Carbon regime structures functional trait trajectories during primary succession in microorganisms

Primary succession is a foundational process in ecology, but how microbial communities shift functionally during succession, and whether these dynamics follow predictable patterns, remains unresolved. We conducted a systematic review of functional primary succession in microorganisms and applied a consistent metagenomic pipeline to evaluate functional richness, rRNA operon copy number (RRN), and average genome size (AGS) over time. We also explored the yield-acquisition-stress life-history framework using functional gene annotations. Across autotrophic systems, RRN tended to decrease and AGS tended to increase during succession, whereas heterotrophic systems exhibited more variable trajectories. These consistent shifts in autotrophic systems suggest a transition from early colonization by copiotrophic taxa with small genomes and high RRN toward later-stage communities with larger genomes, lower RRN, and greater functional versatility. In contrast, heterotrophic systems showed heterogeneous trait trajectories, likely reflecting variation in the timing and predictability of organic inputs. Topic modeling further revealed that early successional stages were enriched in stress-tolerance genes, followed by shifts toward other strategies over time. While certain trait patterns such as RRN and AGS appeared broadly conserved, changes in life-history strategies during succession were context dependent and shaped by resource dynamics and system type. These findings suggest that microbial successional trajectories are structured by differences in resource availability, particularly whether systems are driven by autotrophic inputs or constrained by externally supplied carbon sources.