confirmed. definitive evidence for efficacy comes from the characterization of a biomolecule containing the element with an identifiable and testable function. one problem with identifying efficacy is that some elements are innocuous at low concentrations and are pervasive (examples: silicon and nickel in solid and dust), so proof of efficacy is lacking because deficiencies are difficult to reproduce. ultratrace elements of some minerals such as silicon and boron are known to have a role but the exact biochemical nature is unknown, and others such as arsenic are suspected to have a role in health, but with weaker evidence. element description excess bromine possibly important to basement membrane architecture and tissue development, as a needed catalyst to make collagen iv. bromism arsenic essential in rat, hamster, goat and chicken models, but no research has been done in humans. arsenic poisoning nickel nickel is an essential component of several enzymes, including urease and hydrogenase. although not required by humans, some are thought to be required by gut bacteria, such as urease required by some varieties of bifidobacterium. in humans, nickel may be a cofactor or structural component of certain metalloenzymes involved in hydrolysis, redox reactions and gene expression. nickel deficiency depressed growth in goats, pigs, and sheep, and diminished circulating thyroid hormone concentration in rats. nickel toxicity fluorine fluorine (as fluoride) is not considered an essential element because humans do not require it for growth or to sustain life. research indicates that the primary dental benefit from fluoride occurs at the surface from topical exposure. of the minerals in this table, fluoride is the only one for which the u.s. institute of medicine has established an adequate intake. fluoride poisoning boron boron is an essential plant nutrient, required primarily for maintaining the integrity of cell walls. boron has been shown to be essential to complete the life cycle in representatives of all kingdoms of life. in animals, supplemental boron has been shown to reduce calcium excretion and activate vitamin d. no acute effects (ld50 of boric acid is 2.5 grams per kilogram body weight) chronic effects of long term high dose boron exposure are not fully elucidated lithium based on plasma lithium concentrations, biological activity and epidemiological observations, there is evidence, not conclusive, that lithium is an essential nutrient. lithium toxicity strontium strontium has been found to be involved in the use of calcium in the body. it has promoting action on calcium uptake into bone at moderate dietary strontium levels, but a rachitogenic (rickets-producing) action at higher dietary levels. certain forms of rickets other silicon and vanadium have established, albeit specialized, biochemical roles as structural or functional cofactors in other organisms, and are possibly, even probably, used by mammals (including humans). by contrast, tungsten, the early lanthanides, and cadmium have specialized biochemical uses in certain lower organisms, but these elements appear not to be used by mammals. other elements considered to be possibly essential include aluminium, germanium, lead, rubidium, and tin. multiple mineral ecology minerals can be bioengineered by bacteria which act on metals to catalyze mineral dissolution and precipitation. mineral nutrients are recycled by bacteria distributed throughout soils, oceans, freshwater, groundwater, and glacier meltwater systems worldwide. bacteria absorb dissolved organic matter containing minerals as they scavenge phytoplankton blooms. mineral nutrients cycle through this marine food chain, from bacteria and phytoplankton to flagellates and zooplankton, which are then eaten by other marine life. in terrestrial ecosystems, fungi have similar roles as bacteria, mobilizing minerals from matter inaccessible by other organisms, then transporting the acquired nutrients to local ecosystems.