Spatiotemporal patterns of microbial composition and diversity in precipitation KEN A. AHO, 1,5 CAROLYN F. WEBER, 1 BRENT C. CHRISTNER, 2 BORIS A. VINATZER, 3 CINDY E. MORRIS, 4 RACHEL JOYCE, 2 KEVIN C. FAILOR, 3 JASON T. WERTH, 1 AURORA L. H. BAYLESS-EDWARDS, 1 AND DAVID G. SCHMALE III 3 1 Biological Sciences, Idaho State University, Pocatello, Idaho 83209-8007 USA 2 Department of Microbiology and Cell Science, Biodiversity Institute, University of Florida, Gainesville, Florida 32611 USA 3 School of Plant and Environmental Sciences, Virginia Tech, Blacksburg, Virginia 24061-0331 USA 4 INRA, Plant Pathology Research Unit 407, Montfavet, France Citation: Aho, K. A., C. F. Weber, B. C. Christner, B. A. Vinatzer, C. E. Morris, R. Joyce, K. C. Failor, J. T. Werth, A. L. H. Bayless-Edwards, and D. G. SchmaleIII. 2019. Spatiotemporal patterns of microbial composition and diversity in precipitation. Ecological Monographs 90(1):e01394. 10.1002/ecm.1394 Abstract. Microbes in the atmosphere have broad ecological impacts, including the poten- tial to trigger precipitation through species and strains that act as ice nucleation particles. To characterize spatiotemporal trends of microbial assemblages in precipitation we sequenced 16S (bacterial) and 18S (fungal) rRNA gene amplicon libraries collected from 72 precipitation events in three U.S. states (Idaho, Louisiana, and Virginia) over four seasons. We considered these data from the perspective of a novel metacommunity framework. In agreement with our heuristic, we found evidence for distinct mechanisms underlying the composition and diversity of bacterial and fungal assemblages in precipitation. Specifically, we determined that (1) bacte- rial operational taxonomic unit (OTU) composition of precipitation was strongly associated with macroscale drivers including season and high-altitude characteristics of storms; (2) fungal OTU composition was strongly correlated with mesoscale drivers including particular spatial locations; (3) b-diversity (heterogeneity of taxa among samples) for both bacteria and fungi was largely maintained by turnoverof taxa; however, (4) bacterial assemblages had higher con- tributions to total b-diversity from nestedness (i.e., lower richness assemblages were largely tax- onomic subsets of richer assemblages), due to losses of taxa during dispersal, particularly among potential ice nucleation active bacteria; and (5) fungal assemblages had higher contri- butions to total b-diversity from turnover due to OTU replacement. Spatiotemporal trends in precipitation-borne metacommunities allowed delineation of a large number of statistically sig- nificant indicator taxa for particular sites and seasons, including trends for bacteria that are potentially ice nucleation active. Our findings advance understanding regarding the dispersion of aerosolized microbes via wet deposition, and the development of theory concerning poten- tial assembly rules for bioaerosol assemblages. Key words: atmosphere; biogeography; bioprecipitation; community ecology; metabarcoding; metacom- munity; meteorology; microbial ecology; precipitation. INTRODUCTION Of growing interest to ecologists are the properties of precipitation-borne microbes, particularly species and strains that may trigger precipitation through the forma- tion of ice in clouds (i.e., bioprecipitation; Sands et al. 1982, Morris et al. 2014a, Hummel et al. 2018). At tem- peratures warmer than 38°C, phase transition from liq- uid water to ice requires the presence of ice nucleating particles (INPs; Atkinson et al. 2016, Kanji et al. 2017, Knopf et al. 2018). Various particles can act as ice nucle- ators (Szyrmer and Zawadzki 1997, Despr es et al. 2012, Harrison et al. 2016); however, those most efficient in catalyzing ice formation at warmer temperatures (>15°C) are microbial or biogenic in origin (Murray et al. 2012, Morris et al. 2013, Fr€ ohlich-Nowoisky et al. 2015, 2016, Weber 2015, Hill et al. 2017). Thus, knowl- edge concerning microbes in precipitation is potentially relevant to understanding global precipitation itself. Microbes are aerosolized from all terrestrial and aqua- tic ecosystems (Wilson et al. 2015, Joung et al. 2017), including the surfaces of multicellular organisms, e.g., plants (Lindemann et al. 1982, Burrows et al. 2009a, Despr es et al. 2012), and fluxes of bioaerosols are often generated or affected by anthropogenic activities (Taha et al. 2006, Suski et al. 2018). Once aerosolized, some microbes are well suited for survival in atmospheric envi- ronments. For instance, dormant forms of spore-forming bacteria are commonly isolated from aerosols (Brunet et al. 2017). Still, other microbial taxa are metabolically active in fog (Fuzzi et al. 1997) and supercooled cloud Manuscript received 21 December 2018; revised 26 August 2019; accepted 17 September 2019. Corresponding Editor: Bradley Butterfield. 5 E-mail: ahoken@isu.edu Article e01394; page 1 Ecological Monographs, 90(1), 2020, e01394 © 2019 by the Ecological Society of America