Cobalt Tolerance by Plants

Algae

Stonewort (Chara vulgaris L.) resistant to metal pollution, when cultivated in a natural medium containing CoCl₂ showed high level of cobalt in dry matter as insoluble compounds (120). On the other hand, a copper-tolerant population of a marine brown alga (Ectocarpus siliculosus Lyng.) had an increased tolerance to cobalt. The copper-tolerance mechanism of other physiological processes may be the basis of this cotolerance (121).

Fungi

A genetically stable cobalt-resistant strain, CoR, of Neurospora crassa Shear & Dodge, exhibited an approximately ten-fold higher resistance to CO₂⁺ than the parent strain. The CO₂⁺ toxicity was reversed by Mg²⁺, but not by Fe³⁺, indicating that the CO₂⁺did not affect iron metabolism. Alternatively, the mechanism of resistance probably involves an alteration in the pattern of iron metabolism so that the toxic concentration of cobalt could not affect the process (122). Magnesium (Mg²⁺) may reverse the toxicity of CO₂⁺, either by increasing the tolerance to high intracellular concentration of heavy metal ions or by controlling the process of uptake and accumulation of ions (123). In several mutants of Aspergillus niger growing in toxic concentrations of Zn²⁺, CO₂⁺, Ba²⁺, Ni ²⁺, Fe³⁺, Sn²⁺, and Mn²⁺, the resistance is due to an intracellular detoxification rather than defective transport. Each mutation was due to a single gene located in its corresponding linkage group.

Toxicity of metals is reversed in the wild-type strain by definite amounts of K⁺, NH⁴⁺, Mg²⁺, and Ca²⁺. These competitions between pairs of cations indicate a general system responsible for the transport of cations (124). In Aspergillus fumigatus, cobalt increased thermophily at 45°C and fungal tolerance at 55°C (125).

Higher Plants

In higher plants, cobalt tolerance has been mainly reported in members of ‘advanced’ families such as the Labiatae and Scrophulariaceae growing in the copper-field belt of Shaba (Zaire) (126). Among these plants, Haumaniastrum robertii, a copper-tolerant species, is also a cobalt-accumulating plant. The plant contains abnormally high cobalt (about 4304 µg g^-1 dry weight), far exceeding the concentration of copper. This species has the highest cobalt content of any phanerogam (127). Haumaniastrum katangense and H. robertii grow on substrates containing 0 to 10,000 µg Co g^-1. Although they can accumulate high concentrations of cobalt, an exclusion mechanism operates in these species at lower concentrations of the element in the soil. Uptake of cobalt was not linked to a physiological requirement of the element. The plant–soil relationship for Co was significantly high enough for these species to be useful in the biogeochemical prospecting for cobalt (128). Tolerance and accumulation of copper and cobalt were investigated in three members of phylogenetic series of taxa within the genus Silene (Caryophyllaceae) from Zaire, which were regarded as representing a progression of increasing adaptation to metalliferous soils. Effects of both metals (singly and in combination) on seed germination, seedling and plant performances, yield, and metal uptake from soil culture confirmed the ecotypic status of S. burchelli, which is a more tolerant variant of the nontolerant S. burchelli var. angustifolia. But both the ecotype and metallophyte variants of S. cobalticola are relatively more tolerant to copper than to cobalt.