Anthropogenic activities regulate the assembly strategy of conditionally rare taxa in airborne microbial communities

Abstract

Airborne microorganisms, vital components of particulate matter, usually distribute unevenly and most species belong to rare taxa. Conditionally rare taxa (CRT) are the active members among rare taxa, and occasionally become dominant. However, knowledge of CRT remains limited in airborne microbial community. Here, we explored the spatial distribution and assembly mechanism of airborne bacterial and fungal CRT in regions with different intensities of human activities (urban, suburban, forest). Our results showed that microbial CRT occupied 39.58 %–40.88 % bacteria and 14.09 %–19.89 % fungi, respectively, while they contributed more than half of richness of whole microbial community. Air pollutants showed strong correlations with richness and diversity of airborne microbial CRT. Significantly spatial differences were also observed for CRT composition. The neutral community model and normalized stochasticity ratio revealed that stochastic processes dominated in the assembly of airborne microbial CRT. Furthermore, stochastic processes exhibited an obviously stronger impact on fungal CRT (NST = 0.662) than bacterial CRT (NST = 0.503), likely due to the broader niche breadth and higher migration rate for fungal CRT. Additionally, correlation analysis demonstrated that NO₂ concentration was the most crucial factor regulating the CRT assembly and diversity. Overall, these findings advance our understanding of the assembly processes of airborne microbial CRT, which is critical to decipher their ecological functions.