Compared to soil or aquatic ecosystems, the atmosphere is still an underexplored environment for microbial diversity. In this study, we surveyed the composition, variability and sources of microbes (bacteria and fungi) in the near surface atmosphere of a highly populated area, spanning ~ 4,000 Km² around the city center of Madrid (Spain), in different seasonal periods along two years. We found a core of abundant bacterial genera robust across space and time, most of soil origin, while fungi were more sensitive to environmental conditions. Microbial communities showed clear seasonal patterns driven by variability of environmental factors, mainly temperature and accumulated rain, while local sources played a minor role. We also identified taxa in both groups characteristic of seasonal periods, but not of specific sampling sites or plant coverage. The present study suggests that the near surface atmosphere of urban environments contains an ecosystem stable across relatively large spatial and temporal scales, with a rather homogenous composition, modulated by climatic variations. As such, it contributes to our understanding of the long-term changes associated to the human exposome in the air of highly populated areas.

与土壤或水生生态系统相比，大气仍然是微生物多样性开发不足的环境。在这项研究中，我们调查了人口稠密地区近地表大气中微生物（细菌和真菌）的组成，变异性和来源，范围约为4,000 Km ²在马德里市中心（西班牙）周围，在两年的不同季节内。我们发现丰富的细菌属的核心在空间和时间（大部分土壤来源）上都很健壮，而真菌对环境条件更敏感。微生物群落表现出明显的季节性模式，这是由环境因素（主要是温度和降雨积累）的变化所驱动的，而本地来源的作用则很小。我们还确定了两组季节性季节特征中的分类单元，但没有特定采样点或植物覆盖率的特征。本研究表明，城市环境的近地表大气包含一个在相对较大的时空范围内稳定的生态系统，其组成相当均一，并受气候变化的影响。因此，