Microbes are living organisms too small to see without a microscope, and they include bacteria, archaea, fungi, protists, and (depending on who you ask) viruses. Collectively, they make up the vast majority of life on Earth by sheer number and occupy every habitat from deep ocean trenches to human intestines. What they do is equally vast: microbes produce much of the oxygen you breathe, break down dead matter into nutrients that plants and animals depend on, shape the human immune system, cause infectious disease, and power industries ranging from cheese-making to pharmaceutical manufacturing. The simple version of the answer is that microbes do almost everything, and the longer version is more interesting than most people expect.
The Major Groups of Microbes
When scientists talk about microbes, they are usually referring to several distinct groups of life that happen to share one trait: they are microscopically small. Bacteria are single-celled organisms without a nucleus, and they are probably the first thing that comes to mind when you hear “microbe.” Archaea look similar to bacteria under a microscope but are genetically quite different and often thrive in extreme environments like hot springs and salt flats. Fungi include single-celled yeasts as well as molds and the microscopic stages of mushrooms. Protists are a grab-bag category of single-celled organisms that have a nucleus, including amoebas and the algae that color ponds green. Viruses are sometimes lumped in because they are tiny and cause disease, though many biologists hesitate to call them truly alive since they cannot reproduce on their own.
These groups are not closely related to each other on the tree of life. Archaea are actually more closely related to you than they are to bacteria, despite looking almost identical under a microscope. This diversity matters because it means microbes are not one thing doing one job. They represent multiple independent branches of life, each with radically different chemistry, and that chemical variety is exactly what makes them so influential.
How Much Life on Earth Is Microbial
If you weighed all the living things on the planet, microbes would account for a staggering share. A global census of biomass estimated that bacteria alone represent roughly 70 gigatons of carbon, while archaea add another 7 gigatons. For comparison, all animals on Earth combined amount to about 2 gigatons of carbon. Plants dwarf everything at around 450 gigatons, but that mass is mostly wood and cellulose sitting still in forests. Bacteria and archaea, by contrast, are metabolically active everywhere: in soil, in oceans, in the deep subsurface of the Earth’s crust, and inside the bodies of other organisms.1Proceedings of the National Academy of Sciences. The biomass distribution on Earth
Those numbers only count what researchers have been able to measure. Estimates of microbial life in deep rock and sediment are still being revised upward as drilling projects reach new depths. The point is not the exact tonnage but the scale: microbes are not a minor footnote in the story of life. They are a dominant chapter, and they were here billions of years before anything with eyes or legs showed up.
Microbes and the Air You Breathe
About half of the oxygen in Earth’s atmosphere comes from the ocean, and most of that ocean oxygen is produced by microscopic photosynthetic organisms: phytoplankton, tiny algae, and cyanobacteria. One cyanobacterium in particular, called Prochlorococcus, is the smallest photosynthetic organism known. Despite being invisible to the naked eye, Prochlorococcus alone is responsible for producing up to about 20 percent of the oxygen in the entire biosphere.2National Ocean Service. How much oxygen comes from the ocean?
That single species contributing a fifth of your oxygen supply puts into perspective how dependent complex life is on microbes. Forests get most of the credit in popular imagination, but the ocean’s microbial photosynthesizers are doing comparable work. And their role does not stop at oxygen. Cyanobacteria and other marine microbes also pull carbon dioxide out of the atmosphere and cycle nitrogen, sulfur, and phosphorus through ecosystems. Without these invisible chemical workers, the nutrient cycles that sustain all life on land and in the sea would grind to a halt.
What Microbes Do Inside Your Body
Your body hosts trillions of microbes, mostly bacteria, concentrated heavily in the gut. This community, sometimes called the gut microbiome, is not just passively hitching a ride. Certain bacteria in the large intestine ferment complex carbohydrates that your own digestive enzymes cannot break down, producing short-chain fatty acids in the process. These fatty acids serve as a fuel source for the cells lining the colon and also play a role in regulating the immune system.3National Institutes of Health. Gut Microbiota and Immune System Interactions
That immune connection is one of the most actively researched areas in biology right now. The gut microbiome appears to train and calibrate immune cells, helping the body distinguish between harmless substances and genuine threats. When the microbial community is disrupted, whether by antibiotics, poor diet, or illness, the immune system can become less precise. This is one reason researchers are interested in links between gut bacteria and conditions ranging from allergies to autoimmune diseases. The research is still evolving, and many of the boldest claims about probiotics outpace the evidence, but the basic finding that gut microbes shape immunity is well supported.
Beyond immunity, gut bacteria synthesize vitamins like B12 and K that your body needs. They help metabolize drugs, which is why the same medication can work differently in different people depending on their microbial makeup. Skin microbes help defend against pathogens by occupying space and producing antimicrobial compounds. Microbes in the mouth, the respiratory tract, and the urogenital tract all have their own distinct communities with their own roles. The body, in other words, is not a sterile fortress occasionally invaded by germs. It is a managed ecosystem.
Most Microbes Are Not Trying to Make You Sick
One of the biggest misconceptions about microbes is that they are primarily agents of disease. In reality, pathogens represent only a tiny fraction of all microorganisms.4PubMed Central. Microbiome and Human Health: Current Understanding, Engineering, and Enabling Technologies The overwhelming majority of bacteria, fungi, and archaea you encounter in a given day are harmless or actively helpful. Even within the human body, the resident microbes vastly outnumber the disease-causing ones, and many of the residents are actively working against invaders by competing for resources and space.
Pathogens tend to cause problems not because microbes in general are dangerous but because specific organisms have evolved strategies to exploit host tissues. A bacterium like Mycobacterium tuberculosis, for instance, has a very particular set of tools for surviving inside human immune cells, something the vast majority of bacteria cannot do. Likewise, the fungi that cause serious infections in humans are a tiny subset of the fungal kingdom. Most fungi are busy decomposing leaves and dead wood, which is one of the most ecologically important jobs on the planet.
The framing matters because fear of microbes can lead to counterproductive behavior. Overuse of antibiotics, obsessive sanitization, and avoidance of fermented foods can reduce the diversity of beneficial microbes in and around you. The hygiene hypothesis, while oversimplified in its popular form, does point to real evidence that children raised in environments with more microbial exposure tend to have lower rates of certain allergic and autoimmune conditions. Cleanliness is important, but sterility is neither achievable nor desirable.
Microbes in Food and Industry
Humans have been putting microbes to work for thousands of years, often without knowing it. Bread rises because yeast (a fungus) ferments sugars and produces carbon dioxide gas. Beer, wine, yogurt, cheese, soy sauce, kimchi, and sauerkraut all depend on microbial fermentation. These are not incidental uses. Fermentation was one of the earliest food preservation technologies, and it remains central to cuisines worldwide.
Modern biotechnology has expanded the roster dramatically. One landmark example is the production of human insulin using the bacterium Escherichia coli. Before genetic engineering made this possible, insulin for diabetic patients had to be extracted from pig or cow pancreases, a process that was expensive, limited in supply, and occasionally triggered allergic reactions. Engineering E. coli to produce human insulin opened a new era in biopharmaceutical manufacturing.5PubMed Central. Escherichia coli in the production of biopharmaceuticals
Today, bacteria and yeast are used to manufacture a wide range of products: enzymes for laundry detergent, biodegradable plastics, biofuels, vitamins, amino acids, and specialty chemicals. Wastewater treatment plants rely on microbial communities to break down organic waste. Mining operations use bacteria to extract metals from ore in a process called bioleaching. The textile industry uses microbial enzymes to process fabrics. Whenever an industrial process needs something broken down, built up, or transformed at the molecular level, there is a good chance a microbe can do it cheaper and more sustainably than a chemical reactor.
Life in Extreme Places
Microbes live in places that would kill any animal or plant almost instantly. Thermophilic and hyperthermophilic archaea thrive at temperatures above the boiling point of water near deep-sea hydrothermal vents, where superheated fluid jets out of the ocean floor. Some of these organisms are methanogens, which produce methane by combining hydrogen and carbon dioxide. They are among the most important microbial groups in high-temperature deep-sea environments, where they drive chemical cycles that sustain entire vent ecosystems in complete darkness.6SpringerLink (Marine Life Science & Technology). Microorganisms from deep-sea hydrothermal vents
Other extremophiles live in Antarctic ice, in highly acidic mine drainage with a pH below 1, in salt concentrations that would pickle most cells, and in rock kilometers below the surface where no sunlight has reached in millions of years. These organisms are not just curiosities. Their existence rewrites assumptions about where life can persist, which has direct implications for the search for life on other planets. Mars, Europa (a moon of Jupiter with a subsurface ocean), and Enceladus (a moon of Saturn that vents water vapor into space) all have conditions that overlap with what some Earth extremophiles tolerate. Astrobiologists study these microbes specifically to understand what biosignatures to look for elsewhere in the solar system.
Extremophiles also have practical value. Enzymes from heat-loving microbes are the basis of PCR, the technique used to amplify DNA in everything from forensic labs to COVID-19 testing. The enzyme Taq polymerase, originally isolated from a bacterium living in a Yellowstone hot spring, works precisely because it does not fall apart at the high temperatures the PCR process requires. Discoveries like this illustrate a recurring theme: the stranger the microbe, the more likely it is to have biochemical tools no one has thought to use yet.
The Uncultured Majority
Perhaps the most humbling fact about microbiology is how much remains unknown. Estimates vary, but the vast majority of microbial species on Earth have never been grown in a laboratory. Researchers can detect their DNA in environmental samples, sequence their genomes, and infer what they might be doing, but they have never seen many of these organisms alive in a dish. The reasons are practical: many microbes depend on specific partners, chemical conditions, or nutrient flows that are difficult or impossible to replicate in a lab setting. A bacterium that lives only in a biofilm deep in ocean sediment, relying on chemical gradients and neighboring species to survive, will not grow on a standard agar plate.
This matters because laboratory cultivation has historically been the main way scientists figure out what a microbe does. Without it, researchers rely on genomic inference, which is like reading an instruction manual written in a language you only partly understand. The gap between what is detectable and what is understood means that the catalog of microbial functions described above is almost certainly incomplete. New metabolic pathways, new ecological roles, and new useful enzymes are being discovered regularly as sequencing technology improves and computational tools get better at making sense of genomic data.
Some of the most intriguing recent discoveries have come from metagenomics, the practice of sequencing all the DNA in an environmental sample at once rather than trying to isolate individual species. This approach has revealed entirely new branches on the tree of life, including groups of bacteria and archaea so genetically distinct from known organisms that they required new categories to classify them. These findings suggest that microbial diversity is far richer than even optimistic estimates from a few decades ago, and that the microbes scientists have studied so far are a biased sample of the ones that happen to be easy to grow.
Microbes and Climate
Microbes are not passive bystanders in climate change. They are active participants on both sides of the ledger. Soil bacteria and fungi decompose organic matter, releasing carbon dioxide back into the atmosphere. Methane-producing archaea in wetlands, rice paddies, and the guts of ruminant animals are a major source of methane, a greenhouse gas with far more warming potential per molecule than carbon dioxide. As permafrost thaws in the Arctic, previously frozen organic material becomes available to microbial decomposition, potentially releasing enormous quantities of both carbon dioxide and methane in a feedback loop that accelerates warming.
On the other side, photosynthetic microbes in the ocean absorb vast amounts of carbon dioxide, and some of that carbon sinks to the deep ocean when the organisms die, effectively removing it from the atmosphere for centuries. Certain soil bacteria promote the formation of stable organic compounds that lock carbon into the ground. Researchers are exploring whether microbial communities in agricultural soils can be managed to increase carbon storage, a strategy sometimes called soil carbon sequestration. The idea is promising but complicated, because microbial communities respond to temperature, moisture, soil chemistry, and farming practices in ways that are difficult to predict or control at scale.
Understanding microbial contributions to greenhouse gas fluxes is critical for climate modeling. Models that ignore or simplify microbial activity risk getting the numbers wrong in ways that matter for policy. A wetland that looks like a carbon sink from a plant perspective might be a net carbon source once you account for the methane its microbes produce. Getting the microbial accounting right is one of the quieter but more consequential challenges in climate science.