Where Is Selenium Found in Nature?

Selenium turns up across nearly every compartment of the natural world, from sulfide minerals buried deep in the Earth’s crust to the surface waters of the open ocean, volcanic gas plumes, forest soils, and the tissues of living organisms from bacteria to fish. Its average crustal abundance is low, roughly 0.05 parts per million, but certain geological formations concentrate it to levels hundreds of times higher. Understanding where selenium sits in nature matters because tiny shifts in its availability ripple through ecosystems, agriculture, and human nutrition in ways that are easy to underestimate.

Rocks, Minerals, and the Deep Earth

Selenium rarely forms minerals of its own. Instead, it hitches a ride inside sulfide ores, substituting for sulfur atoms in the crystal lattice of minerals like galena (lead sulfide), pyrite (iron sulfide), and sphalerite (zinc sulfide). Measurements of sulfide ores from deposits in Greece, for instance, found selenium concentrations reaching over 500 parts per million in fine-grained galena, while pyrite samples from a nearby deposit contained around 33 ppm.¹ Those numbers dwarf the crustal average and illustrate how strongly selenium concentrates wherever sulfide mineralization occurs.

Black shales and coal are another major reservoir. Organic-rich sedimentary rocks deposited under oxygen-poor marine conditions tend to lock up selenium alongside sulfur, and the resulting concentrations in black shales, coal, and petroleum run roughly ten to twenty times the crustal average.² Carboniferous shales in Britain and Ireland show this pattern clearly: Bowland Shale samples averaged about 21.5 ppm selenium, compared with a global mean shale value of just 0.6 ppm, and the selenium correlates strongly with sulfur content because it substitutes into sulfide minerals within the rock.³ When these rocks are mined, burned for energy, or weathered at the surface, the stored selenium mobilizes into water and soil, which is a recurring theme in selenium’s environmental story.

Hydrothermal Vents on the Ocean Floor

The deep sea floor offers another concentrated source. At mid-ocean ridges, superheated water circulates through fractured basalt, leaching metals and trace elements before venting back into the ocean. Sulfide chimneys that form around these vents can be strikingly rich in selenium. At the Lucky Strike hydrothermal field on the Mid-Atlantic Ridge, chalcopyrite (a copper-iron sulfide) contained selenium at concentrations ranging from about 213 to 1,640 ppm, reflecting a mix of sulfur leached from basaltic rock and sulfur derived from reduced seawater sulfate.⁴ These vent systems inject selenium into the deep ocean, where it enters the broader marine cycle.

Volcanoes and the Atmosphere

Selenium is volatile compared with most trace elements, and volcanoes are one of the main natural routes by which it enters the atmosphere. Magma chambers concentrate selenium through volatilization, and the gases emitted during eruptions and passive degassing carry it into the air. The resulting fallout enriches nearby soils to levels one to two orders of magnitude above the worldwide soil average of about 0.4 milligrams per kilogram.⁵ Though these volcanic soils often contain a lot of selenium, the element tends to be tightly bound and not very mobile, so high total concentration does not always translate into high availability for plants or water.

Beyond local deposition, volcanic emissions also feed the global atmospheric selenium budget. Once airborne, selenium compounds can travel long distances before settling onto land or ocean surfaces. This atmospheric pathway helps explain why selenium shows up even in soils far from any obvious geological source.

Soils and What Controls Their Selenium Content

Soil selenium varies enormously from one region to the next, and two neighboring fields can have very different levels. A global modeling effort that drew on more than 33,000 soil data points concluded that soil selenium concentrations are dominated by climate-soil interactions rather than parent rock alone.⁶ That finding was somewhat surprising, because earlier, smaller studies had emphasized bedrock geology as the main driver.

The form selenium takes in soil matters as much as the total amount. Selenium cycles through several chemical species in soil, and the balance among them is governed by pH, moisture, oxygen availability, organic matter content, and microbial activity.⁷ Under acidic, waterlogged conditions, selenium tends to be locked into insoluble forms that plants cannot access. In well-drained, alkaline soils, it shifts into more mobile, oxidized forms that move readily into groundwater and plant roots.⁸ This is why some regions with modest total selenium in the soil still produce crops with plenty of it, while others with higher totals end up selenium-deficient in agricultural terms.

The same study that modeled global soil selenium also estimated that up to one in seven people worldwide have low dietary selenium intake, a problem concentrated in regions where soil chemistry keeps selenium unavailable to crops.⁶ Climate change is projected to shift these patterns, potentially widening deficiency zones as rainfall and temperature distributions change.

Selenium in the Ocean

The ocean holds selenium in three dissolved forms: selenite, selenate, and organic selenide. Their distribution follows a nutrient-like profile, meaning they are depleted in surface waters where biology uses them up and enriched at depth where sinking organic matter decomposes and releases them back. Early work in the Pacific documented that organic selenide accounts for roughly 80 percent of total dissolved selenium in surface waters, with the maximum coinciding with peaks in primary productivity.⁹ In deeper waters, the balance reverses: organic selenide becomes undetectable, while selenite and selenate dominate.

Atlantic measurements show a similar vertical structure, with surface depletion and deep enrichment of inorganic selenium. Interestingly, the ratio of selenite to selenate varies with water mass origin: waters sourced from the southern hemisphere, like Antarctic Intermediate Water, are relatively enriched in selenate and carry lower selenite-to-selenate ratios than North Atlantic Deep Water.¹⁰ These differences track the age and chemical history of each water mass.

The first global three-dimensional ocean selenium model identified phytoplankton uptake as the dominant force shaping selenium’s horizontal and vertical distribution. One surprise was that the strongest depletion of inorganic selenium occurred in oligotrophic (nutrient-poor) regions dominated by cyanobacteria, organisms previously thought not to need selenium at all.¹¹ The model estimated a residence time of just over half a year for selenite in the sunlit zone, fast enough that local depletion can drive phytoplankton to switch to using selenate instead. Because phytoplankton community composition responds to temperature and ocean stratification, a warming ocean could reshape selenium availability throughout marine food webs.

How Selenium Moves Through Aquatic Food Chains

Plankton are the first link in the chain that delivers selenium from water to the rest of the food web. In the southern Baltic Sea, measured selenium concentrations in phytoplankton averaged about 1.1 micrograms per gram and in zooplankton about 1.25 micrograms per gram, both substantially higher than the surrounding water concentration of about 0.25 micrograms per liter.¹² This concentration step from water into the base of the food web is the critical gateway.

Freshwater systems show even more dramatic bioaccumulation. A cooling reservoir for a coal-fired power plant in North Carolina received selenium-laden ash effluent, and organisms in the lake accumulated the element anywhere from roughly 500 times (in algal films) to nearly 4,000 times (in fish visceral tissue) above the waterborne concentration.¹³ Similar patterns were measured in a coal power station reservoir in Australia, where fish liver selenium levels reached over 100 micrograms per gram, despite water concentrations of only about 2 micrograms per liter. At those food-chain selenium levels, some fish species suffered birth defects and reproductive failure, while others appeared more resistant.¹⁴

Even after the source of contamination is removed, selenium lingers. A follow-up survey of one such reservoir found that sediment selenium concentrations had dropped by about 65 to 75 percent, but enough remained in the sediment, at 1 to 4 micrograms per gram, to continue contaminating the food organisms of fish and waterbirds for years.¹⁵ This persistence makes selenium a particularly tricky freshwater pollutant: it bioaccumulates efficiently and does not flush out of sediments quickly.

Microbes as Selenium Transformers

Bacteria play an outsized role in selenium’s environmental cycling. Some species reduce dissolved selenate and selenite to elemental selenium, effectively pulling it out of solution and locking it into a solid, less bioavailable form. A bacterium isolated from estuarine sediments was shown to use selenate as an energy source in place of oxygen or sulfate, reducing it to elemental selenium while oxidizing organic carbon. Researchers concluded that this type of dissimilatory selenate reduction is the major removal pathway for selenium oxyanions in oxygen-poor sediments.¹⁶

Other bacteria carry out the same transformation under different conditions. A strain of the common soil and water bacterium Pseudomonas stutzeri was found to rapidly reduce both selenite and selenate to elemental selenium at high concentrations.¹⁷ These microbial processes are not just biogeochemical curiosities. They directly determine how much selenium stays dissolved and available in waterways, soils, and sediments, and they have practical implications for cleaning up contaminated sites.

Plants, Crops, and the Dietary Connection

For most people, the main route of selenium intake is through plant-based foods, especially cereal grains.¹⁸ Plants absorb selenium from the soil primarily through the same transport systems they use for sulfur, and the amount that ends up in the grain depends heavily on what form of selenium is available in the root zone. Whether selenium is actually essential for plants themselves remains an open question; it is clearly essential for animals and humans, and plants serve as the main conduit, but researchers have not definitively shown that plants need it for their own survival.¹⁹

Rice is a particularly important case because it is the staple food for more than half the world’s population. Increasing the selenium content of rice grains through soil amendments or crop breeding is seen as a practical way to boost dietary selenium intake in deficient regions.¹⁸ Finland pioneered a version of this approach decades ago by adding selenium to fertilizers, and several other countries have followed.

Brazil Nuts and Wild Mushrooms

Brazil nuts are famous for their selenium content, and the reputation is well earned, though the actual levels vary widely. A study of Brazil nuts and soils across the Amazon found that selenium accumulation in the nuts generally increased in soils with higher total selenium, but decreased under acidic soil conditions. Soil acidity appears to limit how much selenium the trees can take up, likely because it affects how tightly the element is bound in the soil.²⁰ This means two bags of Brazil nuts from different parts of the Amazon can differ dramatically in their selenium content, and eating a handful of particularly rich ones can push you well past the daily recommended intake.

Wild mushrooms are the other standout natural selenium source that most people do not think about. A comprehensive review of published data found that the Goat’s Foot mushroom averages around 200 micrograms of selenium per gram of dry weight, with maximums approaching 370 micrograms per gram, making it the richest wild source documented. Among more commonly eaten species, the King Bolete averages about 20 micrograms per gram dry weight.²¹ When mushrooms are cultivated on substrates enriched with selenium, they can accumulate even higher concentrations. Selenium-enriched button mushrooms, for example, have been grown with up to 110 micrograms per gram dry weight. Some of this selenium ends up in the form of nanoparticles of elemental selenium inside the mushroom tissue, with particle sizes typically ranging from about 60 to 250 nanometers.²²

Mining, Fossil Fuels, and Human Redistribution

Human activity does not create selenium, but it moves enormous amounts of it from geological reservoirs where it was locked away into soils, water, and air where it becomes biologically active. Coal combustion releases selenium from ancient organic-rich rocks into fly ash and flue gases. Phosphate mining does something similar: in the Western Phosphate Resource Area of the United States, past reclamation practices left weathering waste rock exposed at the surface, promoting selenium release and accumulation in local soils.²³ In both cases, the issue is not that the selenium is new but that it has been moved from deep storage into the active surface environment where organisms encounter it.

Agricultural irrigation in selenium-rich areas creates a different kind of redistribution. In California’s San Joaquin Valley, for example, irrigation drainage concentrated selenium to levels that caused severe wildlife poisoning at the Kesterson National Wildlife Refuge in the 1980s. That episode put selenium contamination on the regulatory map and spurred decades of research into how to manage selenium in agricultural drainage.

Phytoremediation and Wetland Volatilization

The same properties that make selenium move so readily through food chains also offer cleanup opportunities. Plants can absorb selenium from contaminated soil and water, and some convert the inorganic forms into volatile organic compounds, essentially dimethyl selenide, that disperse harmlessly into the atmosphere. This plant-based approach, broadly called phytoremediation, has been studied as a low-cost alternative to engineered treatment systems.²⁴

Constructed wetlands take advantage of both biological volatilization and microbial reduction. In one field trial treating selenium-laden agricultural drainage water, a wetland cell planted with rabbitfoot grass volatilized about 9.4 percent of incoming selenium over a two-year period, with seasonal peaks reaching nearly half of the selenium input during late spring.²⁵ That seasonal swing reflects the temperature and light dependence of the microbial and plant processes involved.

Genetic engineering has pushed phytoremediation further. Transgenic Indian mustard plants overexpressing genes involved in sulfur metabolism accumulated up to about four times more selenium in their leaves than wild-type plants, and some lines tolerated the contaminated soil considerably better.²⁶ Whether transgenic phytoremediation sees wide deployment is still an open question, but the field trial results show the concept works outside the lab.

An Evolutionary Trace Element

Selenium’s distribution in nature is not just a geological and chemical story. Life has depended on it for a very long time, and the evolutionary record shows it. Selenoproteins, proteins that incorporate the amino acid selenocysteine, appear in organisms from bacteria to humans. A comparative study of animal genomes found that even the most primitive animals possess a range and variety of selenoproteins similar to what humans carry, suggesting that the basic selenoproteome was established early in animal evolution and has been mostly conserved since.²⁷

There is, however, a striking split between aquatic and terrestrial life. Across eukaryotes, aquatic organisms tend to retain and sometimes expand their selenoproteomes, while some terrestrial lineages have drastically reduced or even completely lost theirs. Nematodes and insects, for example, underwent massive selenoprotein losses independently in different evolutionary branches.²⁸ Vertebrates are the major exception to this terrestrial-loss trend, hanging onto their selenoproteins despite living on land. The environmental factor that drives selenium use down in terrestrial invertebrates remains unknown, but the pattern suggests that the ocean, where dissolved selenium is constantly recycled by plankton and bacteria, provides a more reliable supply than most terrestrial habitats do. In a sense, the geography of selenium in nature has shaped the molecular toolkit of the organisms living in it.