Biology 381
Pollution Biology
Department of Biological Sciences
University of Alberta
Edmonton, Alberta
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Biology 381 |
| 10. Metals in the environment. |
10.1 Required Reading and other announcements.
No required reading.
10.2 Sources of metal contamination.
The bulk of existing contaminants arise from from combustion of one form or another (smelting, power, transportation).
Because of their toxicological properties, metals are released intentionally as pesticides.
Application of sewage sludge or fertilizer amendments are important sources in agro-ecosystems.
A number of mining activities also give rise to contamination.
10.3 Plant growth in metal contaminated environments.
Species richness and species abundance are dramatically reduced by exposure to metals.
Metals in the environment can be directly toxic to plants.
Metal-contaminated soils are frequently deficient with respect to a number of essential elements such as N, P, and K.
Figure 16,17, Winterhalder (1984).
Low pH can be a factor limiting growth on metal-toxic soils.
Aluminum toxicity may arise if soil pH drops below 5.0.
Figure 21,22, Winterhalder (1984).
Reduced growth of roots can predispose plants to drought injury.
Taylor (unpublished results).
Impoverishment of vegetation can also alter microclimatic conditions.
10.4 Metal resistance.
Some species of plants are more resistance to specific metals than others.
Some plant races are more resistant to specific metals than others (metal-resistant cultivars, metal-resistant ecotypes).
Plants are metal resistant if they can grow in metal-contaminated sites where other plants are incapable of growing, or if they can grow unaffected in the presence of a concentration of a given metal that is toxic to other plants.
Differential response of wheat cultivars to varying aluminum.
Figure 1, Zhang and Taylor (1988).
Differential response of wheat cultivars to varying manganese.
Figure 2, Macfie et al. (1989).
Differential resistance of wheat cultivars to aluminum.
Figure 2, Taylor (1985).Our definition of resistance is a relative one. Resistance really represents a gradient of plant response ranging from susceptible to resistant.
10.5 Evolution of metal-resistant ecotypes.
Do metal-resistant plants belong to a species that is inherently resistant to metals, or have races evolved a special resistance not possessed by the remainder of the species?
Scientific evidence suggests that differences in resistance both between species and between races are typically the result of a process of natural selection (evolution) of metal resistant organisms in a metal-contaminated environment.
The growth of plants on metal-toxic soils provides an interesting view of evolution in action.
The most common technique used to determine if a species has inherent resistance (as opposed to having evolved resistance after exposure to a contaminated environment), is to grow plants collected from contaminated and uncontaminated environments, on both contaminated and uncontaminated substrates.
Example of inherent or constitutional resistance.
Soils collected from...
Plants collected from...
Uncontaminated site Contaminated site Uncontaminated 100 g 100 g Contaminated ? g 75 g Example of resistant ecotypes.
Soils collected from...
Plants collected from...
Uncontaminated site Contaminated site Uncontaminated 100 g 100 g Contaminated ? g 75 g Agrostis capillaris from two abandoned mine sites.
Parys Mountain - contaminated with copper (2060 ug g-1), but with low levels of lead and zinc.
Root length (cm) Resistance index (%) Growth media Parys Mt. Goginan Parys Mt. Goginan Control 82 61 100 100 Copper 56 25 68 41 Lead 33 72 40 118 Zinc 21 17 26 28
Evolution of lead tolerance in Plantago lanceolata along a roadside.
Differential resistance of Plantago lanceolata collected from sites from different sites near a roadway. Figure 1, Wu and Antonovics (1976).
Are there species with an inherent resistance to metals?
Soils collected from...
Plants collected from...
Control (uncontaminated) Sudbury (contaminated) Control Uncontaminated 0.29 + 0.07 g (a) 0.34 + 0.05 g (a) Sudbury Contaminated 0.14 + 0.03 g (b) 0.14 + 0.02 g (b) Letters in parentheses reflect means that are not significantly different.
10.6 "Heavy" Metals?
"Heavy" metals include those metals whose density is 5 g cm-3 or higher, although 4.0 g cm-3 has also been used as the cutoff.
Another definition is any element in the periodic table which is flanked by titanium (Ti), hafnium (Hf), arsenic (As), and bismuth (Bi), but also including selenium (Se) and Tellurium (Te).
Neiboer and Richardson (1980) proposed a classification scheme based upon the chemistry of complex formation.
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Three metal classes (Class A, Class B, and borderline) were proposed reflecting differences in biological activity and toxicity.
Table 2, Nieboer and Richardson (1980.
Class A metals typically form complexes through carboxylate, carbonyl, alcohol, phosphate, and phosphodiester groups.
Class B and borderline metals typically form complexes through sulphydryl, disulphide, thioether, and amino groups, and heterocyclic rings and nucleotide bases.
The biological effects of metals can generally be ascribed to one or more of the following factors.
10.7 Mercury, a Class B metal for the history books.
Mercury in its ionic form (Hg2+) is not an aggressive a toxin. However, it tends to form methylated derivatives.
Methyl mercury is the most toxic of the mercury species.
It tends to interact with SH and S-S groups. Since these groups are ubiquitous and are crucial to the integrity of proteins, this accounts for much of the toxicity of Hg.
Acute ecological effects include reproductive problems, lower fertility, and changes in appearance or behavior, and death.
Mercury from both natural and anthropogenic sources tends to bioaccumulate with increasing trophic levels.
10.8 Cadmium, a Borderline metal and one of the most toxic.
All soils and rocks, including coal and mineral fertilizers, contain some cadmium. It is most commonly associated with zinc, lead, and copper ores.
Cadmium is a by-product of zinc refining and is primarily used in nickel-cadmium batteries.Major anthropogenic sources include zinc mining and smelting operations, coal combustion, discharge from industrial facilities (i.e. steel manufacturing), sewage effluents, and leaching from landfills (batteries) and hazardous waste sites.
Sewage sludge and phosphate fertilizers have also contributed to increased levels of cadmium in agricultural soils.
Cadmium is a borderline metal with strong Class B tendencies. It competes with the more ambivalent zinc for binding sites (S, N) and thus interferes with some of zinc's essential functions as a enzyme co-factor.
Cadmium levels in humans tend to increase with age probably because of chronic exposure.
Cadmium causes bone and joint aches and pains. This symptoms were first described in Japan, as part of the itai-itai ("ouch-ouch") disease. The disease was associated with weak bones that lead to deformities, especially of the spine, and in severe cases, death.
10.9 Aluminum, a Class A metal, is everywhere.
10.10 Additional world wide web information.
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