Synergism and Antagonism
To further complicate
the important
role physical and chemical character—
istics play in sustaining life in the aquatic environment, we must add the
problems
which result from interactions between the various factors consid—
ered thus far. In some cases, the combination of two or more chemicals results
in an additive effect; that is, the effect can be predicted by simply adding the
toxicities of the combining chemicals. In other cases, interactions between the
chemicals, or often between one of the chemicals and the target organism,
make the combination more toxic than can be predicted and result in a syner—
gistic eect. In a third kind of interaction—called an antagonistic effect—the
toxicity of one of the chemicals is reduced. Appendix Table 1 summarizes the
ways
in which various factors inuence the toxicities of a number of chemicals introduced into water by industry. This summary of experimental nd—
ings is not intended to be a complete description of water chemistry, but only
au aid in understanding some of the ways
in which physical, chemical, and bi—
ological factors interact to affect toxicity. It is worth looking at two of the
heavy metals in a little more detail here.
Heavy metals are sometimes added to water by leaching from mineral—
'
bearing rock and soil, but increasingly in recent years they have reached
water from industrial emission. There has been much research designed to de—
termine the toxic effects of these metals on aquatic organisms; as we have
seen, this toxicity may be modied by a variety of factors: the water’s hard—
ness or temperature, the presence of other chemicals, or a combination of one
or more of these.
Zinc is a good example. One of the major determinants of the toxicity of
zinc and other heavy metals is the water’s degree of hardness; as hardness (the
amount of calcium carbonate) increases, toxicity usually decreases. In an ex—
,
periment testing the toxicity of zinc to rainbow trout, sh expoSed to water
with 5 ppm zinc and 320 ppm calcium carbonate survived about three times
as long as fish exposed to same concentration of zinc but only 50 ppm
calcium
carbonate [48]. When zinc appears in water in combination with the salts of
other heavy metals, however, additive or synergistic effects may
result. For
example, the toxic properties of zinc and cadmium are additive, while the
properties of zinc and nickel and of zinc and cyanide are synergistic. Both
copper and zinc are highly toxic, and together, especially in soft water, they
become far more toxic. Fish that survived in water with 8 ppm copper
alone
or in water with 0.2 ppm copper alone, were killed by water with a com—
bination of one ppm zinc plus 0.025 ppm [49].
Biological as well as chemical interactions in the environment can be re—
sponsible for increasing the toxicity of a given substance. Mercury is a case in
point. It is found in a variety of forms—quicksver (elemental, liquid mer—
Oury), elemental mercury vapor, mercuric salts (such as mercuric chloride),
and organic mercury (mercury Which has combined with a carbon compound). Not all of these forms of mercury are equally toxic to people, how60
An Uncommon Friend
.
-
To further complicate
the important
role physical and chemical character—
istics play in sustaining life in the aquatic environment, we must add the
problems
which result from interactions between the various factors consid—
ered thus far. In some cases, the combination of two or more chemicals results
in an additive effect; that is, the effect can be predicted by simply adding the
toxicities of the combining chemicals. In other cases, interactions between the
chemicals, or often between one of the chemicals and the target organism,
make the combination more toxic than can be predicted and result in a syner—
gistic eect. In a third kind of interaction—called an antagonistic effect—the
toxicity of one of the chemicals is reduced. Appendix Table 1 summarizes the
ways
in which various factors inuence the toxicities of a number of chemicals introduced into water by industry. This summary of experimental nd—
ings is not intended to be a complete description of water chemistry, but only
au aid in understanding some of the ways
in which physical, chemical, and bi—
ological factors interact to affect toxicity. It is worth looking at two of the
heavy metals in a little more detail here.
Heavy metals are sometimes added to water by leaching from mineral—
'
bearing rock and soil, but increasingly in recent years they have reached
water from industrial emission. There has been much research designed to de—
termine the toxic effects of these metals on aquatic organisms; as we have
seen, this toxicity may be modied by a variety of factors: the water’s hard—
ness or temperature, the presence of other chemicals, or a combination of one
or more of these.
Zinc is a good example. One of the major determinants of the toxicity of
zinc and other heavy metals is the water’s degree of hardness; as hardness (the
amount of calcium carbonate) increases, toxicity usually decreases. In an ex—
,
periment testing the toxicity of zinc to rainbow trout, sh expoSed to water
with 5 ppm zinc and 320 ppm calcium carbonate survived about three times
as long as fish exposed to same concentration of zinc but only 50 ppm
calcium
carbonate [48]. When zinc appears in water in combination with the salts of
other heavy metals, however, additive or synergistic effects may
result. For
example, the toxic properties of zinc and cadmium are additive, while the
properties of zinc and nickel and of zinc and cyanide are synergistic. Both
copper and zinc are highly toxic, and together, especially in soft water, they
become far more toxic. Fish that survived in water with 8 ppm copper
alone
or in water with 0.2 ppm copper alone, were killed by water with a com—
bination of one ppm zinc plus 0.025 ppm [49].
Biological as well as chemical interactions in the environment can be re—
sponsible for increasing the toxicity of a given substance. Mercury is a case in
point. It is found in a variety of forms—quicksver (elemental, liquid mer—
Oury), elemental mercury vapor, mercuric salts (such as mercuric chloride),
and organic mercury (mercury Which has combined with a carbon compound). Not all of these forms of mercury are equally toxic to people, how60
An Uncommon Friend
.
-