The origin of air masses determines the seasonal recurrence of airborne microorganisms

Microbes represent an important component of atmospheric aerosols with the potential to impact human health and atmospheric dynamics. However, our knowledge on the temporal dynamics and potential sources of airborne microbial populations remains poorly explored, and no studies have addressed these questions with long-term datasets. In this study we applied high-throughput sequencing techniques (16S rRNA and 18S rRNA amplicon) to characterize the microbial communities deposited by precipitation in a high elevation site in the Central Pyrenees (NE Spain) over a period of seven years. Modelled back trajectories and factor analysis of precipitation chemistry data were used to characterize the air masses associated to each precipitation event. Both explained variations among microbial communities.Airborne bacterial and eukaryal communities changed seasonally, but the composition by season was remarkably similar during the whole seven-year period.. Summer and winter communities were the most distinctive ones. Indicative taxa were identified mostly related to these two seasons. Hence, our long-term analysis demonstrates the predictive character of certain microbes. Source-tracking analyses indicated that freshwater, cropland and urban biomes represented important sources of bacteria mostly in summer, while marine and forest biomes prevailed in winter, in agreement with the provenances of air masses associated to precipitation events. Overall, we concluded that the composition of airborne atmospheric microbes is mostly driven by the origin of air masses and, thus, the general precipitation regime in the Mediterranean area. We point out precipitation sampling as a cost-effective and realistic method for monitoring atmospheric microbes.

微生物是大气气溶胶的重要组成部分，对人类健康与大气动力学具有潜在影响。然而，目前学界对空气传播微生物种群的时间动态特征及其潜在来源的认知仍较为匮乏，且尚无研究借助长期数据集对上述问题开展系统性探究。本研究采用高通量测序技术（high-throughput sequencing），对西班牙东北部比利牛斯山脉中部高海拔站点为期七年的降水沉积微生物群落进行表征，测序靶点涵盖16S rRNA与18S rRNA扩增子。本研究结合降水化学数据的后向轨迹模拟与因子分析，对每次降水事件关联的气团进行解析，上述两种方法均可解释微生物群落间的组成差异。空气传播的细菌与真核生物群落呈现季节性变化，但七年观测周期内，各季节的群落组成均表现出高度的一致性。其中夏季与冬季的群落特征最为鲜明，对应的指示类群大多与这两个季节相关。由此，本长期研究证实了部分微生物的预测性特征。源追踪分析显示，淡水、农田与城市生物群系是夏季细菌的主要来源，而海洋与森林生物群系则在冬季占主导，这一结果与降水事件关联气团的起源高度吻合。综上，本研究得出结论：大气空气传播微生物的组成主要由气团起源驱动，进而受地中海区域的整体降水格局影响。研究表明，降水采样是一种兼具成本效益与实用性的大气微生物监测方法。