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Trace Elements in Abiotic and Biotic Environments
as it is the main precursor of vitamin B 12 . Critical Co levels in ruminant diets vary
from 80 to 100 μg/kg. Deficiency of Co may be controlled by the application of Co
salt to soils, and in most cases, the effect of that treatment may last for several years.
However, there are various opinions, and often it is advised to apply Co salts directly
to livestock.
Excess Co in soils is transported mainly to plant leaves. Co toxicities are white
leaves margins and tips, especially of young leaves; several enzymatic processes
of plants may be inhibited. Some plants grown in soils with 25–50  mg Co/kg,
and/or with 140 mg Co/L in soil solution, suffer from excess Co. Plant sensitivity to increased Co contents is various. Commonly reported critical Co levels in
plants are 30,000–40,000 μg/kg. Cereals are known to be the most sensitive plants
to excess Co and may tolerate it at 10,000–20,000 μg/kg. Some plants may develop
a mechanism of Co tolerance, which is observed mainly in metalliferous plant species. Especially Nyssa sylvatica grown in Co-contaminated soil may accumulate
this metal up to 800,000 μg/kg. It is recommended for the phythoextraction of Co
and 60 Co, as well as for the remediation of contaminated soils (Malik et al. 2000).
Among various native plants grown in the polluted area of Northern Europe (Kola
Peninsula), crowberry (Empetrum nigrum) has the highest capacity to accumulate
Co, at the pollution-background ratio of 206 (Reimann et al. 2001).
The mushroom, common chanterelles (Cantharellus cibarius) grown in mountains contained lower amounts of Co, mean 390  μg/kg, than those grown in the
Baltic Sea coast, mean 100 μg/kg (Falandysz et al. 2012).
13.6 HUMANS
Cobalt has been identified in most tissues of the body, with the highest concentrations in the liver. Total body burden is estimated at 1.1–1.5 mg, of which 85% are in
the form of vitamin B 12 , with 0.11 mg in the liver (WHO 2006b). Concentrations in
body fluids are well below the μg/L level; mean concentrations reported in the serum
range from 0.1 to 0.3 μg/L. Considerable differences have been found in the levels of
Co in hair, ranging from 0.4 to 500 μg/kg; 0.05–2.7 μg/L in the blood; 0.1–0.6 μg/L
in the plasma; and 0.1–1.5 μg/L in the urine (Catalani et al. 2011).
Approximately 50% of the Co that enters the gastrointestinal tract will be
absorbed. Cobalt absorption is increased among individuals who are Fe deficient. Its
water-soluble forms are better absorbed than insoluble forms (WHO 2006b).
The only known essential role of Co in animals and humans is being a component
of vitamin B 12 as Co 3+ . Absorbed Co 2+ is not known to have any biological function (EFSA 2012b). In animals with the capacity to synthesize cyanocobalamin, Co
from orally administered Co 2+ is deposited in tissues, in the form of vitamin B 12 .
As a component of vitamin B 12 , Co is essential in the body; therefore, it is found
in the most tissues. Vitamin B 12 is a water-soluble vitamin, with a key role in the
normal functioning of the brain and nervous system, and for the formation of blood.
Vitamin B 12 occurs in foods of animal origin and represents only a small fraction of
Co intake. The recommended dietary allowance (RDA) of vitamin B 12 is 2.4 μg/
day, which contains 0.1 μg of Co (WHO 2004).