The ecology of bacteria in the alfresco atmosphere

Abstract

This MiniReview is concerned with the sources, flux and the spacial and temporal distributions of culturable airborne bacteria; how meteorological conditions modulate these distributions; and how death, culture media, and experimental devices relate to measuring airborne bacteria. Solar radiation is thought to be the planetary driver of the annual (seasonal, where it occurs) and diurnal natural alfresco atmospheric bacterial population cycles. Long‐term climatological and short‐term meteorological events such as storms also ‘randomly’ modulate the populations. The annual cycle may be due largely to the seasonal events of vegetation growth, drying conditions, rainy season, and freezing winter temperatures. The diurnal cycle may also be influenced largely by solar radiation in that the sunrise peak that has been frequently observed may be due to convective updrafts entraining epiphytic and soil bacteria into the atmosphere. At this time they are initially concentrated in a slowly deepening mixed layer but then, as the layer deepens more rapidly in mid‐morning, they become increasingly dilute. When the layer formation slows in the early afternoon, and forms an inversion cap, the bacteria slowly accumulate in the deeper mixed layer until sundown. At sundown atmospheric cleansing processes rid the atmosphere of the larger bacteria associated particles. These processes may include gravitational settling, death due to desiccation, and upward displacement of the warm, light air by clean, cold, heavier air.

Spatial variability in airborne bacterial communities across land-use types and their relationship to the bacterial communities of potential source environments

“…There are likely two main terrestrial sources of bacteria to the near-surface atmosphere: leaf surfaces and soil (dust) (Lindemann and Upper, 1985;Lighthart and Shaffer, 1994;Lighthart and Shaffer, 1995a, b;Lighthart, 1997). However, the relative importance of these primary sources is not known, nor do we know the extent to which airborne bacterial communities overlap with those bacterial communities found in these source environments.…”

“…Of those studies examining biological aerosols, most have focused on the quantification and identification of fungal spores, pollens and dust mite allergens (Beggs and Kerr, 2000). Although there is a history of research examining bacterial distributions in the near-surface atmosphere (Lighthart and Shaffer, 1995a, b;Lighthart, 1997;Tong and Lighthart, 1998), the majority of this work has largely been restricted to surveys of culturable bacteria (those bacteria which can readily be grown and isolated under laboratory conditions). As most bacteria cannot be readily cultured (Pace, 1997), the vast majority of airborne bacteria have effectively been excluded from these culture-based studies (Amato et al, 2005) and comprehensive surveys of airborne bacterial diversity are uncommon.…”

Microbiology and atmospheric processes: research challenges concerning the impact of airborne micro-organisms on the atmosphere and climate

“…which were particularly dominant in the air during a dust event (Kellogg and Griffin, 2006). But Gram-negative bacteria, having a cell wall that is considered to be more fragile than that of Gram-positive bacteria, have also been found (Lighthart, 1997). Among the fungi, spores similar to those from Cladosporium, Aspergillaceae, Alternaria, Botrytis, and various Basidomycetes (Coprinus, Ustilago) have been frequently observed in the atmosphere (Gregory, 1961;Kellogg and Griffin, 2006), but spores of Cladosporium spp.…”

“…This Gram-negative plant pathogenic bacterium is not the most abundant of the micro-organisms present in the atmosphere (Lighthart, 1997), but it will very likely become one of the most highly studied organisms with regard to potential impact on atmospheric processes. This is due in particular to its well-known activity as an ice nucleator at temperatures near zero (Möhler et al, 2007; reviewed by Morris et al, 2004), and to its significant upward flux in the atmosphere (Lindemann et al, 1982), its presence in clouds (Amato et al, 2007b;Sands et al, 1982), its potential activity as a cloud condensation nucleus (Snider et al, 1985), and recent observations about its abundance in snow and rain .…”

“…Micro-organisms are particular abundant during periods favorable for disease of crop plants caused by fungi with aerially disseminated spores (Stakman and Christensen, 1946) and of human activities that are particularly important in releasing microbial particles into the atmosphere such combining and other activities associated with crop harvest (Lighthart, 1997). Concentrations of bacteria, for example, near the canopy level have been observed to range between thousands to 10 8 bacteria m 3 (Lighthart, 1997). Among the bacteria detected in the atmosphere, many are Gram-positive and include spore-formers such as Bacillus and Microbacterium spp.…”