The ecology of bacteria in the alfresco atmosphere

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.