High levels of primary biogenic organic aerosols in the atmosphere in summer are driven by only a few microbial taxa from the leaves of surrounding plants

Primary biogenic organic aerosols (PBOA) represent a major fraction of coarse organic matter (OM) in 1 air. Despite their implication in many atmospheric processes and human health problems, we surprisingly know 2 little about PBOA characteristics (i.e., composition, dominant sources, and contribution to airborne-particles). In 3 addition, specific primary sugar compounds (SCs) are generally used as markers of PBOA associated with Bacteria 4 and Fungi but our knowledge of microbial communities associated with atmospheric particulate matter (PM) 5 remains incomplete. This work aimed at providing a comprehensive understanding of the microbial fingerprints 6 associated with SCs in PM10 (particles smaller than 10μm) and their main sources in the surrounding environment 7 (soils and vegetation). An intensive study was conducted on PM10 collected at rural background site located in an 8 agricultural area in France. We combined high-throughput sequencing of Bacteria and Fungi with detailed 9 physicochemical characterization of PM10, soils and plant samples, and monitored meteorology and agricultural 10 activities throughout the sampling period. Results shows that in summer SCs in PM10 are a major contributor of 11 OM in air, representing 0.8 to 13.5% of OM mass. SCs concentrations are clearly determined by the abundance of 12 only a few specific airborne Fungi and Bacteria Taxa. These microbial are significantly enhanced in leaf over soil 13 samples. Interestingly, the overall community structure of Bacteria and Fungi are similar within PM10 and leaf 14 samples and significantly distinct between PM10 and soil samples, indicating that surrounding vegetation are the 15 major source of SC-associated microbial taxa in PM10 in rural area. 16

It should be noted that leaf samples were collected only once, four weeks after the end of PM and soil sampling, while the major crops were still on site.

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Daily PM10 samples were analyzed for various chemical species using subsampled fractions of the collection filters 150 and a large array of analytical methods. Detailed information on all the chemical analysis procedures have been 151 reported previously (Golly et al., 2018;Samaké et al., 2019b;Waked et al., 2014). Briefly, SCs (i.e. polyols and ionic chromatography (IC, Thermo Fisher ICS 3000, USA). Free-cellulose concentrations were determined using 155 an optimized enzymatic hydrolysis (Samaké et al., 2019a) and the subsequent analysis method of the resultant 156 glucose units with an HPLC-PAD (Golly et al., 2018;Samaké et al., 2019b;Waked et al., 2014). Organic and of soil in 15 ml of sterile saturated phosphate buffer for 15 min. About 2 mL of the resulting extracts were centrifuged for 10 min at 10,000g, and 500 µL of the resulting supernatant were used for DNA extraction using the NucleoSpin Soil Kit (Macherey-Nagel, Düren, Germany) following the manufacturer's original protocol after To extract DNA from either endophytic or epiphytic microorganisms, aliquots of leaf samples (about 25-30mg) Table S2 for details on the rarefaction depths). Non-metric 269 multidimensional scaling (NMDS) ordination analysis was performed to decipher the temporal patterns in airborne    ). Glycerol was also observed in our samples, but with concentrations frequently below the quantification limit.

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A Spearman's rank correlation analysis based on the daily dynamics was used to examine the relationships between 297 SC species. As shown in Table 1, sorbitol and inositol are well linearly correlated (R = 0.57, p < 0.001). Herein, 298 sorbitol (R = 0.59, p < 0.001) and inositol (R = 0.64, p < 0.001) are significantly correlated to Ca 2+ . It can also be 299 noted that all other SC species are highly correlated with each other (p < 0.001) and that they are weakly correlated 300 to the temporal dynamics of sorbitol and inositol (Table 1)

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Results for nine-week daily measurements indicate that SCs together represent a large fraction of OM, contributing 305 between 0.8 to 13.5% to OM mass in summer. Glycerol is not presented because its concentration was generally below

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The NMDS (non-metric multidimensional scaling) ordination exploring the temporal dynamics of microbial 364 community beta diversity among all PM10 aerosol samples revealed significant temporal shifts of community structure for both Fungi and Bacteria (Fig. 4).
distinct clusters of PM10 samples. With one exception (A23), all air samples with higher SC concentration levels (A5 to A20, see Table S2    415 associated with SC species are generally more abundant in the leaves than in the topsoil samples (Fig. 5). In order 416 to further explore and visualize the similarity of species compositions across local environment types, we 417 conducted an NMDS ordination analysis (Fig. 6). As evidenced in Fig. 6, the beta diversities of fungal and bacterial Interestingly, the beta diversities of fungal and bacterial MOTUs in leaf samples and those in airborne PM10 are 420 generally not readily distinguishable, with similarity becoming more prominent during atmospheric peaks of SC 421 concentration levels (Fig. 6). However, the overall beta diversities in airborne PM10 and in leaf samples are

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Ellipses represent 95% confidence intervals for the cluster centroids. Circular and triangular shapes highlight air PM10 441 samples respectively with background and peak SC concentrations.

Discussion
Very few studies exist about the interactions between air microbiome and PM chemical profiles (Cao et al., 2014; airborne microbial fingerprints associated with SC species in PM10 and to identify the dominant sources of SCs in 446 a continental rural area extensively cultivated.

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The total concentrations of SC quantified in the atmospheric PM10 over our study site accounted for 0.8 to 13.5% 456 of the daily OM mass. This is remarkable considering that less than 20% of total particulate OM mass can generally 457 be identified at the molecular level . Hence, our results for a nine week-long period indicate that SC could be a

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Therefore, sorbitol and inositol are most likely associated with microorganisms from soil resuspension.

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With the exception of sorbitol and inositol, all other SC species measured in air samples at our sampling site are

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Microbial abundance and community structure in samples from the surrounding environment can provide further 573 useful information on sources apportionment and importance. Our data indicates that the airborne microbial genera 574 most positively correlated to SC species are also distributed in surrounding environmental samples from both 575 surface soils and leaves, suggesting a dominant influence of the local environments for microbial taxa associated 576 with SC species, as opposed to long-range transport. This observation makes sense since actively discharged 577 ascospores and basidiospores are generally relatively large airborne particles with short atmospheric residence 578 time (Elbert et al., 2007;Womack et al., 2015), limiting the possibilities of long-range dissemination. Accordingly, 579 the majority of previous studies investigating the potential sources of air microbes identified the local surface 580 environments (e.g., leaves, soils, etc.) to have more important effects on airborne microbiome structure in field 581 crop areas (Bowers et al., 2011;Wei et al., 2019b;Womack et al., 2015). This is all the more the case in our study, 582 with homogeneous crop activities for 10's to 100's of km around the site.

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In the present study, microbial diversity and richness observed in the surface soils are generally higher than those

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To that end, we combined high-throughput sequencing of Bacteria and Fungi with detailed physicochemical 620 characterization of PM10 soils and leaf samples collected at a continental rural background site located in a large 621 agricultural area in France.

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The main results demonstrate that the identified SC species are a major contributor of OM in summer, accounting 931 Zhu, C., Kawamura, K., and Kunwar, B.: Organic tracers of primary biological aerosol particles at subtropical