The chemical and biological components of rainwater: A case study for the habitability of the atmosphere

Phillip Lloyd Rust

Particles from the earth’s surface are routinely borne into the atmosphere where they have residence times that generally run from days to weeks. Once in the atmosphere, particulate matter interacts with moisture to play a dominant role in controlling rainwater chemistry. Microbes from the earth’s surface, either free floating or attached to inorganic particles, are admitted into the atmosphere for extended periods of time. Atmospheric conditions are generally considered to be too inhospitable for long term microbial survival, but life is relatively abundant in the atmosphere. Despite this abundance, atmospheric ecology is still a little understood component of the atmosphere and the earth’s extended biosphere. How microbes endure, adapt and utilize scarce resources in the atmosphere is likewise poorly understood. This thesis attempts to shed some light on the following questions. What species of microbes are more likely to survive in the atmosphere? What species are more prevalent? Are they pathogenic? Is there a relationship between the chemical and biological components of rain water? In the present study, 52 rainwater samples were collected at eight locations around the globe to be studied chemically and biologically. Select samples were chosen for a phospholipid fatty acid (PLFA) analysis for insights into the microbial population size and diversity present in the samples, as well as knowledge of their stress/starvation levels. Population levels were found to be high, with cell counts ranging from 5.43E+03 to 1.49E+06 cells/ml, and were dominated by Proteobacteria and secondly by eukaryotes. Some samples possessed small amounts of anaerobic metal reducers, SRB (sulfate reducing bacteria), and Actinomycetes. Culturing experiments on LB (Luria-Bertani) or Starkey media led to the identification of 74 organisms by their 16S rRNA gene. The culturing results were dominated by Gram-positive organisms from soil and freshwater sources. 44% of the microbes cultured were pathogenic or opportunistic pathogens of fungi, plants and animals. Elemental sulfur (S8) was analyzed in all the samples as a possible source of microbial UV protection using fluorescent spectroscopy. S8 was not ubiquitous, and thus readily available, but it was detected in some samples. The other chemical constituents in rainwater can also vary greatly between locations on the globe, with Ca2+, K+ , Mg2+ and Na+ being major cations. Sulfate levels ranged from 4.3-63 µeq/L and nitrate levels ranged from 81-608 µeq/L. The differing concentrations of these important microbial nutrients did not demonstrate an observed effect on microbial population densities or on the types of organisms found. A comparison of the microbial communities with the chemistry of the rainwater showed no correlation between types of microbes present and chemistry. Although limited amounts of the nutrients and micronutrients needed by organisms can be found in rainwater, these limited nutrients may only assist microbe survivability for short durations, rather than meet the requirements necessary for long-term atmospheric residence times

The chemical and biological components of rainwater: A case study for the habitability of the atmosphere

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