Abundance of airborne heterotrophic protists in ground level air of South Dakota

The abundance of heterotrophic protists (as protozoan cysts) in the near-surface tropospheric air of Rapid City, SD, was determined using a filtration sampler capable of collecting particles in the 2.0 to 20.0 μm size range (sampling rate ca. 16 l min−1). The number of viable cysts collected on the filter was estimated after enrichment cultivation in soil extract medium. Samples were mainly collected in the spring, fall and winter of 1997 and abundances throughout the year ranged from below detection (<0.05 m−3) to 1.08 cysts m−3 with an overall mean of 0.25 m−3 (SE=0.05). Not all of the isolates were identified to species but, over the study, 25 different morphotypes were found. Flagellates and naked amoebae were the commonest protozoa (as cysts) found; ciliates were rare. This reflects the abundance of these groups in soils, the likeliest source of most of the airborne protozoa in this study. The concentration of cysts generally increased as a function of the total particles in the atmosphere. The concentration of both particulates and cysts exhibited a wide range of variability, but highest values of both occurred on days with higher winds and lower relative humidity.