What If All Microbes Disappeared From The Earth?

Have you ever imagined what would happen if microbes suddenly disappeared? You may think that it would be a miracle to get rid of all those nasty little invisible beings, that we would finally take off masks for good (and forever). However, I must warn you: indeed, losing microbes could be our very end. Let me explain to you why.

“Life would not long remain possible in the absence of microbes.”

Louis Pasteur, the French biologist who is often regarded as the father of modern microbiology, was the first to demonstrate that infectious diseases are caused by microbes. Yet, he once stated that “Life would not long remain possible in the absence of microbes.” A little bit contradictory? No way!

Back in the 19th century, Pasteur not only made remarkable breakthroughs in the causes and prevention of diseases but also proposed the principles of fermentation for food preservation by linking fermentation to yeasts. As a matter of fact, for thousands of years, preservation by fermentation has been used by early human societies to ensure long-term food preservation. The earliest archaeological evidence of human-prepared fermentation dates from ~7,000 BC – an alcoholic drink made from fruit, rice, and honey. This ancient art of using microbes in the preparation of foods and beverages (zymology) is part of the intangible heritage of every human community in the world (Anagnostopoulos & Tsaltas, 2019).

Pasteur had recognized the values of micro-organisms for an essential part of human life: feeding, even without the wide knowledge of the essential roles micro-organisms play as the engines of life on Earth. Nowadays, we know that the importance of microbes for feeding goes beyond producing and preserving food. It is related to the micro-organisms themselves that are ingested and take part in our gut microbiome, as well as to many nutritional requirements and dietary components produced by microbes. Gut microbiota is known to contribute to a multitude of aspects of host biology, such as nutrition and metabolism, immune protection, brain development, and behaviour (Amato et al., 2021).

Of course, today it is possible to synthesize nutritional requirements so that we could spend life without the need to have direct contact with gut microbes or fermented foods and beverages. At the same time, I ask you: how many pills do you think we might need for this?

Importantly: the microbial nutritional requirements are necessary not only for humans and other animals. Half of all phytoplankton, the base of aquatic food webs, require B12 vitamin (cobalamin) for living. They, along with all the life forms that depend on them, would likely perish from nutrient and cofactor starvation without microbes.

The second genome

Many organisms depend on microbial partners to perform very basic activities in their everyday lives. Like ruminants, both domestic (e.g., cows, sheep, and goats) and wild (e.g., giraffes, deers, and antelopes), and termites, dependent on the microbial activity of symbionts to digest their cellulose diet, many animals have co-evolved with microbial partners and would starve in their absence (as would all the other life forms that depend on those to survive).

Many other life forms depend on the nutritional components of microbial origin, just like us, or even use their products, such as antimicrobials or toxins, to protect or defend themselves, or even to hunt. The number of microbial cells living on the surface or inside animals is so high and the use of microbial genes and functions is so important that they are referred to as “the second genome”.

To illustrate this, I will give a few examples. The pufferfish, recognized by their habit of expanding when threatened, uses a bacterial substance to make tetrodotoxin, a lethal substance that poisons any predator that tries to eat it. Also, the predatory ant lion uses toxins produced by bacteria to paralyze prey. Many corals thrive in the ocean reefs thanks not only to symbiotic microalgae but also to symbiotic bacteria that protect them from pathogens and bleaching.

Another aspect refers to all the life forms in the dark corners of the oceans, like abyssal life forms that depend on microbial chemosynthesis or symbiosis to occupy such deep zones. Some species of tubeworms (Annelida, Sabellida) form dense populations in hydrothermal vents and depend on intracellular chemosynthetic bacteria to have energy and nutrients to survive. This is another example of not-so-popular animals that would struggle in the absence of microbes.

Curiously: In a reference to the deep zones in which those tubeworms are found, one of those symbiont bacteria was named Endoriftia persephone (actually, Candidatus Endoriftia persephone; the “candidatus” refers to microbes that have been described but not cultivated yet). This is a reference to the Greek Goddess Persephone, who was abducted by Hades, God of the Underworld, and became the queen of the Underworld.

How could we breathe? Or even eat?

A rather simple fact could be enough to show you how we wouldn’t last a minute without microbes: mitochondria and chloroplasts are essentially symbiont microbes that live in close association with eukaryotic cells. So, most eukaryotic life (like us, humans, crops, and other pets and domestic animals) would essentially die without them.

Indeed, micro-organisms alone have populated, dominated, and shaped our planet for about 3.7 billion years! Yep, a “microbes only” world! When plants and metazoans appeared, 700-800 million years ago, microbes had already existed for about 3 billion years! All those billion years of microscopic activities set the ground for the expansion of life (Tavares, 2022), including the emergence of eukaryotic life forms after establishing successful symbioses with ancestral aerobic (Alphaproteobacteria) and photosynthetic (Cyanobacteria-like) prokaryotes that later evolved to mitochondria and chloroplast, respectively (check out the Endosymbiotic Theory).

Some proof of this endosymbiotic origin also helps to illustrate how a “non-microbial world” would affect mitochondria and chloroplasts directly. These subcellular organelles are vulnerable to some antibiotics, just like bacteria, and retain some microbial characteristics such as their own multiple copies of circular DNA and bacterial-type ribosomes.

However, if we ignore this microbial ancestry of mitochondria and chloroplasts, there are still many topics regarding our relationship with microbes that illustrate why we cannot exterminate them.

Food production

Surely, agriculture has played a crucial part in human history. From the Neolithic to the Green Revolution, it has profoundly changed our societal paths and our relationship with nature. Nevertheless, transforming plants into crops is profoundly interconnected with nitrogen fixation by soil bacteria. Biological nitrogen fixation provides the basic component, nitrogen fixed as ammonia, for the synthesis of nucleic acids, amino acids, and other cellular components. Thus, this basal process would be severely affected in the absence of microbes. In fact, plants require nitrogen as a limiting nutrient; thus, photosynthesis would also be compromised, as would the survival of plants, after all.

In a theoretical exercise on the theme, the scientists Jack Gilbert and Josh Neufeld claimed that this absence of microbes wouldn’t be noticed immediately, though. Most of our agricultural practices depend on artificial fertilizers, produced via the Haber-Bosch process, an artificial fixation of nitrogen. In the absence of nitrogen-fixing bacteria, we could increase artificial production and fertilize lots of areas worldwide, which would be easier in the absence of bacteria that normally use fixed nitrogen, such as denitrifying and ammonia-oxidizing bacteria (Gilbert, Neufeld, 2014).

Rather simple? Of course, it is not!

Although the atmosphere is full of nitrogen to be artificially fixed, nowadays we already experience the formation of dead zones in oceans and lakes. In those areas, the excess of nitrogen and phosphorus from fertilizers and wastewater leads to eutrophication, a process in which algae blooms in response to the excess of nutrients, forming a layer that prevents photosynthesis in the waters below. Those oxygen-deprived waters (hypoxic areas) don’t support the survival of tiny organisms as well as bigger ones. A supposed intensification of the use of nitrogen fertilizers, to compensate for the loss of microbial fixation, would increase this process. Another important point is the depletion of atmosphere nitrogen reserves, which means that even this artificial process would eventually stop.

Lords of decay

Have you ever been through a sanitation workers’ strike? During those strikes, what is more easily noticed is the accumulation of garbage on the streets (and the bad smells). The disappearance of microbes would have a similar effect. In the absence of these tiny decomposers, dead organic matter would accumulate in the environment, from our sewage systems to organic matter in each and every ecosystem.

The grazing food web supports most of the known energy fluxes in most known ecosystems. It is based on energy production by photosynthetic organisms, such as plants and phytoplankton, the producers. Nevertheless, it produces great amounts of biomass - lots of debris, unconsumed parts, and dead bodies (dead animals, branches, faeces, etc) - rich in nutrients that are normally recycled by decomposers and detritivores from detrital food chains. As most decomposers are microbes (Archaea, Bacteria, and Fungi), without them dead materials would accumulate everywhere and, more importantly, would trap lots of nutrients, which are essential for the grazing food chains that are the base of our whole way of life. A huge biogeochemical tragedy!

At a molecular level, such dead biomass represents a huge reservoir of biogeochemical waste that would disrupt the biogeochemical recycling and lead to a gradual return of elements to the geological material. For example, phosphorus, which is a non-renewable element, would begin to disappear. This would have a great impact on marine primary production and on ocean productivity, dependent on phosphorus regeneration in the water column (Gilbert, Neufeld, 2014).

Living in a “microbeless” world

If you read everything so far, I hope you got to realize how important microbes are for our daily lives on many levels so that, in a pros and cons list, their role as pathogens could be compensated. However, I would be insincere if I didn’t mention that we could survive in their absence. Nowadays, there are many technological advances that would allow us to survive despite the extinction of many other species or the heavy reduction in their population sizes. Indeed, our very population would be reduced as there would be reduced oxygen levels, reduced crop supplies linked to reduced nutrients, reduced livestock, increased greenhouse effect, and accumulation of wastes, to cite a few consequences.

According to the exercise of Gilbert and Neufeld (2014), it would take some time for us to notice the vanishing of microbes. However, eventually, we would become aware of the absence of microbial diseases and the increase of other diseases, such as cancers and mental disorders. Even if we consider them optimists, the authors predicted biogeochemical asphyxiation and a complete societal collapse in about a year, mainly related to the catastrophic collapse of the food supply chain. Starvation, disease, civil war, and anarchy are some of their predictions. Citing them ipsis litteris “the quality of life on this planet would become incomprehensibly bad, life as an entity would endure”.

Now I ask you: would you like to live in a world without microbes?

FEMS MicrobiologySeptember 20, 2022