Abstract
Primary biological aerosols are transported over large distances, are traveling in various media such as dry air masses, clouds or fog, and eventually deposited with dry deposition, especially for larger particles, or precipitation like rain, hail or snow. To investigate relative abundance and diversity of airborne bacterial and fungal communities, samples have been collected with a liquid impinger (Coriolis µ) from the top of Mount Sonnblick (3106 m asl, Austrian Alps) from the respective sources under a temporal aspect over four seasons over the year to include all climatic conditions. Bacterial and fungal samples (16S rRNA and ITS) were sequenced using Illumina MiSeq paired-end sequencing, investigated for relative abundance by qPCR(16S rRNA and 18S rRNA) and ice nucleation activity. Results show that there is no stable free tropospheric air microbial community and air mass origin was different for the four sampling periods which exerted influence on the microbial composition of the atmosphere although a core microbiome could be identified consisting of 61 bacterial OTUs and eight fungal genera. Differentiation between seasons was stronger pronounced in air than in precipitation, with rain being most different and variable of precipitation types, indicating distinct forces driving microbial fate in the air. Microorganisms precipitated with snow, hail or rain or being transported by clouds differ in their species composition from free tropospheric air masses and do not mirror the air community structure. They were more diverse, distinct in composition, 16S:18S ratio and abundance from free-floating PBA. Hence, snow or cloud samples are not suitable proxies for free tropospheric air microbiome composition, since separation processes in aerosolization, transport and scavenging occur. The microbial composition of arriving precipitation or clouds represents only a part of the microbial air composition communities of the cumulative sources of origin. Relative abundance and composition of ice nucleation-active bacteria showed a higher share of relative % reads of known ice nucleation-active bacteria present in all wet phases compared to air. Results propose a separation of IN-active reads with higher shares occurring in precipitation. This study presents the first comparison of free tropospheric bacterial and fungal abundance and diversity in time series of air over several seasons in contrast to various precipitation forms in the free troposphere.
Highlights
Knowledge about and scientific interest in the composition and abundance of primary biological aerosols (PBA) is rapidly increasing (180% increase in publications per year from 2000 to 2018, search term: ‘‘airborne bacteria’’)
Differentiation between seasons was stronger pronounced in air than in precipitation, with rain being most different and variable of precipitation types, indicating distinct forces driving microbial fate in the air
Microorganisms precipitated with snow, hail or rain or being transported by clouds differ in their species composition from free tropospheric air masses and do not mirror the air community structure
Summary
Knowledge about and scientific interest in the composition and abundance of primary biological aerosols (PBA) (e.g., airborne bacteria and fungi) is rapidly increasing (180% increase in publications per year from 2000 to 2018, search term: ‘‘airborne bacteria’’). Microbial composition in air masses within the PBL is affected by ground emissions and mixing of air masses (Bowers et al 2011; Carotenuto et al 2017; Burrows et al 2009a, b). The PBL, laminar flowing air masses are stratified in the free troposphere (Wekker et al 2015). Studies focusing on free tropospheric air microbial communities often present data from single campaigns or few flights (Schmale and Ross 2015; Techy et al 2010; Jimenez-Sanchez et al 2018; Smith et al 2018; Maki et al 2017; Xia et al 2013; Zweifel et al 2012)
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