Diatoms live in nearly every wet environment on Earth, from open oceans and freshwater lakes to soil surfaces, polar sea ice, and even industrial water systems. These single-celled algae are among the most widespread organisms on the planet, responsible for about 20% of all global photosynthesis and producing roughly one in every five breaths of oxygen you take.
Oceans and Freshwater
The two largest diatom habitats are marine and freshwater environments, and the species found in each are distinctly different. Marine diatoms tend to be dramatically larger, spanning almost nine orders of magnitude in cell volume. The biggest ocean species can be a thousand times the volume of the largest freshwater diatoms. This size gap isn’t random. In the ocean, nitrogen is often the limiting nutrient, and its pulsed supply favors the evolution of very large cells that can store reserves. In lakes and rivers, phosphorus limitation and shallower water columns push optimal cell sizes downward, resulting in smaller diatoms with a narrower range of sizes.
Despite these differences, diatoms dominate primary production in both realms. In the ocean alone, they account for roughly 40% of all marine photosynthesis. Freshwater diatoms are equally important to their ecosystems, forming the base of food webs in rivers, streams, ponds, and lakes worldwide.
Floating vs. Attached to Surfaces
Within any body of water, diatoms occupy two very different niches. Planktonic diatoms drift in the sunlit upper waters, where they’re the most studied group. But benthic diatoms, the ones living on sediment, rocks, and other submerged surfaces, are just as productive. Research in the Bay of Brest found that bottom-dwelling diatoms can generate as much total biomass as their floating counterparts. Benthic species are also far more diverse, dominated by elongated, boat-shaped forms rather than the round, disc-like shapes common among planktonic species.
These two groups also operate on different seasonal schedules. Benthic diatoms are typically the first to bloom each year, making up about 60% of total diatom biomass through early spring. They provide food and energy to coastal ecosystems before planktonic species take over later in the season. Benthic diatoms are also remarkably adapted to dim conditions, thriving at light levels that would be too low for most planktonic species. Some can even migrate into sediment and back out again, positioning themselves where conditions are best.
From Surface Waters to the Deep Ocean
Diatoms need light to photosynthesize, so their active growth is concentrated in the upper sunlit layer of the ocean, typically the top 200 meters. But healthy, intact diatom cells have been found far deeper than anyone expected. Researchers have recovered living photosynthetic diatoms at depths down to 4,000 meters in the deep ocean, carried there by sinking particles and ocean currents. These cells aren’t actively growing in the dark, but their presence at such depths confirms that diatoms play a major role in transporting carbon from surface waters to the deep sea floor, a process central to the ocean’s carbon cycle.
Polar Sea Ice
Some of the most extreme diatom habitats are in the Arctic and Antarctic, where specialized species live inside and on the surface of sea ice. These ice diatoms are the primary producers in polar ecosystems before the spring bloom begins in open water, making them a critical food source for everything from tiny zooplankton to fish and seals.
Ice diatoms have developed remarkable adaptations for survival. They can remain active and mobile at temperatures as low as minus 15°C, a record for movement in any complex cell. They secrete specialized proteins and sticky substances that prevent ice crystals from damaging their cells and help them grip ice surfaces. In lab tests, ice diatoms withstood shear forces roughly 90 times greater than what it took to detach related temperate species from ice. They also glide along ice surfaces, something their warm-water relatives simply cannot do, by producing a modified mucilage with unusual adhesive properties.
These diatoms navigate through tiny brine channels that form within sea ice, which become passable at temperatures above minus 5°C. As spring warming increases ice porosity, the diatoms spread through the ice and may seed blooms beneath it, jumpstarting the polar food web before the ice fully melts.
Soil, Moss, and Rock
Diatoms are not strictly aquatic. Many species survive and reproduce in terrestrial environments including soil surfaces, mosses, wet rock faces, and walls. Scanning electron microscopy of soil samples reveals diatoms as a visible part of the microbial community at the soil surface, living alongside bacteria and cyanobacteria. Researchers have counted roughly 7,500 diatom cells per square centimeter of topsoil, making them surprisingly abundant even in thin surface layers.
These terrestrial diatoms need moisture but not standing water. Morning dew, rain, and the water held within moss cushions or rock crevices is enough. They contribute to soil nutrient cycling and form part of a complex microbial community that supports broader ecosystem health.
Why Silica Controls Where Diatoms Thrive
Unlike most algae, diatoms build their cell walls from silica, the same compound found in glass and sand. This means their distribution is tied not just to light and typical nutrients like nitrogen and phosphorus, but also to dissolved silica concentrations in the water around them. In every ocean region studied so far, silica availability periodically or chronically limits diatom growth.
Off the California coast, dissolved silica drops from over 21 micromoles per liter near shore to just 2 micromoles offshore, and diatom populations shift accordingly. When silica runs low, diatoms have a clever workaround: they build thinner cell walls, reducing their silica content by three to four times without significantly slowing their growth rate. This flexibility lets them keep dividing at near-maximum speed until silica drops below about 1 micromole per liter. Below 0.5 micromoles, though, growth stalls and community structure shifts toward other types of algae.
Coastal upwelling zones, where deep nutrient-rich water rises to the surface, tend to be diatom hotspots precisely because they deliver fresh supplies of silica along with nitrogen and phosphorus. The open ocean’s central gyres, by contrast, are silica-poor and support far fewer diatoms.
Industrial and Engineered Systems
Diatoms also colonize man-made environments wherever water and light intersect. Cooling towers at power plants, wastewater treatment facilities, and agricultural drainage systems all host diatom populations. In cooling tower water, diatom species like those in the genera Navicula and Denticula form biofilms that actually help remove dissolved silica, phosphorus, and nitrate from the water. One experimental biofilm reactor using diatoms removed nearly 67% of reactive silica and 83% of colloidal silica from power plant blowdown water, while also reducing phosphorus by over 90% and nitrate by nearly 70%.
Engineers are now exploring diatom-based treatment systems as a low-impact alternative to chemical water treatment. A projected system for a 500-megawatt power plant could save over 1.4 billion liters of water annually while producing 57 tonnes of silica-rich diatom shells, along with harvestable lipids and proteins, turning a waste stream into usable materials.