In managing the living part of soil, micro-organisms are
possible the most relevant consideration. Soil micro-organisms are in charge
for the highest percentage of recycling nutrients available in the soil. The
life processes of these microbes are also controlled by these metals (eg.,
calcium, chromium, cobalt, iron, copper, magnesium, potassium, sodium,
manganese, zinc and nickel). These metals are important source of trace
nutrients and are used for oxidation and reduction processes within these
organisms. The microbes conversion of metals serve various functions and can be
categorized into two main divisions:

·     
transformations
from inorganic to organic form and vice versa and

·     
redox
transformations of inorganic forms (Turpeinen,
2002).

Microorganisms obtain energy from the oxidation of sulphur,
iron, arsenic and manganese (Santini et
al., 2000). However, metals reduction can occur by dissimilatory reduction
which involves microbes using metals as terminal electron acceptors during
anaerobic respiration (Turpeinen, 2002).
For instance, in microbial anaerobic respiration, chromium oxyanions are used
as terminal electron acceptors. Microbes can also have reduction mechanisms
which are not related to respiration but are rather known to impart resistance
to metals (Turpeinen, 2002). A
typical example is the aerobic and anaerobic reduction of Cr5+ to Cr3+.

 

Microbial processes either dissolve or solubilize metals
increasing their potential toxicity and bioavailability or immobilize them
reducing bioavailability and toxicity of the metals. The term redox conditions
in microbial systems means the microbial terminal electron accepting processes
that usually take place in the microorganism. Thus, in the presence of oxygen,
aerobic conditions dorminate and metabolism in microbes occurs with oxygen as
the terminal electron acceptor. Other species that can be used as electron
acceptors to generate energy for growth and maintenance are oxides and
hydroxides of manganese (IV), nitrates, carbon dioxide and sulphates. Heavy
metals usually come in contact with organic contaminants in polluted sites. Under
anaerobic soil conditions, redox potential show a negative correlation with
microbial activity. Low redox potential was measured for increased soil
moisture due to the complete or partial displacement of oxygen from soil and
the rapid utilization of oxygen by microbes. Microbial activity in aerobic
soils was mainly affected by redox potential. In arable soils, the moisture
content indirectly reduce the redox potential by increasing the microbial
activity (Volk, 1993). In other
studies, a link between redox potential and moisture content of the soil has
been established.

 

2.8 Metabolic impacts of
heavy metals on animals

 

There
are so many different metabolic implications that heavy metals have on animals.
The most important impacts are;

·     
Immune system degradation

·     
Enzyme inhibition

·     
Organic specific degradation and

·     
Neuron signal interference.

The capability of some
these heavy metals to imitate other essential metals in enzymatic processes is
one of the fundamental issues in cell metabolic interference. For instance,
zinc or calcium in enzymatic processes is displaced often by lead which can
result in minor impacts like reduction in fitness or even deadly consequence
when very high doses are exchanged. Hexavalent chromium for example can
cause organ failure for example can cause organ failure for doses as low as 50
ug/kg. The strong oxidative effect of this metal is responsible for its high
toxicity. When hexavalent chromium is transported via the bloodstream, it
impairs important organs like the kidney, liver and blood cells by oxidation,
and causing complete shutdown of liver and renal organs when in high
concentrations. Cadmium can cause lung damage, acute liver and renal failure,
cause pneumonitis and pulmonary edema in mammals.

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