INTERACTION BETWEEN
ELEMENTS AND
TERRESTRIAL/MARINE SYSTEM
Chemical Interactions
It is rare that organisms in the environment
are exposed to a single toxicant. Typically,
multiple toxicants are present, and can
interact in a variety of ways.
The United States Environmental Protection Agency has
one of the more detailed and precise definitions of toxic
interaction, designed to facilitate risk assessment.[15]
In their guidance documents, the no-interaction default
assumption is dose addition, so synergy means a mixture
response that exceeds that predicted from dose addition.
The EPA emphasizes that synergy does not always make a
mixture dangerous, nor does antagonism always make
the mixture safe; each depends on the predicted risk
under dose addition.
Classification of Chemical Interactions:
1. Additive:
2. Synergistic:
3. Potentiation:
4. Antagonism:
1. Additive:
Combined effect of chemicals is equal to the sum
of each chemical's effect alone (4+4=8).
AKA, Independent Joint Action
Most commonly observed effect is additive
egs., combinations of chemicals in same class
(organophosphate insecticides, PAHs, narcotics).
2. Synergistic:
Combined effect of chemicals is greater than the sum
of each chemical's effect alone (4+4=20).
AKA, Super additive
egs.
carbon tetrachloride + ethanol
ethanol + smoking
Dispositional effects:
enhance uptake (eg., ETOH + oral medications)
3. Potentiation:
Toxicity of one chemical is increased in the presence of
another, non-toxic chemical (0+4=10).
egs.
isopropanol + carbon tetrachloride
croton oil + DMBA
4. Antagonism:
Combined effect of chemicals is less than the sum of
each chemical's effect alone.
Chemicals can interfere with each other or one can
interfere with the other (4+4=5; 4+ (-4)=0; 4+0=1).
Functional
produce opposite effects
caffeine + alcohol
Chemical
"inactivation"
chemical reaction produces less toxic compound
chelators (EDTA)
antitoxins
Dispositional
alter disposition of compound (uptake, elimination,
biotransformation)
inhibit uptake (milk, activated carbon, oils)
induction of enzyme systems
Receptor
"blockers"
block or compete for receptors
eg., atropine blocks cholinergic receptor
Examples from terrestrial system
There are basically two kinds of interactions between
nutrients. SYNERGISM is a positive effect between
nutrients and ANTAGONISM is a negative effect
between nutrients.
Two or more elements working together to create an
overall improved physiological state in the plant is called
physiological synergism
while, excess of one nutrient reducing the uptake of
another nutrient is called physiological antagonism.
These interactions depend on soil type, physical
properties, pH, ambient temperatures and
proportion of participating nutrients. There is a
highly controlled selectivity process involved in
uptake of nutrients by plants and that is the reason
why the plant does not contain the same ratio of
nutrients inside the plant as found
in the soil.
Synergism
Many soil scientists, plant physiologists and plant
biochemists have tried to clarify the much complicated
relationships between nutrients.
Some of these relationships are straight forward but,
most are not. A few examples from agricultural
laboratory research and field based experiments have
shown us
that an:
· Optimum supply of nitrogen ensures optimum uptake of
potassium as well as phosphorus, magnesium, iron, manganese
and zinc from the soils.
· Optimal levels of copper and boron improve nitrogen uptake
by plant.
· Optimal levels of molybdenum improve utilization of
nitrogen as well as increases uptake of phosphorus.
· Optimal levels of calcium and zinc improve uptake of
phosphorus and potassium.
· Optimal levels of sulphur increases the uptake of manganese
and zinc.
· Optimal levels of manganese increases uptake of copper.
Antagonism
· Excessive amounts of nitrogen reduce the uptake of
phosphorus, potassium, iron and almost all secondary and
micronutrients like calcium and magnesium iron, manganese,
zinc and copper.
· Excessive amounts of phosphorus reduces uptake of
cationic micronutrients like iron, manganese, zinc and copper.
· Excessive amounts of potassium reduce uptake of
magnesium to a greater extent and calcium to a lesser extent.
· Excessive amounts of calcium reduces uptake of iron.
· Excessive Iron reduces zinc uptake.
· Excessive zinc reduces manganese uptake.
The examples above show that
the interrelationships between nutrients in the plant
system are quite complicated and interdependent.
More research will need to be done on the
molecular levels to elucidate the actual relationships
if possible.
1. Toxic Unit Approach (Sprague, 1970).
Additive Effect --
(x)TUA + (1-x)TUB = 1
Synergistic Effect --
(x)TUA + (1-x)TUB > 1
Antagonistic Effect --
(x)TUA + (1-x)TUB < 1
2. Toxic Interaction Scale (S) (Marking and Dawson, 1975).
2. Toxic Interaction Scale (S) (Marking and Dawson, 1975).
Toxic interaction scale is useful for binary mixtures of chemicals. The raw scale is
non-linear, and calculated as:
, where LC50(a) and LC50(b) are the LC50 values for chemicals "a" and "b",
respectively, LC50(a|b) is the LC50 value of chemical "a" in the presence of "b"
and LC50(b|a) is the LC50 value of chemical "b" in the presence of "a".
•Marking and Dawson modified "S" to conform to a linear scale, centered around
zero:
Indicative values of "S":
S=1
additive
S<1
synergistic
S>1
antagonistic