If all bacteria were to be removed from the earth suddenly then life would likely appear to go on as normal for humans at first, though the impact would soon be felt. Firstly, the removal of all disease-causing pathogenic bacteria would be noted, meaning no more illness from both mild non-life-threatening bacterial infection and more severe diseases such as tuberculosis or Ebola.
The production of antibacterial drugs and treatments would no longer be necessary, lifting the threat posed by multi-drug resistant bacteria. However, other pathogenic microorganisms such as fungi would still pose a threat to human health, as would viruses, and the lack of competition from bacteria may allow these sources of disease to obtain a greater foothold against human health.
It is well known that bacteria play a role in the normal functioning of macroscopic life, as, given their pervasiveness throughout the environment, they have adapted to fill a wide range of niches that other life has depended on, forming symbiotic relationships. However, macroscopic life does not necessarily depend on the presence of bacteria, as has been demonstrated by the generation of germ-free animals and plants for research purposes.
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Germ-free organisms are produced by a variety of methods, though generally involve early sterilization of the egg or embryo using antibiotics and subsequent rearing in sterile conditions, receiving bacteria-free food and water. These organisms frequently exhibit lacking immune and digestive functions, with some reports also indicating that mice reared in these conditions also bear impaired brain development.
Reduced motility in the bowel causes a greatly enlarged cecum in some cases, with deadly results, and underdeveloped lymph nodes result in fewer immune cells circulating in the blood. The heart, lungs, and liver of germ-free animals are also often observed to be undersized compared to their ordinary counterparts.
Otherwise, however, these animals generally live healthy and, on average, slightly longer lives than non-sterile animals of the same type. Early experimentation with sterile animals saw many die from complications surrounding malnutrition, but modern nutritional engineering has allowed the development of foods that can be digested by these animals in the absence of a gut microbiome.
Some macroscopic life, however, has evolved such a deeply symbiotic relationship with their microbiome that they cannot function without them, and in this case, the removal of all bacteria from the earth would result in the quick extinction of these species. The ruminants such as goats, sheep, and cows all depend on the fermentation of plant products in a specialized stomach prior to ingestion, breaking down complex carbohydrates like cellulose into a form that the animal can digest. These bacteria directly consume a large portion of the carbon, phosphorous, and in particular, nitrogen that the ruminant ingests, though are in turn digested by the ruminant to gain these nutrients.
A wide range of plants and fungi would be similarly affected by the loss of their bacterial symbiotic partners, while in some respects many would see a benefit, with their chief competitors gone. One additional group that would be eliminated almost immediately upon the removal of bacteria would be bacteriophages, viruses that infect and replicate in bacteria, without which they would have no host.
Perhaps the most essential function of bacteria to the wider ecosystem is in nitrogen fixation in soil, where gaseous nitrogen molecules are converted to ammonia by nitrogenase enzymes endogenous to some species. Molecular nitrogen composes the majority of the earth’s atmosphere but is inaccessible to most organisms owing to the triple covalent bond between atoms. Nitrogen fixation by bacteria completes the nitrogen cycle, providing nitrogen to plants that are then consumed by other life, the nitrogen eventually being returned to the environment.
With regards to nitrogen fixation, a sudden ceasing of all bacterial activity would be catastrophic to plant life, and then the remainder of life that depend on said plants. Human agriculture would be impacted similarly, though it may be possible to avoid a devastating loss of crops by fertilizing fields using soil with nitrogen fixated using the Haber process.
The Haber process is currently used to produce ammonia and is the staple process by which fertilizer is made. The process requires very high temperature and pressure, however, and if it was required that humanity produce all of the needed nitrogen fixed soil in this way then global energy concerns would become even direr, not to mention the potential effect that the accumulation of greenhouse gasses in the atmosphere combined with the removal of atmospheric nitrogen may have.
Early bacteria that engaged in photosynthesis to produce oxygen are possibly the source of the greatest extinction event to have occurred in the history of the earth: the great oxidation event. During this time around 2.4 billion years ago oxygen came to major permanent prominence in the atmosphere and oceans, driven by the evolution of photosynthesis in cyanobacteria, which quickly came to dominate the earth’s oceans. This relatively sudden shift in the composition of the water and air caused the mass extinction of many organisms not well adapted to the newly corrosive environment, and given that the majority of oxygen is still produced in the ocean by bacteria their removal may have dire consequences. Since the first appearance billions of years ago, however, several other organisms have evolved to photosynthesize and produce oxygen, such as plants, and thus their loss may not be felt as strongly by now.
While insects, fungi, and other decomposers play a role and may initially contribute most greatly to the removal of waste from the environment, the process could not be complete without bacteria, which bring a unique set of biomolecular tools to the table. Vast reservoirs of organic compounds that cannot be broken down without the aid of enzymes found only in bacteria would eventually accumulate, and essential components of the ecosystem would become irrecoverably locked in place.
For example, phosphorous would sequester into oceanic sediment and become unavailable to life. Fungi or other microorganisms may eventually fill these niche roles and provide a new balance and new ecosystem, though not before significant upheaval.
In conclusion, if bacteria were to be wiped from the face of the earth then humans may not be initially affected in any significantly detrimental manner, likely facing digestive issues owing to the loss of the gut microbiome but otherwise benefiting from the absence of pathogenic bacteria.
Animals and plants that more strongly depend on bacteria to live would die quickly, which includes many species that humans depend on for food: cows, goats, and sheep. Over time sequestration of essential nutrients in accessible forms would cause more widespread death, as plants and animals steadily dwindled in number. In this case, the oceans would become the major stores of nitrogen, and thus perhaps life would be concentrated to coastal regions.
Small pockets of humanity may be able to persist thanks to modern methods of fertilizer production, but without further intervention would also likely fail in time. Over much longer periods other life forms may evolve to fill the ecological niches previously occupied by bacteria, though the biosphere would have changed notably by this point, with new symbiotic and antagonistic relationships developing.