Coupling of fog and marine microbial content in the near-shore coastal environment

M E Dueker,G D O'Mullan,K C Weathers,M Uriarte,A R Juhl

doi:10.5194/bg-9-803-2012

Abstract

Abstract. Microbes in the atmosphere (microbial aerosols) play an important role in climate and provide an ecological and biogeochemical connection between oceanic, atmospheric, and terrestrial environments. However, the sources and environmental factors controlling the concentration, diversity, transport, and viability of microbial aerosols are poorly understood. This study examined culturable microbial aerosols from a coastal environment in Maine (USA) and determined the effect of onshore wind speed and fog presence on deposition rate, source, and community composition. During fog events with low onshore winds (<2 m s−1) the near-shore deposition of microbial aerosols (microbial fallout) decreased with increasing wind speeds, whereas microbial fallout rates under clear conditions and comparable low wind speeds showed no wind speed dependence. Mean aerosol particle size also increased with onshore wind speed when fog was present, indicating increased shoreward transport of larger aerosol particles. 16S rRNA sequencing of culturable ocean surface bacteria and microbial aerosols deposited onshore resulted in the detection of 31 bacterial genera, with 5 dominant genera (Vibrio, Bacillus, Pseudoalteromonas, Psychrobacter, Salinibacterium) making up 66 % of all sequences. The sequence library from microbial aerosol isolates, as with libraries found in other coastal/marine aerosol studies, was dominated at the phylum level by Proteobacteria, with additional representation from Firmicutes, Actinobacteria and Bacteroidetes. Seventy-five percent of the culturable microbial aerosols falling out under foggy conditions were most similar to GenBank-published sequences detected in marine environments. Using a 97 % similarity cut-off, sequence libraries from ocean surface and fog isolates shared eight operational taxonomic units (OTU's) in total, three of which were the most dominant OTU's in the library, representing large fractions of the ocean (28 %) and fog (21 %) libraries. The fog and ocean surface libraries were significantly more similar in microbial community composition than clear (non-foggy) and ocean surface libraries, according to both Jaccard and Sorenson indices. These findings provide the first evidence of a difference in community composition and microbial culturability of aerosols associated with fog compared to clear conditions. The data support a dual role for fog in enhancing the fallout of viable microbial aerosols via increased gravitational settling rates and decreased aerosolization stress on the organisms, which may include relief from UV inactivation, desiccation, and oligotrophic microconditions. This study provides a strong case for ocean to terrestrial transport of microbes and a potential connection between water quality and air quality at coastal sites.

Highlights

Current global estimates of bacterial concentrations in the atmosphere range from 1 × 104 to 6 × 105 cells m−3 depending on ecosystem type (Shaffer and Lighthart, 1997; Bauer et al, 2002; Burrows et al, 2009)
Microbial aerosol fallout decreased as wind speeds increased (Fig. 2), whereas fallout rates under clear conditions showed no relationship to wind speed (R2 = −0.03, p = 0.400) under the low wind conditions examined in this study
Within the Proteobacteria, Gammaproteobacteria dominated the microbial aerosol sequence library at this site (Fig. 4), and Vibrionales made up 37 % of that class

Summary

Introduction

Current global estimates of bacterial concentrations in the atmosphere (microbial aerosols) range from 1 × 104 to 6 × 105 cells m−3 depending on ecosystem type (Shaffer and Lighthart, 1997; Bauer et al, 2002; Burrows et al, 2009). There is growing evidence that microbial aerosols impact climate by serving as fog, cloud, and ice nucleators (Posfai et al, 1995; Hamilton and Lenton, 1998; Bauer et al, 2003; Ariya et al, 2009; Ekstrom et al, 2010). The atmospheric transport of microbes between ecosystem types, such as from African desert soils to the Caribbean Sea (Prospero et al, 2005), or from ocean surface to land in coastal systems (Dueker et al, 2011), has important implications for our understanding of microbial biogeography and genetic exchange among seemingly isolated microbiomes

Methods

Results

Discussion

Conclusion