Metabolically active bacteria detected with click chemistry in low organic matter rainwater.

Rain contains encapsulated bacteria that can be transported over vast distances during relatively short periods of time. However, the ecological significance of bacteria in "precontact" rainwater-rainwater prior to contact with non-atmospheric surfaces-remains relatively undefined given the methodological challenges of studying low-abundance microbes in a natural assemblage. Here, we implement single-cell "click" chemistry in a novel application to detect the protein synthesis of bacteria in precontact rainwater samples as a measure of metabolic activity. Using epifluorescence microscopy, we find approximately 103-104 bacteria cells mL-1 with up to 7.2% of the observed cells actively synthesizing protein. Additionally, our measurement of less than 30 μM total organic carbon in the samples show that some rainwater bacteria can metabolize substrates in very low organic matter conditions, comparable to extremophiles in the deep ocean. Overall, our results raise new questions for the field of rainwater microbiology and may help inform efforts to develop quantitative microbial risk assessments for the appropriate use of harvested rainwater.

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This document has been reviewed in accordance with U.S. Environmental Protection Agency policy and approved for publication. Any mention of trade names, manufacturers or products does not imply an endorsement by the United States Government or the U.S. Environmental Protection Agency. EPA and its employees do not endorse any commercial products, services, or enterprises. The United States Environmental Protection Agency through its Office of Research and Development funded and managed the research described here. The funder provided support in the form of salaries for authors [RG, MCH, CAB], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section. This does not alter our adherence to PLOS ONE policies on sharing data and materials.