Effect of Chemical Speciation on Toxicity of Cadmium to Grass Shrimp, Palaemonetes pugio: Importance of Free Cadmium Ion William G. Sunda*, David W. Engel, and Roger M. Thuotte
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National Marine Fisheries Service, Southeast Fisheries Center, Beaufort Laboratory, Beaufort, N.C. 28516
Experiments were carried out in diluted seawater media at different salinities (4-29%o) and different concentrations of the chelator NTA (nitrilotriacetic acid) to determine the relationship between the chemical speciation of cadmium and the toxicity of cadmium to Palaemonetes pugio, grass shrimp. After four days of exposure to a given concentration of CdCl2, shrimp mortality decreased with increasing salinity and increasing concentration of NTA. The protective effect of high salinity or NTA was attributable to the complexation of cadmium. Mortality was related to the measured free cadmium ion concentration that, in turn, was determined by the total concentration of cadmium and by the level of complexation by either chloride ion or NTA. Fifty percent mortality occurred at a free cadmium ion concentration of ~4 X 10-7 M.
Increased industrialization has led to contamination of aquatic systems by toxic metals such as cadmium, lead, copper, and zinc. To understand the impact of these metals on aquatic organisms, it is necessary to know both the chemical speciation of the metal in solution and the toxicity of the various chemical species. The toxicity of copper to several species of algae (1,2) and to a microcrustacean, Daphnia magna (3), was shown to be directly related to free cupric ion rather than total copper concentration. In these experiments, copper was complexed to varying degrees by chelators, trishydroxymethylamino methane and ethylenediaminetetraacetic acid, and to inorganic ligands, pyrophosphate and carbonate. An increase in the complexation of copper decreased the toxicity of copper by decreasing the concentration (or activity) of free cupric ion. Similar quantitative relationships between toxicity and free metal concentrations have not been demonstrated previously for metals other than copper. Experiments described in this paper test the hypothesis that the acute toxicity of cadmium to grass shrimp, Palaemonetes pugio, is related to free cadmium ion rather than the total dissolved cadmium concentration. The experiments were conducted in seawater media of varied salinity in which cadmium was complexed to differing degrees by chloride ion and/or by the chelator NTA (nitrilotriacetic acid). By varying the degree of complexation, we could control the free cadmium ion concentration independent of total cadmium.
Materials and Methods Shrimp were taken from the Newport River estuary near Morehead City, N.C., with a dip net and kept for at least two.
days in flowing seawater at 22 °C. The shrimp were acclimated to the test salinities (4-29%o) for 24 h prior to the start of the
experiments. During the preexperimental period the shrimp were fed minced clams. Experiments were carried out at 22 ± 1 °C in linear polyethylene buckets containing 2000 mL of medium. Test media consisted of membrane filtered (0.45 µ) Gulf Stream seawater diluted with deionized distilled water to salinities of 4-29%o. Three separate experiments were performed. In the first two, shrimp mortality was determined for different combinations of cadmium chloride concentration and salinity in the absence of NTA. Each bucket in these experiments contained 10 shrimp. In the third experiment, salinity was held constant at 5%o and the concentration of CdCl2 and NTA was varied. This experiment had 15 shrimp per container. One millimole per liter of NaHCOs was added to media having salinities ] + [CdNTA-]
[Cd2+]KCdNTA+ [Ca2+]KCaNTA+ [Mg2+]KMgNTA (3)
Cd
=
[Cd2+]
^1/acdci
NTAtKcunta [Cd2+] KcdNTA + [Ca2+]KcaNTA + [Mg2+] Equation 4 is derived from Equations 1-3
+
(4)
-
KMgNTAJ
B. Experimentally measured aCdci values (Equation 2) that are valid in dilute seawater at 22 °C
Salinity 5 8 16
20 29
[Cl-]
log «cdci
0.078 0.126 0.25 0.32 0.46
-0.67 -0.82 -1.05 -1.15 -1.36
for NTA complexes at 0.1 M ionic strength (equivalent to a salinity of 5%o) and 20 °C. Constants taken from Sillen and Marte (19) C. Stability constants 11
Metal
log K
Cd
9.83 6.41 5.41
Ca Mg 3
Cdr, NTAr total cadmium or NTA concentration. [ ] = molar concentration of the enclosed chemical species. Kc
