The microbial world represents the vast, unseen majority of life on Earth. These minute entities have been present on the planet for an estimated 3.5 to 4 billion years, dominating the biosphere for much of that time. They are the original architects of Earth’s atmosphere and biogeochemical cycles, laying the foundation for all subsequent life forms. Despite their minuscule size, the collective biomass and sheer genetic diversity of microbes dwarf that of all plants and animals combined. This ancient biological community fundamentally governs the habitability of the planet.
Defining the Microbial World
Microbial life is generally defined as any organism that exists as a single cell or small cluster of cells and requires magnification to be observed. This definition spans three of the Earth’s domains of life: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are both prokaryotes, meaning their cells lack a membrane-bound nucleus and other internal organelles. However, these two domains are distinct from one another, separated by billions of years of evolution.
Archaea possess unique biochemical features, such as cell membranes constructed from ether-linked lipids, which differ from the ester-linked lipids found in Bacteria. While the cell walls of Bacteria often contain peptidoglycan, this substance is absent in Archaea. Bacterial cells are typically around one micrometer in size. Viruses can be ten times smaller, residing in the nanometer range.
The third domain, Eukarya, also contains numerous microbes, including the single-celled Protists and microscopic Fungi. Protists are highly diverse, encompassing algae and protozoa, with some species reaching up to 100 micrometers in diameter. Fungi are classified as microorganisms when they exist in their unicellular form, like yeast, or as microscopic filamentous structures known as molds.
Global Distribution and Survival
Microorganisms are ubiquitous, inhabiting every environment on Earth, from the atmosphere to the deep subsurface. They thrive in the air, soil, and water, with some scientists estimating that a significant portion of the planet’s total bacterial and archaeal biomass is concentrated deep within the Earth’s crust. Their adaptability allows them to form complex communities in virtually all ecological niches.
Extremophiles are organisms that flourish in conditions incompatible with most life. For instance, thermophiles and hyperthermophiles thrive in scorching environments like hot springs and deep-sea hydrothermal vents, where temperatures can exceed 100 degrees Celsius. They survive by possessing specialized enzymes, known as extremozymes, which maintain function despite the intense heat.
Other extremophiles include psychrophiles, which grow in sub-zero environments like Arctic ice, using proteins that act like a form of biological antifreeze to prevent ice crystal formation. Halophiles flourish in highly saline water bodies, such as the Dead Sea, by utilizing organic compounds called osmo-protectants to balance internal osmotic pressure and prevent cellular dehydration. Furthermore, some microbes possess robust DNA repair mechanisms that allow them to survive in environments with high levels of radiation.
Foundational Roles in Earth’s Ecosystems
Microbes are the primary drivers of global biogeochemical cycles, which continuously recycle the elements needed to sustain all life. Decomposition involves breaking down dead organic matter and waste products. Through enzymatic action, complex organic molecules like cellulose are converted back into simpler compounds, releasing carbon dioxide, water, and essential nutrients like nitrogen and phosphorus back into the environment.
The nitrogen cycle relies entirely on microbial action. Atmospheric nitrogen gas ($\text{N}_{2}$) is unusable by plants and animals. Certain bacteria, including species of cyanobacteria, perform nitrogen fixation, converting $\text{N}_{2}$ into ammonia ($\text{NH}_{3}$), which can then be assimilated by plants.
Microbes also govern the carbon and sulfur cycles, significantly influencing the planet’s climate and geochemistry. Photosynthetic cyanobacteria, for example, are abundant in the oceans and contribute substantially to global carbon fixation, converting carbon dioxide into organic matter and forming the base of many marine food webs. Recent discoveries have also revealed bacteria, known as MISO bacteria, that “breathe” iron minerals by oxidizing toxic sulfide in marine sediments, connecting the sulfur and iron cycles and helping to prevent the expansion of oxygen-depleted “dead zones.”
Direct Impact on Human Life
The human microbiome is the microbial community inhabiting the body, primarily in the gastrointestinal tract. These microbes collectively contribute millions of genes, extending the metabolic and functional capacity of the human host. The gut microbiota aids in the digestion of complex carbohydrates that human enzymes cannot break down and produces important metabolites, such as short-chain fatty acids and certain vitamins.
Beyond digestion, the microbiome plays a significant role in the development and maturation of the immune system. Disturbances in this microbial community have been associated with a range of conditions, including autoimmune disorders and chronic inflammatory diseases. There is also increasing evidence of a “gut-brain axis,” where microbial metabolites can influence neurological function and behavior.
Conversely, some microbes are pathogens, organisms that cause infectious disease by invading a host. Infections can be acquired exogenously from external sources like contaminated food or water, or endogenously from a person’s own microbiota when immune defenses are compromised. Understanding the mechanisms by which pathogens cause disease is fundamental to developing effective antimicrobial treatments.
Microbes are harnessed for technological and industrial applications. Fermentation, a process driven by yeasts and bacteria, is used to produce foods like yogurt, cheese, and bread, as well as alcoholic beverages. Many antibiotics, such as penicillin, were originally discovered as metabolic products of fungi and bacteria. The therapeutic transfer of healthy microbial communities, such as Fecal Microbiota Transplantation, is now used with high efficacy to treat recurrent infections caused by the bacterium Clostridioides difficile.