Selenium (Se) is a naturally occurring trace element whose presence in the environment is highly variable, carrying significant implications for life on Earth. Its distribution creates geographical regions of both abundance and scarcity. Selenium’s behavior and mobility across different environmental compartments are dictated by its four distinct oxidation states. Understanding where selenium is found begins with the rocks that make up the planet’s crust.
Geological Origin and Soil Reservoirs
The primary, static reservoir for selenium is the Earth’s crust, where it is globally distributed at an average concentration of approximately 0.09 parts per million (ppm). The concentration in the soil, which is the direct source for the terrestrial food web, is determined almost entirely by the underlying parent rock material. This geological history establishes the baseline for local selenium levels, which vary dramatically across the globe.
Much of the planet’s selenium is locked within sedimentary rocks, particularly ancient black shales and phosphate-rich formations. These geological layers, formed from the deposition of organic matter, often act as selenium “hotspots” that can yield soils with elevated concentrations, sometimes exceeding 7.0 milligrams per kilogram (mg/kg). Conversely, soils derived from weathered igneous rocks, such as granite, or young, sandy sediments are low in selenium content, creating vast areas of scarcity.
The global range of selenium in surface soils generally falls between 0.01 and 2.0 mg/kg, with most areas averaging between 0.2 and 0.4 mg/kg. These geological variations lead to distinct geographical patterns. For example, the Great Plains of the United States are known for high-selenium soils derived from Cretaceous shales. In contrast, large parts of Northern Europe, like Finland and Scotland, have low-selenium soils due to their acidic parent materials.
Movement Through Water and Atmosphere
Once selenium is released from the bedrock or soil through weathering, it begins a journey through the hydrosphere, where its mobility is governed by its chemical form. While four oxidation states exist, the most common and biologically relevant in water are selenite (+4) and selenate (+6). The local environment’s acidity (pH) and oxidation-reduction (redox) conditions determine which form predominates and how far it can travel.
Selenate is highly soluble in water and is not readily bound to soil particles, making it easily leached into groundwater, rivers, and streams. This high mobility makes selenate the dominant species in well-aerated, neutral to alkaline soils and surface waters. In contrast, selenite is far less mobile because it strongly adsorbs onto the surfaces of iron and aluminum oxides, effectively immobilizing it in the soil and sediment.
The atmosphere provides a secondary, globally important pathway for selenium distribution. Natural emissions primarily come from volcanic activity, which releases significant amounts of the element, often in volatile forms. For instance, a single active volcano can emit tons of selenium compounds annually, which are then dispersed through the air before settling back onto the land and water surfaces.
Natural Incorporation into Biological Systems
The final stage of selenium’s natural distribution involves its entry into living organisms, beginning with plant uptake from the soil. Plants act as the bridge, converting inorganic selenium forms (selenite and selenate) into organic compounds like selenomethionine and selenocysteine, which are then transferred up the food chain. Plant species can be categorized based on their ability to accumulate the element, which is directly linked to the selenium content of the local soil.
Non-accumulator plants, which include most common crops and grasses, contain less than 100 milligrams of selenium per kilogram of dry weight. These plants absorb selenium, often via the same transporters used for sulfur, but they are not specialized for high-level accumulation and can suffer toxicity if soil levels are too high. In contrast, hyperaccumulator plants, such as certain species of Astragalus (milk vetch), can accumulate high concentrations, sometimes exceeding 1,000 mg/kg dry weight without showing signs of poisoning.
This difference in plant accumulation directly impacts animal populations, especially grazing livestock. In high-selenium regions, animals consuming hyperaccumulator or even secondary accumulator plants can suffer from selenium poisoning. Conversely, in geographically deficient areas, the low concentration in local forage can lead to nutritional deficiencies in livestock and, subsequently, in humans who rely on locally grown food.