Effect of cadmium and zinc salts on energy metabolism and survival of Unio tumidus Philipsson.

Abstract

Energy metabolism of aquatic invertebrates is one of the most important targets of environmental pollutants, in particular, heavy metals. In this study, we determined changes in survival, oxygen uptake and hepato-pancreas glycogen level of the bivalve mollusk Unio tumidus Philipsson following chronic exposure of Cd and Zn salts (chlorides and sulfates). The concentrations of Cd salts were equal to 0.001, 0.01 and 0.1 mg/l and concentrations of Zn salts were equal to 0.01, 0.1, 1.0 mg/l (maximal contamination level for Cd is 0.005 mg/l and for Zn is 0.01 mg/l). Survival was registered daily whereas oxygen consumption and glycogen level were determined on days 3, 7 and 14. Changes in survival of U. tumidus along the course of Cd and Zn salt concentrations was not monotonic; the sulfates were more inhibitory than the chlorides. No correlation was found between changes in survival and changes in the oxygen uptake and the glycogen level. By analysis of data in the literature, it was possible to construct a general scheme of adaptation of aquatic invertebrates' energy metabolism to heavy metals impact: (1) "exhaustive" or "economical" activation of aerobic metabolism (with or without depletion of energy resources); (2) change to "exhaustive" anaerobic mechanism (with depletion of glycogen); (3) change to "economical" anaerobic metabolism (involving mechanisms of glycogen expenditure economy); (4) late-term activation of aerobic metabolism due to need for binding and excretion of accumulated metals. Different severity of the pollutants' impact may lead to prevalence of different stages of this scheme. Our results revealed most of these stages in U. tumidus: "economical" activation of aerobic metabolism, change to "exhaustive" and "economical" anaerobic metabolism, and late-term activation of aerobic metabolism. It can be suggested that Cd is more toxic due to late-term decrease in oxygen consumption whereas Zn resulted only in transient early-term decrease. On the other hand, mechanisms of glycogen economy during anaerobic metabolism were involved in the effect of Cd and not involved in the effect of Zn. Concerning the effect of anions, chlorides promoted transient activation of aerobic metabolism, while sulfates promoted the passage to anaerobic metabolism. Thus, the effect of sulfates seems to be more inhibitory than the effect of chlorides.