A cloud is a visible mass of minute water droplets or frozen crystals suspended in the atmosphere. Clouds form when water vapor cools and condenses around tiny airborne particles known as aerosols. The intriguing question is whether these vast collections of water and particles can also contain biological material, such as DNA. The atmosphere, including the clouds within it, is not a sterile environment but a complex, high-altitude ecosystem that carries life. While a cloud itself is not a living organism, it functions as a temporary host for biological components that carry genetic material.
The Physical Composition of Clouds
Cloud formation begins with the cooling of air, which allows water vapor to reach saturation. Water molecules do not readily combine to form droplets; they require a surface for condensation. These surfaces are provided by microscopic particles called cloud condensation nuclei (CCN).
These non-biological aerosols are diverse, originating from sources like wind-blown mineral dust, sea salt ejected from ocean spray, and soot from combustion. Sulfates and inorganic pollutants also serve as common condensation points. Water vapor condenses onto these inert particles to create the billions of droplets or ice crystals that constitute a cloud. This initial process is purely physical and chemical.
Microbes: The Biological Aerosols
The atmosphere is continually seeded with biological aerosols, or bioaerosols, which are particles released from terrestrial and marine ecosystems. These include living and non-living components such as bacteria, fungal spores, pollen grains, viruses, and cell fragments. Bioaerosols are lofted into the air from sources like soil, oceans, and plants through wind turbulence, often reaching altitudes where clouds form.
Once airborne, some biological particles play a direct role in weather phenomena. Bioaerosols can act as highly efficient cloud condensation or ice nuclei. They influence precipitation by providing a surface for water to freeze or condense upon.
A well-studied example is the bacterium Pseudomonas syringae, a common plant pathogen. This organism possesses specialized ice-nucleating proteins (INPs) on its outer membrane. These proteins act as a template, causing water to freeze at remarkably warm temperatures, sometimes as high as -2°C to -3.8°C. This ability facilitates the formation of ice crystals and snow, which are precursors to rain, in cold clouds where pure water would otherwise remain supercooled. The involvement of biological entities in the water cycle demonstrates that life has a functional impact on atmospheric processes.
Detecting Genetic Material in the Atmosphere
Since clouds are populated by microbial life, they inherently contain the genetic material of these organisms, including DNA and RNA. The presence of these airborne microbes means their collective genetic blueprint, or metagenome, is suspended within the cloud water. Scientists confirm this presence by collecting cloud water and analyzing its contents.
Researchers employ advanced sequencing techniques, notably metagenomics, to filter and extract nucleic acids from the sampled cloud water. This process involves sequencing the recovered genetic material to identify the diversity of bacteria, fungi, and viruses present. Metagenomic analysis allows for the comprehensive identification of the microbial community’s composition and functional potential.
Scientists also study the ratio of RNA to DNA, which indicates the metabolic activity of microbes surviving in the harsh cloud environment. This research reveals that these organisms are not dormant but are actively expressing genes to cope with the cold and oxidative stress found at high altitudes. Understanding the genetic content of clouds is important for climate modeling, as biological particles influence cloud formation and precipitation patterns. It also offers insights into the global transport of microorganisms and potentially disease-causing agents.