simple carbon regimes and provide a foundation for future mechanistic study.