Distribution

Microorganisms and Lower Plants

Algae

Cobalt is essential for many microorganisms including cyanobacteria (blue-green algae). It forms part of cobalamin, a component of several enzymes in nitrogen-fixing microorganisms, whether freeliving or in symbiosis. It is required for symbiotic nitrogen fixation by the root nodule bacteria of legumes (3). Soybeans grown with 0.1 µg L^-1 cobalt with atmospheric nitrogen and no mineral nitrogen showed rapid nitrogen fixation and growth (4). Cobalt is distributed widely in algae, including microalgae, Chlorella, Spirulina, Cytseira barbera, and Ascophyllum nodosum. Alginates, such as fucoiden, in the cell wall play an important role in binding cobalt in the cell-wall structure (5,6).

Bioaccumulation of heavy metals in aquatic macrophytes growing in streams and ponds around slag dumps has led to high levels of cobalt (7). Certain marine species such as diatoms (Septifer virgatus Wiegman) and brown algae Sargassum horneri (Turner) and S. thunbergii (Kuntze) from the Japanese coast act as bioindicators of cobalt (8). Accumulation has been shown to be controlled by salinity of the medium with bladder wrack (brown alga, Fucus vesiculosus L.) (9).

The cell walls of plants, including those of algae, have the capacity to bind metals at negatively charged sites. The wild type of Chlamydomonas reinhardtii Dangeard, owing to the presence of its cell wall, was more tolerant to metals such as cobalt, copper, cadmium, and nickel, than the wallless variant (10). When exposed to metals, singly in solutions for 24 h, cells of both strains accumulated the metals. Absorbed metals not removed by chelation with EDTA–CaC12 wash were considered strongly bound. Cobalt and nickel were present in significantly higher amounts loosely bound to the walled organism than in the wall-less ones. It was concluded that metal ions were affected by the chelating molecules in walled algae, which limited the capacity of the metal to penetrate the cell. Thus, algae appear to contain a complex mechanism involving internal and external detoxification of metal ions (10).

In a flow-through wetland treatment system to treat coal combustion leachates from an electrical power system using cattails (Typha latifolia L.), cobalt and nickel in water decreased by an average of 39 and 47% in the first year and 98 and 63% in the second year, respectively. Plants took up 0.19% of the cobalt salts per year. Submerged Chara (a freshwater microalga), however, took up 2.75% of the salts, and considerably higher concentrations of metals were associated with cattail roots than shoots (11).

Fungi

In fungi, cobalt accumulates by two processes. The essential process is a metabolically independent one presumably involving the cell surface. Accumulation may reach 400 mg g^-1 of yeast and is rapid in Neurospora crassa Shear & BO Dodge (12,13).

In the next step, which is metabolism dependent, progressive uptake of large amounts of cations takes place. Two potassium ions are released for each CO₂⁺ ion taken up in freshly prepared yeastcell suspensions. The CO₂⁺ appears to accumulate via a cation-uptake system. Its uptake is specifically related to the ionic radius of the cation (14). Accumulated cobalt is transported (at the rate of 40 µg h^-1 100mg^-1 dry weight of N. crassa) mainly into the intercellular space and vacuoles (13,15). Acidity and temperature of media are factors involved in CO₂⁺ uptake and transport. In N. crassa, Mg²⁺ inhibits CO₂⁺ uptake and transport, suggesting that the processes of the two cations are interrelated. In yeast cells exposed to elevated concentrations of cobalt, uptake is suppressed, and intercellular distribution is altered (15).

Yeast mitochondria passively accumulate CO₂⁺ in levels linearly proportional to its concentration in the medium. The density of mitochondria is slightly increased and their appearance is altered, based on observations with electron microscopy (16). The more dense mitochondria are exchanged by hyphal fusion in the fully compatible common A and common AB matings of tetrapolar basidiomycetes Schizophyllum commune Fries, but not in the common B matings (17). Toxicity and the barrier effect of the cell wall inhibit surface binding of CO₂⁺. As a result, isolated protoplasts from yeast-like cells of hyphae and chlamydospores of Aureobasidium pollulans were more sensitive to intracellular cobalt uptake than intact cells and chlamydospores (18).

Moss

The absorption and retention of heavy metals in the woodland moss Hylocomium splendens Hedw followed the order of Cu, Pb>Ni>Co>Zn, and Mn within a wide range of concentrations and was independent of the addition of the ions (19).

Higher Plants

Cobalt is not known to be definitely essential for higher plants. Vitamin B₁₂ is neither produced nor absorbed by higher plants. It is synthesized by soil bacteria, intestinal microbes, and algae. In naturally cobalt-rich areas, cobalt accumulates in plants in a species-specific manner. Plants such as astragalus (Astragalus spp. L.) may accumulate from 2 or 3 to 100 mg kg^-1 dried plant mass. Cobalt occurs in a high concentration in the style and stigma of Lilium longifolium Thunb. It was not detected in the flowers of green beans (Phaseolus sativus L.) and radishes (Raphanus sativus L.) though the leaves of the latter contain it. It was shown to occur in high amounts in leafy plants such as lettuce (Lactuca sativa L.), cabbage (Brassica oleracea var. capitata L.), and spinach (Spinacea oleracea L.) (above 0.6 ppm) by Kloke (20). Forage plants contain 0.6 to 3.5 mg Co kg^-1 and cereals 2.2 mg kg^-1 (21). Rice (Oryza sativa L.) contains 0.02 to 0.150 mg kg^-1 plant mass (22).

Cobalt chloride markedly increases elongation of etiolated pea stems when supplied with indole acetic acid (IAA) and sucrose, but elongation is inhibited by cobalt acetate. Cobalt in the form of vitamin B₁₂ is necessary for the growth of excised tumor tissue from spruce (Picea glaucaVoss.) cultured in vitro. It increases the apparent rate of synthesis of peroxides and prevents the peroxidative destruction of IAA. It counteracts the inhibition by dinitrophenol (DNP) in oxidative phosphorylation and reduces activity of ATPase and is known to be an activator of plant enzymes such as carboxylases and peptidases (4). The CO₂⁺ ion is also an inhibitor of the ethylene biosynthesis pathway, blocking the conversion of 1-amino-cyclopropane-1-carboxylic acid (ACC) (23).