It's generally agreed that life on this planet would not be possible if it weren't for microbes. In a fascinating thought experiment, a pair of biologists scrutinized this assumption to find out. As their paper makes clear, a microbe-free world would be a strange place, indeed.
There's no question about it — microbes play a critical role in our bodies and in virtually every ecosystem. As Louis Pasteur once said, "Life would not long remain possible in the absence of microbes."
To test Pasteur's assumption, researchers Jack Gilbert, an environmental biologist at Argonne National Laboratory, and Josh Neufeld, a biologist at the University of Waterloo, considered a number of different scenarios in which microbes were to suddenly become absent, including the human gut microbiome, a planet devoid of bacteria and archaea, and a complete absence of all microbial life. The researchers excluded organelles, such as mitochondria and chloroplasts, because most life would cease to exist without them.
Perhaps surprisingly, the researchers conclude that "it would be false to claim that macroscopic life cannot exist without microbes," and that "life would persist in the absence of microbes," though the quantity and quality would be reduced dramatically.
Gilbert and Neufeld contend that macroorganisms, like humans, can still digest food in the absence of microbes, writing that: "Animals can spend their entire lives absent of microbial flora because all required dietary components can be synthesized chemically, without the need for a biological precursor."
But the sudden disappearance of microbes across the world's ecosystems would be a different story.
The planet's oceans and soils would enter into a period of stagnation owing to the complete loss of biogeochemistry. Worse, without bacteria to play a role in the nitrogen fixation process, most global photosynthesis would come to a grinding halt within a year. At the same time, there would be no microbes to break down massive amounts of accumulating waste. The biomass would accumulate, "creating vast reservoirs of biogeochemical waste that no biological entity could transform, at least initially." It would mark the end of biogeochemical recycling — a process on which all life depends.
And as for our ability to breathe, the authors write:
How much of global atmospheric oxygen is accounted for by bacterial activity? Oxygenic photosynthetic progenitors transformed the world's atmosphere from anoxic to oxic during the Great Oxygenation Event, beginning approximately 3,000,000,000 years ago. Prochlorococcus and Synechoccocus are now two of the world's most abundant cellular life forms, filling the ocean to varying degrees from pole to pole, generating oxygen as a byproduct of sunlight-driven photosynthesis. If these great oxygen sources vanished from the world's oceans, lakes, surface soils, and plant surfaces, then what would happen? Perhaps surprisingly, it is unlikely that anything problematic for aerobic life would happen for at least a few hundred thousand years. Assuming humans could distribute nitrogen globally, algae and plants could be expected to continue generating a proportion of available atmospheric oxygen, potentially as high as 50%. Existing pools of atmospheric oxygen might satisfy the demand for aerobic metabolism among surviving organisms, possibly for decades or centuries. If this were the case, then asphyxiation of aerobic life would not be likely in the near term.
At the same time, however, the deletion of microbes would result in the complete absence of all forms of microbial disease, including Ebola, malaria, the common cold, ulcers, and many, many more.
All this said, the authors make the claim that the roles of microbes "are not necessarily irreproducible." They write:
When you next hear someone claim that we cannot live without microorganisms, it would be appropriate to ask them to qualify the statement. Would we still be able to eat and digest food? Yes. Would life be extinguished in the absence of Bacteria and Archaea or in a world without any microbes? Not immediately, not all life, and potentially not for a long time.
In short, we argue that humans could get by without microbes just fine, for a few days. Although the quality of life on this planet would become incomprehensibly bad, life as an entity would endure.
A rather interesting conclusion given that our warming planet is in the early stages of the 6th Great Mass Extinction, and that humanity is at the dawn of molecular nanotechnology and synthetic biology.
Read the entire study at PLOS | Biology: "Life in a World without Microbes".