Bacterial utilization of formic and acetic acid in rainwater

Rain samples were collected aseptically, during 1983 and 1984, in Charlottesville, Virginia to determine the ability of bacteria in precipitation to utilize formate and acetate. The total number of bacteria, as counted by Acridine Orange Direct Counts, was one to two orders of magnitude greater from April to September (10⁵ cells ml⁻¹) than during the rest of the year (10³−10⁴ cells ml⁻¹). Formate and acetate concentrations ranged between 6–23 and 3–9 μM, respectively and were higher from June to September. Heterotrophic uptake on the day of collection was not different from the controls, but after incubation at room temperature for a minimum of three days, the turnover rate constants were 0.14 and 0.17 h⁻¹ for formate and acetate, respectively. Total bacterial counts increased an order of magnitude during that interval. These turnover rate constants were used to calculate losses of 44 and 24 μmoll⁻¹day⁻¹ of formic and acetic acid, respectively. Turnover times were 1.5 and 34 days for formate and acetate, respectively. This study demonstrated that there are viable microorganisms in the atmosphere capable of utilizing formate and acetate for growth.