Bacterial abundance and viability in rainwater associated with cyclones, stationary fronts and typhoons in southwestern Japan

The abundance and viability of bacterial cells in rainwater at a suburban site in southwestern Japan between October 2014 and September 2015 were measured and their distinctiveness, according to synoptic weather systems, i.e., cyclones (cold fronts), stationary fronts (including Meiyu and non-Meiyu fronts) and typhoons, was examined. On average, the cell concentration of bacteria in the rainwater was 2.3 ± 1.5 × 104 cells mL−1, and bacterial viability, the ratio of viable cells to total cells, was 80 ± 10%. In the rainwater of cyclones when clouds were induced by the intrusion of continental air, the bacterial concentration was higher (3.5 ± 1.6 × 104 cells mL−1) and the viability was lower (75 ± 8%) than in the rainwater of other types. In the rainwater of Meiyu fronts and typhoons when clouds were significantly influenced by marine air, bacterial concentrations were 1.5 ± 0.5 × 104 and 1.2 ± 0.3 × 104 cells mL−1, and bacterial viabilities were 84 ± 7% and 85 ± 7%, respectively. In the rainwater of non-Meiyu stationary fronts, the bacterial concentration was 2.4 ± 1.6 × 104 cells mL−1, and the viability was 78 ± 14%. Abundant bacteria were associated with ions nss-SO42−, nss-Ca2+, and NO3− in rainwater, but bacterial concentrations did not correlate with the ratios of airborne particle concentrations to the precipitation amounts. Further investigations with correlation and principal component analysis combining bacteria and ion species revealed that bacteria in the rainwater were likely enclosed in clouds at the stage of cloud formation in addition to below-cloud removal, and bacteria involved in the rainwater did not show confirmable growth.