Prokaryotic cells are found in virtually every environment on Earth, from deep ocean trenches to the upper atmosphere, while eukaryotic cells make up all animals, plants, fungi, and a wide range of single-celled organisms in nearly every habitat. The two cell types overlap in many environments, but prokaryotes dominate the extremes, and eukaryotes dominate in size and complexity.
The Three Domains of Life
All cellular life falls into three major domains. Two of them, Bacteria and Archaea, are prokaryotic, meaning their cells lack a nucleus and are structurally simpler. The third domain, Eukarya, includes every organism whose cells contain a membrane-bound nucleus: animals, plants, fungi, and a vast group of single-celled organisms often called protists. This classification, introduced by Carl Woese based on genetic sequencing, revealed that bacteria and archaea look similar under a microscope but are as fundamentally different from each other as either is from eukaryotes.
Where Prokaryotic Cells Live
Bacteria are arguably the most widespread organisms on Earth. They saturate soil, freshwater, and ocean water. They coat surfaces in your home, blanket the seafloor, and drift through the atmosphere attached to dust particles and marine organic matter. Research at high-altitude observatories in Spain has shown that tens of millions of bacteria are deposited per square meter per day above the atmospheric boundary layer, carried across continents by wind before settling back down with rain or dust.
Below the surface, prokaryotes extend deep into Earth’s crust. Microbial communities have been found in groundwater 2.8 kilometers underground in South African gold mines, and samples from boreholes 4 to 5 kilometers deep have turned up living bacteria. Temperature increases by roughly 25°C for every kilometer of depth, which sets a practical limit: no known microorganism could survive below about 5 kilometers, where temperatures become too extreme even for extremophiles.
Archaea are especially prominent in the harshest environments. In hydrothermal vents on the ocean floor, bacteria grow well up to about 68°C, but above 90°C, archaea take over. One species, found near deep-sea vents, can grow at 122°C under the intense pressure that keeps water liquid at those temperatures. Archaea also thrive in highly acidic hot springs, hypersaline lakes, and the deepest ocean trenches, where pressure exceeds 1,000 atmospheres. These pressure-loving organisms (piezophiles) have been found at the bottom of the Mariana Trench, nearly 11,000 meters below the surface.
But prokaryotes aren’t limited to exotic places. The ocean’s surface teems with photosynthetic cyanobacteria like Prochlorococcus and Synechococcus, tiny cells less than a micrometer across. These are among the most abundant organisms in the sea and produce a significant share of the planet’s oxygen.
Where Eukaryotic Cells Live
Eukaryotic organisms cover the planet’s land and water in forms ranging from microscopic algae to blue whales. Animals occupy every continent and nearly every aquatic habitat. Plants dominate terrestrial ecosystems as the primary producers, using photosynthesis to convert sunlight into energy. Fungi are everywhere organic matter exists: in soil, on decaying wood, inside living plants as symbionts, and even in the air as spores. One honey mushroom colony in Oregon’s Malheur National Forest spans 9.6 square kilometers, making it one of the largest known organisms.
Single-celled eukaryotes are just as widespread. In freshwater, photosynthetic algae account for nearly half of the microbial community. In soil and marine environments, consumer protists (organisms that feed on bacteria or other cells) dominate instead. In the open ocean, eukaryotes tend to be larger than prokaryotes. Marine prokaryotes are mostly picoplankton-sized, with cell diameters under 1 micrometer, while eukaryotes dominate the nanoplankton and microplankton size classes. Even in the smallest size range, photosynthetic picoeukaryotes can make up 11 to 70% of the tiny photosynthetic community by mass.
Prokaryotes Living Inside Eukaryotes
One of the most remarkable places prokaryotic cells are “found” is inside eukaryotic cells themselves. Mitochondria, the structures that generate energy in nearly all eukaryotic cells, descended from an ancient bacterium that was engulfed by a host cell roughly two billion years ago. Chloroplasts, which carry out photosynthesis in plant and algae cells, originated from cyanobacteria captured in a similar event. These organelles still carry their own small genomes, a remnant of their free-living bacterial ancestors.
This means every plant cell contains structures that were once independent prokaryotes, and every animal cell depends on mitochondria that trace back to a bacterial symbiont. The line between “where prokaryotes are found” and “where eukaryotes are found” is, at the molecular level, blurred inside every complex cell on the planet.
The Human Body as a Shared Habitat
Your own body is a striking example of coexistence. A standard 70-kilogram adult carries roughly 30 trillion human cells and about 38 trillion bacterial cells, giving a ratio of approximately 1.3 bacterial cells for every human cell. That’s far lower than the old estimate of 10 bacteria for every human cell, which persisted in textbooks for decades before being revised in 2016. The total mass of all those bacteria is only about 0.2 kilograms, since individual bacterial cells are so much smaller than human cells.
Most of these prokaryotes live in your gut, but they also colonize your skin, mouth, respiratory tract, and urinary tract. Your human cells, all eukaryotic, form the tissues and organs. So within one body, both cell types coexist in enormous numbers, occupying different niches just as they do across the rest of the planet.
Overlapping and Distinct Ranges
In moderate environments like forests, lakes, agricultural soil, and ocean surfaces, prokaryotic and eukaryotic cells are found side by side, often depending on each other. Bacteria break down organic matter that feeds plants. Fungi form networks around tree roots to exchange nutrients. Protists graze on bacteria, keeping their populations in check.
The gap appears at the extremes. Above about 60°C, eukaryotic life drops off sharply, but prokaryotes persist well beyond 100°C in pressurized environments. At the bottom of the ocean, in deep rock fractures, and in highly acidic or highly alkaline water, prokaryotes are often the only cellular life present. A few eukaryotes do tolerate acid, including certain algae and yeasts found in acid mine drainage, but their range is narrow compared to the archaea that flourish there.
In short, eukaryotic cells are found wherever you see visible life: every animal, plant, fungus, and the countless protists in water and soil. Prokaryotic cells are found in all of those places too, plus they extend into environments no eukaryote can tolerate, from boiling hydrothermal vents to rock kilometers underground to dust particles drifting through the upper atmosphere.