Why are microbes everywhere? Theory of global connectedness

When I had my own lab, I was regularly contracted to develop microbial cultures to decontaminate polluted soils or waters. I would identify a source of bugs that break down toxicants, then develop a treatment process based on them.  One of my first jobs was to detoxify soils contaminated with pentachlorophenol (PCP), a wood preservative so toxic and persistent that it is banned worldwide. I looked at places where PCP could be found – in compost and pulp mill sludge – and worked out ways to isolate and grow PCP-degraders from them. I developed a process to detoxify large volumes of contaminated soil, the largest hazardous waste site in British Columbia at the time. A few years later, I was contracted to remove selenium from coal ash leachate. To find the right bugs, I sampled quiescent sections of a stream that received the leachate, where marsh plants grew, and isolated selenium-reducing bacteria from their sediments. These isolates were incorporated in a process that removed selenium from leachate.

I always found microbes that break down contaminants, even in remote places like the Arctic or Atacama desert. After a few years, I started wondering why there are microbes wherever I looked. The question was salient when I searched for microbes that detoxify metal-cyanide complexes at a gold mine in the Yukon. Cyanide is commonly found in the environment, but not metal-cyanide complexes. These compounds are fairly unreactive, which means that the bugs breaking them down are specialists, not run-of-the-mill cyanide-degraders. How did metal-cyanide degraders get to this remote mine and make a home there for themselves?

One possible answer is aeolian dust. Dust-borne microbes, including pathogens, have been shown to travel large distances. There are well-established, seasonal patterns of global dust migration, with airborne dust crossing both the Atlantic and the Pacific oceans. This dust can distribute microbes to new environments across the globe, where some will occasionally grow if they land in a hospitable environment. Like a global MixMaster® that redistributes microbes everywhere, air currents that transport dust act like a planetary-scale redistribution system.

This process looks an awful lot like natural selection. Similarly to the process of evolution by natural selection - in which random mutations in a population are tested in their environment - this process entails random dispersal of microbes throughout the globe, followed by testing in new environments. Microbes that can survive will propagate elsewhere on the planet, whereas others will perish. This is a type of global natural selection that ties geology and biology.

I like this idea, though I don't know if it has any merit. I like that it came to me by asking a banal question – how do microbes end up everywhere, even in remote places – and arriving at an interested answer.

I hope that others will look into it.