Trace element
A trace element is a chemical element that occurs in minute quantities, typically at concentrations of less than 100 parts per million (ppm) or 0.01% of the total composition, in living organisms, soils, rocks, waters, and other natural materials.[1][2] These elements function primarily as cofactors in enzymatic reactions and metabolic processes, with some being indispensable for life while others can be harmful in excess.[3] In human nutrition and biology, trace elements are classified as essential, probably essential, or non-essential based on their roles in health. Essential trace elements, required in daily amounts of 1–100 mg or less (comprising less than 0.01% of body weight), include iron, zinc, copper, cobalt, selenium, iodine, manganese, molybdenum, chromium, and fluoride.[4][5][6] Iron facilitates oxygen transport in hemoglobin, zinc supports immune function and DNA synthesis, and selenium acts as an antioxidant in selenoproteins.[3] Iodine is vital for thyroid hormone production, while deficiencies in these elements can cause severe disorders such as anemia (from iron deficiency), goiter (from iodine lack), or impaired growth and reproduction (from zinc shortfall).[3][7] Non-essential trace elements, such as lead, cadmium, mercury, and arsenic, occur naturally or through environmental exposure but lack proven biological necessity and can be toxic even at low levels, leading to neurological damage, kidney dysfunction, or cancer.[3] Probably essential elements like silicon contribute to bone health and connective tissue formation, though their requirements are less well-defined.[8] Humans obtain trace elements mainly through diet—from sources like seafood (for iodine and selenium), red meat (for iron and zinc), and grains—supplemented by water and soil-derived uptake in plants.[3] Balancing intake is critical, as both deficiency and toxicity pose public health risks, particularly in regions with poor soil quality or industrial pollution.[4] In geochemistry and environmental science, trace elements serve as indicators of geological processes, pollution levels, and ecosystem health, with their low concentrations analyzed using techniques like atomic absorption spectroscopy to assess bioavailability and cycling in the environment.[1]Fundamentals
Definition and Characteristics
Trace elements are chemical elements that occur in minute quantities, typically at concentrations less than 0.01% by weight (or less than 100 mg/kg) in biological tissues, or up to 0.1% (1000 mg/kg) in geochemical contexts such as soils, rocks, or environmental matrices like water.[9][3] These low levels distinguish them from more abundant components, and they encompass both essential and non-essential types, with essential ones required for physiological processes.[3] Key characteristics of trace elements include their atomic properties, where many belong to the transition metals (e.g., exhibiting variable oxidation states and d-block electron configurations) or metalloids (e.g., displaying intermediate metallic and non-metallic behaviors).[10] Their solubility in aqueous environments is influenced by chemical speciation, with factors such as soil or water pH playing a critical role—acidity often increases solubility for metals like iron and zinc, while alkalinity promotes precipitation.[11] Bioavailability, or the fraction available for uptake by organisms, is further modulated by interactions with organic matter, which can form chelates that enhance or inhibit dissolution depending on the ligand type.[12] In contrast to macroelements (also called major elements), which are required in concentrations exceeding 1000 mg/kg in plant tissues (e.g., nitrogen, phosphorus, potassium), trace elements are needed below 100 mg/kg in biological systems and typically serve catalytic rather than structural roles in biochemical systems.[3] This quantitative threshold highlights their specialized functions, as macroelements form bulk components like proteins or cell walls, whereas trace elements participate in trace-level reactions. Common examples of trace elements include iron, zinc, copper, iodine, and selenium.[3]Historical Context
The recognition of trace elements in biological systems began in the 19th century with early chemical analyses of minerals in animal nutrition. Swedish chemist Jöns Jacob Berzelius identified iron in blood in the early 1800s, establishing its presence in vital tissues and laying groundwork for understanding mineral roles in physiology.[13] Subsequent observations in the latter half of the century highlighted the importance of trace minerals for growth in animals, as researchers detected small quantities of elements like copper and zinc in biological samples, shifting focus from macronutrients to these minor components.[14] Key breakthroughs in the early 20th century confirmed the essentiality of specific trace elements. In the 1910s, American physician David Marine conducted pioneering trials demonstrating iodine's role in preventing goiter, with large-scale supplementation studies in schoolchildren from 1916 to 1917 showing dramatic reductions in thyroid enlargement.[15] By the 1930s and 1940s, British biochemists David Keilin and Thaddeus Mann discovered the enzyme carbonic anhydrase in erythrocytes in 1939 and its zinc component in 1940, contributing to the understanding of zinc's biochemical role; zinc's dietary essentiality had been demonstrated in animals earlier in the decade.[16][17] Advancements in the mid-20th century expanded the trace element paradigm in both nutrition and environmental contexts. In 1959, researchers Klaus Schwarz and Walter Mertz at the U.S. Department of Agriculture identified chromium as an essential factor for glucose metabolism in rats, marking a significant step in recognizing ultratrace elements' roles.[18] Concurrently, the 1956 outbreak of Minamata disease in Japan, caused by methylmercury contamination from industrial wastewater, underscored the toxic potential of trace elements in the environment, prompting global awareness of bioaccumulation risks.[19] During this period, terminology evolved from earlier terms like "microelements," used in plant nutrition contexts, to "trace elements" as the standard in animal and human studies by the mid-20th century, reflecting refined understanding of their low concentrations and biological impacts.[20] This historical progression informed modern classifications distinguishing essential from non-essential trace elements.Classification
Essential Trace Elements
Essential trace elements are chemical elements required by living organisms in very small amounts to support fundamental biological processes such as growth, reproduction, and metabolism. The criteria for determining essentiality were formalized by Arnon and Stout in 1939, stating that an element qualifies as essential if it is necessary for completing the organism's life cycle, cannot be replaced by another element, and is directly involved in nutrition or metabolism.[21] These criteria, originally developed in the context of plant nutrition, ensuring that only elements with irreplaceable functional roles are classified as essential.[3] In biological systems, the primary essential trace elements include iron, zinc, copper, manganese, iodine, selenium, molybdenum, chromium, and cobalt, each fulfilling specific roles that cannot be substituted. Iron is crucial for oxygen transport in hemoglobin and myoglobin, as well as for electron transfer in cytochromes and other redox reactions.[3] Zinc serves as a cofactor for over 300 enzymes involved in DNA synthesis, protein folding, and immune function, stabilizing protein structures and participating in catalytic sites.[3] Copper facilitates electron transport in cytochrome c oxidase, a key enzyme in the mitochondrial respiratory chain, and supports iron metabolism through ceruloplasmin.[3] Manganese acts as a cofactor in enzymes like superoxide dismutase for antioxidant defense and, in plants, is vital for the oxygen-evolving complex in photosystem II during photosynthesis.[3] Iodine is indispensable for the synthesis of thyroid hormones thyroxine and triiodothyronine, which regulate metabolism and development.[3] Selenium incorporates into selenoproteins such as glutathione peroxidase, which neutralizes reactive oxygen species to protect cells from oxidative damage.[3] Molybdenum functions as a cofactor in enzymes like xanthine oxidase for purine catabolism and, in plants and microbes, nitrogenase for nitrogen fixation.[3] Chromium enhances insulin action to improve glucose uptake and metabolism, though its essentiality remains under some debate in certain contexts.[3] Cobalt, primarily as a component of vitamin B12 (cobalamin), is essential for methylmalonyl-CoA mutase in fatty acid metabolism and methionine synthase in one-carbon transfers.[22] Biochemically, these elements integrate into metabolic pathways through specific mechanisms that underscore their indispensability. For instance, iron's incorporation into heme groups during heme synthesis enables its role in oxygen binding and transport, a process mediated by enzymes like ferrochelatase.[3] Zinc's tetrahedral coordination in enzyme active sites, such as in carbonic anhydrase for CO2 hydration, exemplifies its catalytic versatility across hydrolytic, redox, and transfer reactions.[3] Copper's redox cycling between Cu(I) and Cu(II) states in cytochrome c oxidase drives proton pumping for ATP production, while in plants, manganese clusters in photosystem II facilitate water oxidation to produce oxygen.[3] Iodine's iodination of tyrosine residues in thyroglobulin forms the hormonal precursors, and selenium's selenocysteine residue in glutathione peroxidase catalyzes the reduction of hydrogen peroxide.[3] Molybdenum's pterin-based cofactors enable electron transfer in nitrogenase for N2 reduction to ammonia, and chromium's coordination with oligopeptides may stabilize insulin-receptor interactions to potentiate glucose transport.[3] Cobalt in vitamin B12 undergoes homolytic cleavage to generate radicals for isomerase activity in metabolic rearrangements.[22] These mechanisms highlight how trace elements enable precise biochemical control, with deficiencies disrupting core physiological functions across organisms.Non-essential Trace Elements
Non-essential trace elements are chemical elements present in biological systems at low concentrations but without an established nutritional requirement for growth, reproduction, or maintenance of health in humans or most organisms.[3] Unlike essential trace elements, they lack specific biochemical roles and dedicated transport or metabolic pathways, often entering cells incidentally through similarity to essential ions or environmental exposure.[23] Common examples include aluminum (Al), cadmium (Cd), lead (Pb), arsenic (As), and mercury (Hg), which can exert pharmacological effects at low doses or toxicity at higher levels.[24] These elements can be categorized as potentially beneficial or as ultra-trace disruptors. Potentially beneficial non-essential elements, such as vanadium, may mimic physiological processes without being required; for instance, vanadium compounds act as insulin mimetics by activating glucose uptake and glycogen synthesis pathways in mammalian cells, potentially aiding diabetes management.[25] Similarly, fluoride at low concentrations strengthens tooth enamel by promoting remineralization, though it is not vital for systemic metabolism and becomes toxic in excess.[26] In contrast, ultra-trace disruptors like arsenic, lead, and cadmium interfere with essential element functions; arsenic (as arsenate) competes with phosphate in energy metabolism, forming unstable analogs that uncouple oxidative phosphorylation and disrupt ATP production.[27] Specific interactions highlight their disruptive potential. Lead ions substitute for calcium in bone hydroxyapatite and signaling pathways, leading to impaired mineralization and altered cellular responses due to similar ionic radii and charge.[28] Cadmium, accumulated via dietary or environmental routes, binds to sulfhydryl groups in proteins, mimicking zinc or calcium in enzymes and contributing to severe osteomalacia in conditions like Itai-itai disease, where chronic exposure causes bone pain and deformities through renal tubular damage.[29] Mercury and aluminum similarly disrupt enzyme activity and membrane integrity, amplifying toxicity in combination with other metals.[30] Research on non-essential trace elements continues to evolve, with ongoing debates about borderline cases like nickel, which is essential in microbial urease enzymes but lacks confirmed roles in human physiology, though deficiency studies suggest possible influences on growth and reproduction.[31] These elements underscore the importance of exposure limits, as their incidental biological impacts range from subtle modulation to profound interference.[32]Biological Roles
In Human and Animal Physiology
Trace elements play critical roles in human and animal physiology, primarily as cofactors in enzymatic reactions, structural components, and signaling molecules. In humans, iron is essential for erythropoiesis, where it serves as the central component of heme in hemoglobin, enabling oxygen transport in red blood cells. Iron is also a key constituent of myoglobin, facilitating oxygen storage and release in muscle tissues. Zinc supports immune function by regulating intracellular signaling pathways in innate and adaptive immune cells, including T-cell activation and cytokine production. Additionally, zinc is vital for DNA synthesis, acting as a cofactor for enzymes like DNA and RNA polymerases that ensure accurate replication and repair. Copper contributes to connective tissue formation through its role in lysyl oxidase, a copper-dependent enzyme that catalyzes the oxidative deamination of lysine and hydroxylysine residues in collagen and elastin, promoting cross-linking and tissue stability. In animals, trace element requirements can vary significantly across species due to differences in metabolism and microbial interactions. For instance, ruminants such as cattle exhibit higher manganese needs, with requirements reaching 40 ppm in pregnant and lactating cows to support rumen microbial activity, which aids in fiber digestion and overall nutrient utilization. These microbes rely on manganese for superoxide dismutase activity, protecting against oxidative stress in the anaerobic rumen environment. Homeostatic regulation of trace elements involves specialized absorption and transport mechanisms to maintain optimal levels. In the human intestine, iron absorption occurs primarily via the divalent metal transporter 1 (DMT1), a proton-coupled symporter on the apical membrane of enterocytes that facilitates ferrous iron (Fe²⁺) uptake from the lumen. Once absorbed, iron binds to transferrin in the plasma for systemic distribution to tissues, particularly the bone marrow for hemoglobin synthesis. Copper homeostasis similarly depends on enterocyte transporters, with subsequent binding to ceruloplasmin in the blood; this ferroxidase oxidizes ferrous iron to ferric iron (Fe³⁺) for transferrin loading while transporting about 95% of circulating copper to target organs like the liver. Interactions among trace elements can influence their bioavailability and physiological effects. High zinc intake, for example, induces metallothionein expression in enterocytes, which preferentially binds copper and reduces its absorption, leading to potential copper deficiency. This antagonism highlights the need for balanced trace element intake to prevent disruptions in metabolic pathways.In Plant and Microbial Systems
Trace elements play crucial roles in plant and microbial systems, often differing from their functions in animals by supporting processes like photosynthesis, cell wall structure, and symbiotic nitrogen fixation rather than primarily vertebrate metabolism. In plants, these elements are absorbed from soil and integrated into enzymatic complexes essential for growth and nutrient assimilation, while in microorganisms, they enable anaerobic metabolisms and enzyme catalysis vital for environmental nutrient cycling. Deficiencies can impair these systems, leading to reduced productivity in agriculture and ecosystems. In plants, manganese is integral to the oxygen-evolving complex within photosystem II, where it facilitates water oxidation during photosynthesis by cycling through oxidation states in a Mn4Ca cluster.[33] Boron maintains cell wall integrity by cross-linking rhamnogalacturonan-II (RG-II) pectic polysaccharides, enhancing mechanical strength and supporting turgor-driven cell expansion.[34] Molybdenum serves as a cofactor in nitrate reductase, enabling the reduction of nitrate to nitrite as the first step in nitrogen assimilation, which is critical for protein synthesis and overall plant growth.[35] Microorganisms rely on specific trace elements for key metabolic enzymes. Cobalt is essential in methanogenic archaea, where it forms the core of corrinoid cofactors like coenzyme M methyltransferase, facilitating methyl group transfer in the methanogenesis pathway from substrates such as methanol or acetate.[36] Nickel is required as a catalytic center in bacterial urease, which hydrolyzes urea to ammonia and carbon dioxide, aiding nitrogen recycling in soil bacteria and contributing to pH regulation in microbial environments.[37] Uptake and transport mechanisms ensure efficient acquisition of trace elements in these systems. In plants, zinc is absorbed by root cells through ZIP family transporters, which facilitate influx across plasma membranes in response to soil availability and pH conditions.[38] Iron plays a symbiotic role in legume-rhizobia interactions, where it is supplied by the host plant to bacteroids for nitrogenase activity, an enzyme complex containing iron-sulfur clusters essential for converting atmospheric nitrogen to ammonia.[39] Agriculturally, trace element management enhances crop yields by addressing deficiencies. Chelated iron fertilizers, such as Fe-EDDHA, are applied to calcareous soils to prevent chlorosis in crops like soybeans and fruit trees, improving iron availability and chlorophyll synthesis without rapid precipitation.[40] Similar micronutrient amendments for manganese, boron, and molybdenum support nitrogen assimilation and photosynthesis in deficient regions, boosting overall productivity.[41]Environmental and Geochemical Distribution
Occurrence in Soils and Water
Trace elements occur naturally in soils primarily through the weathering of parent rocks and subsequent pedogenic processes. The concentrations of these elements vary widely depending on the geological substrate; for instance, soils derived from mafic rocks tend to have higher levels of elements like chromium and nickel, while those from sedimentary rocks may show elevated cadmium and lead. Weathering intensity influences release rates, with chemical weathering in humid climates mobilizing more soluble trace elements into the soil matrix. Soil pH plays a critical role in their speciation and availability, as acidic conditions (pH < 5.5) enhance the solubility of metals like aluminum and cadmium, whereas neutral to alkaline pH favors precipitation and adsorption onto clay minerals or iron oxides.[42][43][44] Global geochemical surveys provide baseline concentrations for common trace elements in uncontaminated soils, typically expressed in milligrams per kilogram (mg/kg). According to data from the U.S. Geological Survey's analysis of surficial materials across the conterminous United States, representative geometric mean averages include arsenic at 7.2 mg/kg, cadmium at 0.2 mg/kg, copper at 25 mg/kg, lead at 19 mg/kg, and zinc at 60 mg/kg.[45][46] These values align with broader international databases, where zinc often ranges from 50 to 100 mg/kg and cadmium remains below 1 mg/kg in most non-anthropogenically influenced profiles. Volcanic soils, however, can exhibit elevated levels due to ash deposition, with mercury concentrations sometimes exceeding 0.1 mg/kg in regions like the Pacific Ring of Fire.[47][48]| Trace Element | Average Concentration (mg/kg) | Source |
|---|---|---|
| Arsenic | 7.2 | USGS PP 1270[45] |
| Cadmium | 0.2 | USGS SIR 2017-5118[46] |
| Copper | 25 | USGS PP 1270[45] |
| Lead | 19 | USGS PP 1270[45] |
| Zinc | 60 | USGS PP 1270[45] |