Diatoms live in virtually every body of water on Earth, from open oceans to puddles, and even in moist soil that never qualifies as a true water body. These single-celled algae are one of the most widespread organisms on the planet, generating roughly 20 percent of the oxygen produced on Earth each year. Their range extends from tropical seas to polar ice, from mountain streams to hot spring margins, and from lake bottoms to the thin film of water coating forest moss.
Oceans and Coastal Waters
The ocean is where diatoms are most abundant. They float in the sunlit upper layers of the water column as part of the phytoplankton, and they’re especially concentrated in nutrient-rich coastal ecosystems, upwelling zones (where deep, cold water rises to the surface), and high-latitude waters like the Southern Ocean surrounding Antarctica. Coastal waters consistently support the highest densities. As you move from shore into the open ocean, diatom numbers drop significantly, though they remain present across all ocean basins.
Different species prefer different depths. Some thrive near the surface, while others concentrate at the deep chlorophyll maximum, a layer typically 50 to 150 meters down where light is dimmer but nutrients are more available. Seasonal blooms can temporarily make diatoms dominant even in the open ocean. The North Atlantic spring bloom is one of the most dramatic examples, turning vast stretches of water greenish-brown as diatom populations explode.
Freshwater Lakes, Rivers, and Ponds
Diatoms are among the most abundant and diverse photosynthetic organisms in freshwater. They colonize lakes of all sizes, from deep temperate reservoirs to shallow Arctic ponds less than a meter deep. Rivers and streams host their own communities, often distinct from those in still water. Studies in the Arctic have found that pond and lake habitats support different diatom assemblages than streams do, with dozens of species unique to each type of environment. In one survey of a remote Arctic archipelago, researchers identified 72 species found only in still-water habitats and 42 found only in flowing water.
Water chemistry plays a major role in determining which species show up. Conductivity, pH, and temperature are the strongest drivers of freshwater diatom communities. Nutrient levels, particularly nitrate and nitrite, also shape which species dominate. This sensitivity to water chemistry is so reliable that scientists routinely use diatom species as biological indicators of water quality. The mix of species in a stream sample can reveal pollution levels, acidity changes, and nutrient loading more accurately than a single snapshot of water chemistry.
On Surfaces: Rocks, Plants, and Sediment
Not all diatoms drift freely. A large portion of species are benthic, meaning they live attached to surfaces rather than floating in open water. These diatoms coat submerged rocks (forming the slippery brown film you’ve felt when stepping on river stones), cling to aquatic plants, or live in the top layer of lake and ocean sediment. Some attach using mucilage stalks or pads, while others glide slowly across surfaces. The benthic zone offers stability compared to the open water column, with less fluctuation in temperature, salinity, and light.
Inside Polar Sea Ice
One of the most remarkable diatom habitats is sea ice itself. Polar ice caps cover up to 13 percent of the ocean surface and contain networks of tiny brine channels, essentially veins of salty liquid running through the ice. Diatoms live inside these channels, forming visible brown layers within ice cores. Despite permanent darkness for parts of the year, subfreezing temperatures, and extreme salinity swings, ice diatoms remain active.
Research from a 2023 expedition in the Chukchi Sea found that Arctic pennate diatoms can actively glide on ice surfaces and within ice channels at temperatures as low as negative 15°C. They achieve this through specialized adaptations, including changes to their cell membranes that keep them fluid in extreme cold and metabolic adjustments that maintain energy production. These diatoms appear to select specific depths within ice cores where the balance of light, nutrients, and salinity is most favorable. When ice becomes permeable above negative 5°C, they can migrate through the brine network, spreading to new locations and seeding under-ice blooms.
Soil, Moss, and Wet Rock
Diatoms don’t require a true water body. Numerous species survive and reproduce in terrestrial environments, including soils, mosses, wet rock faces, and cave walls. All they need is a thin film of moisture. Moss wetlands host particularly diverse diatom communities, with certain genera found almost exclusively in these habitats. These terrestrial diatoms tend to be smaller and more tolerant of drying than their aquatic relatives, though they still depend on periodic wetting to remain active.
Hot Springs and Extreme Environments
Diatoms generally thrive between 10°C and 45°C, but they’ve been documented near geothermal features around the world. Hot springs in Russia, Thailand, India, and Yellowstone National Park have all yielded diatom samples. However, the relationship is more nuanced than it first appears. Research at the Puga hot spring in Ladakh, India, where water temperatures reach 84°C at the vent, found that the diatoms present were sparse and low in diversity. The evidence suggests these diatoms weren’t actually growing in the scalding water. Instead, they were thriving in the cooler margins and temperature gradients surrounding the vents, where conditions drop closer to their tolerable range. Above roughly 55°C, diatom abundance drops sharply.
Why Silica Controls Their Range
One factor limits diatoms that doesn’t affect most other algae: they build their cell walls out of glass. Each diatom constructs an intricate shell from dissolved silica in the surrounding water, and without enough silica, they can’t divide. In regions where silica runs low, diatoms have a workaround. Off the coast of California, researchers found that diatom populations in upwelling zones build thinner glass walls when silica becomes scarce, sacrificing structural protection to keep dividing. Thinner walls make them more vulnerable to being eaten, but the strategy allows a larger total population to persist on a limited pulse of silica. This tradeoff helps explain why diatoms dominate in upwelling zones where nutrients arrive in unpredictable bursts.
Fossilized Deposits in Rock and Sediment
Because diatom shells are glass, they persist long after the organism dies. Over millions of years, accumulated diatom shells form thick deposits of diatomaceous earth, a chalky sedimentary rock. In the United States, three main types of deposit exist: marine deposits that formed near ancient continental margins, freshwater deposits from prehistoric lakes and marshes, and sediment still accumulating in modern lakes and bogs.
The largest U.S. deposits are in the western states. Lompoc, California, holds the single largest source, formed from ancient marine diatoms. Nevada, Oregon, Washington, and eastern California contain major freshwater deposits from ancient lakes. Smaller, currently uneconomic deposits exist in Florida, New Hampshire, New York, Maryland, and Virginia. Outside the U.S., significant deposits are mined underground in Chile, China, and France. In Iceland, diatomaceous mud is dredged from the bottom of Lake Mývatn, where diatoms continue to accumulate today.