Diurnal and Nocturnal Microbial Concentrations in the Air

The atmosphere is home to a diverse array of microbes and some of these microbes can be pathogens of humans, animals,and plants or act as allergens. Given the dynamic nature of atmospheric conditions, we would expect the amounts and types of microbes found in the near-surface atmosphere to differ during the day (diurnal) versus the night (nocturnal). We quantified how microbial abundances in the near-surface atmosphere change throughout the course of the day in a semi-arid urban environment in Boulder, Colorado (elevation ~1660m). We collected air samples for 8 consecutive days, at 2 liters per minute, for approximately 12 hours, from 0700 to 1900(diurnal samples) and 1900 to 0700 (nocturnal samples). We assessed microbial abundances in each sample using both a cultivation-independent approach, measuring the amounts of bacterial DNA in each collected sample via 16S rRNA qPCR, and with a cultivation-based approach, measuring the number of colony-forming units (CFUs). We found that bacterial DNA concentrations were significantly higher during the night, with approximately three times higher DNA yields from samples during the night than during the day. The cultivation-based assays yielded similar results, with more CFUs recovered from the nocturnal samples than the diurnal samples, however this result was not significant. Our results suggest that the lower temperatures and higher humidity during the night lead to higher bacterial concentrations, likely due to increased survival and persistence in the near-surface atmosphere. The observed diurnal and nocturnal patterns, and the impact on microbial concentrations in the air, should be considered when designing and interpreting results from aeromicrobiology studies. Furtherinvestigation is needed to determine if the patterns we identified are also observed during other seasons and how the specific types of microbes, including known allergens, might vary diurnally and nocturnally.