The classic vision of Earth from space is a bluish planet painted with an ever changing, deeply textured wash of white clouds. Often we peer between the gaps in these clouds, looking for the recognizable continents and oceans of the surface, because that's our domain, and the obvious domain of life.
But life doesn't stop at the rocks and liquids of Earth, it permeates the atmosphere too. Birds, insects, plants, and fungi all exploit the world-spanning fluid of the air and its currents and turbulence. It also seems that the vast microbial biosphere extends well into this domain.
On the face of things it's not surprising that there are single-celled organisms floating through the air. At scales of a few micrometers a bacterium, for instance, is easily lofted into the jumble of atmospheric molecules. Plants, oceans, land, and human urban areas are constantly spewing microbes. Soil erosion lofts soil microbes, ocean evaporation lofts marine microbes, and every coughing spluttering animal helps inject microscopic organisms into the air.
Atmospheric sampling suggests that there is an appreciable biological load at least up and into the bottom of Earth's stratosphere at around 7 kilometers altitude at polar regions all the way up to about 20 kilometers at the equator, with seasonal variation. These measurements are not easy, in part because the number of organisms in a given volume is quite low by surface standards - between around 100 to 10,000 cells in every cubic centimeter. But also because of the sheer genomic diversity.
A big question is whether or not microbial species that frequently end up airborne also take advantage of this - or indeed have evolved to exploit not just the global transport system of the atmosphere but some of its other properties.
So called 'rain-making' bacteria have been in the news over the years. Any kind of precipitation of water tends to involve the nucleation or seeding of droplets or crystals of condensing water vapor. Since biological particulates (not just things like bacteria but also biologically produced compounds like dimethyl sulfide made by phytoplankton that turns into atmospheric sulfate particles) make up somewhere between 20% and 70% of atmospheric aerosols, it seems that life can play a big role. Indeed, there is evidence that phytoplankton blooms in the Southern Ocean can seed their own cloud cover. Globally it looks like biological aerosols boost cloud droplet numbers by as much as 60%.
But it also seems that lofted species are doing more than just physically interacting with Earth's hydrological cycle (a big enough deal in its own right). There is evidence that there are metabolically active bacteria in the atmosphere. This was not a sure thing, microbes tend to work best together in physically associated colonies mingling with other species.
Over the years researchers have seen that certain cloud-borne species, if cultured in a lab, could certainly be altering the chemistry of atmospheric compounds involving carbon, nitrogen, and oxygen. Other studies, that attempt to measure the in-situ metabolisms, suggest that species in the family of Acetobacteraceae could be active. These ferment ethanol to acetic acid - and ethanol is (perhaps surprisingly) typically present in Earth's atmosphere, as part of the complex chemical mix that circulates around us. Other species utilize sunlight and use simple organic acid compounds to grow; the kinds of organic acids that wildfires produce.
As with much cutting-edge science, there are more questions than answers at the moment. But there seems to be evidence that airborne, metabolically active microbes are directly engaged in the core biogeochemical cycles of the Earth - churning through organic compounds as they float around the planet.
Just as it took us a long time to recognize the ubiquity and scale of the subsurface biosphere of our world, we may have to further expand biology's scope to include the rich but largely invisible terrain of the air above our heads.