The invisible majority — bacteria, archaea, protists, and fungi that decompose, cycle nutrients, produce oxygen, cause disease, sustain ecosystems, and make modern biotechnology possible
Most life on Earth is invisible to the naked eye. Beneath every square centimeter of soil, inside every drop of pond water, on every surface of your body, and throughout the air around you, there are organisms so small they were unknown to science until microscopes were invented in the seventeenth century. These are the microorganisms — and they are not peripheral to life on Earth. They are central to it.
Microorganisms decompose dead matter and return nutrients to ecosystems. They fix nitrogen from the atmosphere into forms plants can use. They live inside your gut and help you digest food. They drive the fermentation that produces bread, cheese, and yogurt. They cycle carbon through the atmosphere and oceans. They were the first living things on Earth and remain, by any measure — number of individuals, number of species, total mass of living material — the dominant form of life on the planet.
This chapter introduces the three domains and their microbial members — bacteria, archaea, and protists — and then turns to fungi, which are not microorganisms in the strict sense but share many features with the microbial world and are studied alongside it. Together, these groups reveal the extraordinary range of life's strategies for surviving, reproducing, and shaping Earth systems.
A patient takes a broad-spectrum antibiotic for a week and then develops a secondary intestinal infection. A soil scientist finds that removing all microorganisms from a field causes crop yields to collapse within two seasons. A biochemist traces the oxygen in Earth's atmosphere back to ancient cyanobacteria. What connects these three observations? By the end of this chapter, you will understand why microorganisms are not merely agents of disease but are the invisible infrastructure on which all complex life depends.
For most of human history, the microbial world was invisible and unsuspected. People knew that food spoiled, wounds became infected, and bread rose with yeast — but the agents responsible were unknown. When Antonie van Leeuwenhoek looked through a simple microscope in the 1670s and observed what he called "animalcules" in pond water and dental scrapings, he opened a window onto a world no one had imagined.
Today, biologists estimate that Earth harbors somewhere between one trillion and one quadrillion species of microorganisms — most of them not yet described by science. The number of individual bacterial cells in and on a single human body is comparable to, and possibly exceeds, the number of human cells. A single gram of fertile soil may contain more than one billion bacterial cells from thousands of species.
It is tempting to think of microorganisms primarily as causes of disease. Some certainly are. But the vast majority of microorganisms do not harm humans at all. Many are essential to human life. Without microorganisms, life as we know it would not exist.
Microorganisms are measured in micrometers (µm), where one micrometer equals one millionth of a meter. A typical bacterium is 1–10 µm in length. A eukaryotic cell is generally 10–100 µm. A human hair is about 70 µm wide. Viruses are even smaller — typically 20–300 nanometers (one nanometer = one thousandth of a micrometer). Light microscopes can resolve cells in the 1–100 µm range; electron microscopes are needed for finer structures and most viruses.
Bacteria are prokaryotic organisms — single-celled life forms that lack a membrane-bound nucleus and membrane-bound organelles. Their genetic material, a single circular chromosome, floats freely in the cytoplasm in a region called the nucleoid. Bacteria also frequently carry small extra circles of DNA called plasmids, which often carry genes for antibiotic resistance or other adaptive traits.
Bacteria are among the oldest and most successful life forms on Earth, with fossil evidence suggesting they have existed for at least 3.5 billion years — more than three quarters of Earth's history.
A bacterial cell is surrounded by a plasma membrane, and most bacteria also have a rigid cell wall outside the membrane. The composition of the cell wall differs between the two major categories of bacteria, a difference revealed by the Gram stain procedure.
| Category | Cell Wall Structure | Gram Stain Result | Medical Significance |
|---|---|---|---|
| Gram-positive | Thick peptidoglycan layer | Purple (retains dye) | Often more susceptible to penicillin-class antibiotics |
| Gram-negative | Thin peptidoglycan + outer membrane | Pink/red (counterstained) | Outer membrane blocks some antibiotics; often requires different drugs |
| Shape | Term | Example |
|---|---|---|
| Spherical | Coccus (plural: cocci) | Streptococcus |
| Rod-shaped | Bacillus (plural: bacilli) | Escherichia coli |
| Spiral | Spirillum / Spirochete | Treponema pallidum |
Cocci that stay connected after dividing form characteristic arrangements: pairs (diplococci), chains (streptococci), or clusters (staphylococci). These arrangements help with identification. Streptococcus forms chains; Staphylococcus aureus forms grape-like clusters.
Bacteria reproduce primarily by binary fission: a cell grows, replicates its chromosome, and divides into two genetically identical daughter cells. Under ideal conditions, some bacteria can divide every twenty minutes. Growth is always limited by nutrients, space, and waste products.
Bacteria also transfer genetic material between cells through three processes that are not reproduction:
These mechanisms allow beneficial genes — especially antibiotic resistance genes — to spread rapidly through and between bacterial populations, a major driver of the antibiotic resistance crisis.
| Category | Energy Source | Carbon Source | Example |
|---|---|---|---|
| Photoautotroph | Light | CO₂ | Cyanobacteria |
| Chemoautotroph | Inorganic chemicals | CO₂ | Deep-sea vent bacteria |
| Photoheterotroph | Light | Organic compounds | Some purple bacteria |
| Chemoheterotroph | Organic chemicals | Organic compounds | Most bacteria; decomposers; pathogens |
Bacteria also differ in their relationship to oxygen. Obligate aerobes require oxygen. Obligate anaerobes are poisoned by oxygen and live only in oxygen-free environments such as deep soil layers and sealed wounds. Facultative anaerobes — including E. coli — use oxygen when available but can switch to fermentation or anaerobic respiration when it is not.
| Group | Features | Examples |
|---|---|---|
| Proteobacteria | Gram-negative; largest phylum; enormous metabolic diversity | E. coli, Salmonella, nitrogen-fixing Rhizobium |
| Firmicutes | Gram-positive; includes endospore formers | Bacillus, Clostridium, Lactobacillus |
| Actinobacteria | Gram-positive; filamentous; produce many antibiotics | Streptomyces |
| Cyanobacteria | Photosynthetic; produce oxygen; fix nitrogen | Nostoc, Anabaena |
| Spirochetes | Gram-negative; spiral; motile | Treponema, Borrelia (Lyme disease) |
For most of the twentieth century, biologists divided life into two groups: prokaryotes (bacteria) and eukaryotes. In 1977, Carl Woese and George Fox analyzed ribosomal RNA sequences — genetic sequences shared by all living things but varying enough to show evolutionary relationships — and discovered that some prokaryotes were so fundamentally different from bacteria that they deserved their own domain. These organisms are now called archaea (singular: archaeon).
Archaea look like bacteria under a light microscope — they are prokaryotic, typically single-celled, and lack a nucleus. But at the molecular and biochemical level, they differ profoundly from bacteria: their cell membranes use different lipids, their ribosomes are biochemically distinct, and their DNA replication and transcription machinery more closely resembles that of eukaryotes than bacteria. Genetic evidence indicates that eukaryotes share a more recent common ancestor with archaea than with bacteria, making archaea our closest microbial relatives.
| Group | Environment | Key Feature | Example |
|---|---|---|---|
| Thermophiles | Boiling hot springs, hydrothermal vents | Thrive above 60°C | Sulfolobus (acidic hot springs, >80°C) |
| Halophiles | Very salty lakes, salt flats | Require high salinity | Halobacterium (Dead Sea) |
| Methanogens | Swamps, cow guts, deep-sea sediments | Produce methane as byproduct; obligate anaerobes | Methanobacterium |
Archaea were initially found only in extreme environments and were considered curiosities. Research using molecular tools has since shown that archaea are common in temperate oceans, in ordinary soil, and in the human gut. No archaea have been confirmed as human pathogens — a fact that remains scientifically interesting and not fully explained.
The discovery of archaea led to a revision of fundamental life classification. All life is now organized into three domains:
Protists are eukaryotic organisms that are not fungi, plants, or animals. This is a definition by exclusion — protists are grouped together not because they form a natural evolutionary lineage but because they are the eukaryotes left over after fungi, plants, and animals are identified. Modern classification places the various protist lineages in separate groups, most of which diverged very early in eukaryotic evolution, but the term remains useful for introductory study.
Protists include single-celled organisms that move by pseudopods, cilia, or flagella; large multicellular seaweeds; photosynthetic phytoplankton that drive marine food webs; and parasites that cause some of the world's most devastating diseases.
Protozoa are single-celled heterotrophic protists that obtain nutrients by ingesting other organisms or organic matter.
Algae are photosynthetic protists. They are not plants — they lack the complex tissue organization of true plants — but like plants, they use light, water, and carbon dioxide to produce sugars, releasing oxygen as a byproduct.
| Group | Key Features | Ecological Importance | Example |
|---|---|---|---|
| Diatoms | Single-celled; silica shells; intricate glass-like frustules | ~20% of Earth's atmospheric oxygen; base of aquatic food webs | Numerous species in oceans and freshwater |
| Dinoflagellates | Two flagella; many bioluminescent; some produce toxins | Marine primary producers; red tides cause harmful algal blooms | Karenia brevis |
| Green algae | Closest relatives of land plants; diverse forms | Freshwater and marine primary producers; ancestors of plants | Chlamydomonas, Volvox |
| Brown algae | Almost exclusively marine; largest protists | Giant kelp forests shelter entire ecosystems | Macrocystis pyrifera (up to 50 m) |
| Red algae | Primarily marine; contain red pigments | Agar used in biology labs; food in Asian cuisines | Porphyra (nori) |
When diatoms die, their silica shells sink and accumulate in ocean and lake sediments. Thick deposits — called diatomaceous earth — are mined and used as filtration media, mild abrasives, and pesticides. The abundance of diatom fossils also allows scientists to reconstruct past climate and water conditions from sediment cores.
Slime molds spend part of their lives as independent amoeba-like cells. When food becomes scarce, thousands of cells aggregate and move as a mass, then form a fruiting body that releases spores — remarkably coordinated behavior in organisms with no nervous system.
Water molds (oomycetes) resemble fungi in appearance but are genetically distinct. Phytophthora infestans, a water mold, caused the Irish potato famine of the 1840s by destroying potato crops across Ireland and contributing to widespread starvation and mass emigration.
Fungi are a kingdom of eukaryotic organisms that includes yeasts, molds, and mushrooms. They are not plants — they lack chloroplasts and cannot photosynthesize. They are not animals — they are mostly stationary and do not ingest food. Fungi are heterotrophs that feed by absorption: they secrete enzymes into their surrounding environment, break down organic matter externally, and absorb the resulting nutrients.
The body of most fungi consists of a network of thread-like filaments called hyphae (singular: hypha). A mass of hyphae is called a mycelium. Hyphae are surrounded by cell walls made of chitin, the same tough polysaccharide that forms the exoskeletons of insects. Fungi are more closely related to animals than to plants at the molecular level.
Fungi reproduce through spores — small, hardy reproductive cells dispersed by air, water, or animals. Asexual reproduction produces vast numbers of spores without fusion of nuclei. Sexual reproduction involves the fusion of hyphae from two compatible mating types and eventually the production of sexual spores. The mushrooms familiar to us are the above-ground fruiting bodies that release spores; the actual organism lives as mycelium in soil or wood below. Yeasts reproduce primarily by budding — a small outgrowth forms, grows, and separates from the parent cell.
| Group | Characteristics | Examples |
|---|---|---|
| Zygomycetes | Simple; produce thick-walled zygospores | Black bread mold (Rhizopus) |
| Ascomycetes | Largest group; produce spores in sac-like asci | Yeasts, Penicillium, morels, truffles |
| Basidiomycetes | Produce spores on club-shaped basidia | Mushrooms, puffballs, rusts, smuts |
| Chytrids | Aquatic; motile spores with flagella | Batrachochytrium (threatens amphibians) |
| Glomeromycetes | Form mycorrhizal associations with plant roots | Arbuscular mycorrhizal fungi |
A lichen is a symbiotic partnership between a fungus and a photosynthetic partner — either a green alga, a cyanobacterium, or both. The fungal partner provides structure, retains water, and absorbs mineral nutrients. The photosynthetic partner produces sugars through photosynthesis. Lichens colonize bare rock, arctic tundra, and desert surfaces where few other organisms can survive, and they are sensitive indicators of air quality — many species are killed by sulfur dioxide and other pollutants.
Students often classify fungi as plants because they are stationary and grow in soil. Fungi are not plants. They do not photosynthesize, their cell walls contain chitin (not cellulose), and they feed by absorption rather than producing their own food. Molecular evidence places fungi closer to animals than to plants on the tree of life.
A pathogen is an organism or agent that causes disease. Bacteria, protists, and fungi can all be pathogens. Understanding how pathogens cause disease and spread is central to public health.
Bacterial pathogens cause disease through toxins or direct invasion. Clostridium botulinum produces botulinum toxin, which blocks nerve signaling and causes paralysis. Vibrio cholerae produces a toxin that floods the gut with water and ions, causing severe diarrhea. Mycobacterium tuberculosis infects and survives inside macrophages — the very immune cells meant to destroy bacteria.
Protist pathogens include Plasmodium (malaria), Trypanosoma (sleeping sickness and Chagas disease), and Giardia (intestinal illness). Fungal pathogens include Candida albicans (thrush), Aspergillus species (lung infections in immunocompromised patients), and the chytrid fungus Batrachochytrium dendrobatidis, which has driven dozens of amphibian species to extinction.
| Route | Description | Example Pathogen |
|---|---|---|
| Direct contact | Touching an infected person or surface | Staphylococcus aureus |
| Droplets/aerosols | Respiratory pathogens spread by coughing or breathing | Influenza virus; Mycobacterium tuberculosis |
| Foodborne/waterborne | Ingesting contaminated food or water | Vibrio cholerae, Giardia |
| Vector transmission | A carrier organism (mosquito, tick) transmits pathogen while feeding | Plasmodium (mosquito); Borrelia (tick) |
| Vertical transmission | Parent to offspring during pregnancy, birth, or breastfeeding | HIV; Treponema pallidum |
Antibiotics are compounds that inhibit the growth of or kill bacteria by targeting structures unique to bacteria: cell wall synthesis, bacterial ribosomes, bacterial DNA replication enzymes, or membrane integrity. This selectivity allows antibiotics to kill bacteria without harming eukaryotic host cells (in most cases).
Antibiotic resistance arises by natural selection. Resistance genes are not created by antibiotic exposure — random mutations produce them in the absence of antibiotics. When a population is exposed to an antibiotic, sensitive individuals are killed and resistant individuals survive and reproduce. Over time, the population shifts toward resistance.
Resistance genes can spread rapidly through and between bacterial populations by conjugation, transformation, and transduction. Overuse — such as using antibiotics for viral infections, not completing treatment courses, or using them in agriculture at subtherapeutic doses — accelerates the evolution and spread of resistant strains.
"Antibiotics create resistance genes." Antibiotics do not cause mutations or create resistance — they select for resistance genes that already exist in the population due to random mutation. Antibiotic exposure determines which bacteria survive and reproduce; it does not alter the bacteria's DNA to produce resistance.
Nitrogen-fixing bacteria convert atmospheric nitrogen gas (N₂) into ammonia (NH₃) and other nitrogen compounds that plants can absorb and use to build proteins and nucleic acids. Without biological nitrogen fixation, most terrestrial ecosystems would be severely nitrogen-limited. Nitrogen-fixing bacteria include free-living soil bacteria such as Azotobacter and symbiotic bacteria — most importantly, Rhizobium species that live inside root nodules of legumes (beans, peas, clover).
Cyanobacteria not only photosynthesize but also fix nitrogen, making them especially important in aquatic ecosystems. Phytoplankton in ocean surface waters — bacteria, protists, and algae — are responsible for roughly half of Earth's total photosynthesis and form the base of marine food webs.
The microbiome is the community of microorganisms living in and on a host organism. The human gut microbiome includes trillions of microbial cells that perform functions the human body cannot perform on its own.
The legume–Rhizobium symbiosis allows legume crops to grow in nitrogen-poor soils without synthetic fertilizer. Mycorrhizal fungi associate with the roots of most land plants, dramatically increasing the effective surface area for water and nutrient absorption. Modern agriculture is increasingly exploring how to use beneficial microorganisms to reduce dependence on synthetic fertilizers and pesticides.
Decomposition is the biological breakdown of dead organic matter into simpler inorganic compounds, carried out primarily by bacteria and fungi. Without decomposition, nutrients would be locked in dead tissues and unavailable for new life. Decomposition is the process that closes nutrient cycles.
Bacteria are generally more important in decomposing soft tissues, while fungi dominate the decomposition of tough materials containing cellulose and lignin — the structural polymers of plant cell walls. The ability to digest lignin is rare in the living world. White rot fungi break down both cellulose and lignin, completely decomposing wood. Brown rot fungi break down primarily cellulose, leaving lignin fragments behind. Without wood-rotting fungi, dead plant material would accumulate and nutrients would be permanently locked away.
| Nutrient Cycle | Role of Microorganisms | Key Process |
|---|---|---|
| Carbon cycle | Decomposers release CO₂ from dead organic matter through cellular respiration | Decomposition returns carbon to atmosphere as CO₂ |
| Nitrogen cycle | Nitrogen-fixing bacteria make atmospheric N₂ available; decomposers release nitrogen from dead organisms; denitrifying bacteria return N₂ to atmosphere | Nitrogen fixation, ammonification, nitrification, denitrification |
| Phosphorus cycle | Decomposers release phosphorus from organic matter in soil and sediments | Mineralization of organic phosphorus makes it available for plant uptake |
Decomposition is fastest in warm, moist, oxygen-rich conditions. Cold, dry, or anaerobic conditions dramatically slow it — which is why food is refrigerated, why ancient organic material is preserved in peat bogs, and why the permafrost of arctic regions stores vast quantities of ancient organic carbon.
A symbiosis is a close, long-term relationship between two different species. Microorganisms are involved in an extraordinary range of symbioses:
In nature, bacteria often form biofilms — structured communities of cells embedded in a self-produced matrix of polysaccharides, proteins, and nucleic acids attached to a surface. Dental plaque is a biofilm. So are the slippery coatings on stream rocks and the communities on medical implants.
Bacteria in biofilms communicate using chemical signals in a process called quorum sensing, coordinating gene expression, metabolic activity, and response to antibiotics based on population density. Bacteria within biofilms are much more resistant to antibiotics than free-living bacteria — a serious challenge in treating hospital-acquired infections on catheters, implants, and wound surfaces.
Microbial communities in the ocean process enormous quantities of carbon. The marine microbial loop describes how dissolved organic carbon released by phytoplankton is taken up by bacteria, which are then consumed by protists, transferring energy through the food web in pathways not recognized until relatively recently. This "invisible" energy channel — invisible because it involves organisms too small to see — may account for a substantial fraction of energy flow in oceanic food webs.
Humans have used microorganisms in food production for thousands of years — long before anyone understood that living organisms were responsible. Fermentation is the metabolic process by which microorganisms transform organic compounds, often producing alcohol, acids, or gases as byproducts.
| Food Product | Microorganism | Process |
|---|---|---|
| Bread | Saccharomyces cerevisiae (yeast) | Ferments sugars; CO₂ causes dough to rise |
| Beer and wine | Yeast | Ferments sugars in grain or fruit juice; produces ethanol and CO₂ |
| Yogurt | Lactobacillus, Streptococcus thermophilus | Bacteria ferment lactose to lactic acid; lowers pH and thickens product |
| Blue cheese | Penicillium roqueforti (fungus) | Mold ripens cheese; produces characteristic flavor and blue-green color |
| Vinegar | Acetobacter | Oxidizes ethanol to acetic acid |
| Sauerkraut and kimchi | Lactic acid bacteria | Ferment vegetables anaerobically; acid preserves food |
Chapter 16 introduced genetic engineering and recombinant DNA technology. The production of human insulin in E. coli is a direct application of the techniques studied there — inserting a gene into a bacterial host cell and using the bacterium as a biological factory. Microorganisms are the workhorses of modern biotechnology precisely because they grow quickly, are easy to maintain, and can be genetically modified to produce virtually any protein.
For each description, identify whether it best describes a bacterium, an archaeon, or a protist. Explain your reasoning.
Complete the table by identifying one disease and one benefit associated with each microbial group. Then answer the question below.
| Group | One Disease Example | One Benefit Example |
|---|---|---|
| Bacteria | [student answers here] | [student answers here] |
| Protists | [student answers here] | [student answers here] |
| Fungi | [student answers here] | [student answers here] |
After completing the table: Why is it misleading to characterize microorganisms primarily as agents of disease? Support your answer with at least two specific examples from this chapter.
Each item contains an error in biological reasoning. Identify the error and write a corrected explanation.
Why is this statement incorrect? What makes archaea a separate domain from bacteria?
Correct this statement using the concept of natural selection. What does antibiotic exposure actually do?
Identify two specific differences between fungi and plants that disprove this claim.
Give a specific example that disproves this claim and explain why protists are difficult to define as a group.
Correct this misconception by describing three beneficial functions of the gut microbiome.
"Bacteria are always harmful." The vast majority of bacteria do not cause disease in humans. Most play beneficial or neutral roles — decomposing organic matter, fixing nitrogen, supporting soil fertility, producing food, and maintaining human health as part of the microbiome. Only a small fraction of bacterial species are pathogens.
"Archaea are extremophile bacteria." Archaea are not bacteria. They represent a completely separate domain of life, distinct from bacteria at the level of membrane chemistry, ribosomal structure, and the molecular machinery for DNA replication and gene expression. While many archaea live in extreme environments, they are also common in temperate oceans, soil, and the human gut.
"Fungi are plants." Fungi do not photosynthesize, have cell walls made of chitin (not cellulose), and feed by absorption rather than producing their own food. Fungi are more closely related to animals than to plants at the molecular level.
"Protists are all tiny single-celled organisms." Many protists are indeed single-celled and microscopic, but the group also includes large multicellular seaweeds such as giant kelp, which can reach fifty meters in length — among the largest organisms on Earth.
"Antibiotic resistance is created by antibiotics." Antibiotics do not create resistance genes — random mutations produce these genes whether or not antibiotics are present. Antibiotic exposure does not cause resistance; it selects for resistant individuals already present in the population.
Chapter 21 turns to plants — the photosynthetic organisms that make terrestrial life possible. You have already learned how photosynthesis works at the cellular level and how green algae represent the photosynthetic ancestors of land plants. Chapter 21 will examine how plant structure connects to plant function: how roots anchor, absorb, and store; how stems transport and support; how leaves capture light and exchange gases; how flowers attract pollinators; and how seeds disperse offspring. You will also see how plants respond to their environments through growth, tropisms, and chemical signals. The chemistry-of-life chapters, the cell biology chapters, and the evolution chapter will all contribute to understanding why plants are structured as they are and how they came to dominate terrestrial ecosystems.