Cellulolytic responses to heavy metal accumulation in Corbicula fluminea and Mudalia dilatata

Abstract

Cellulolytic responses of the Asiatic clam, Corbicula fluminea and a snail, Mudalia dilatata, to selected constituents of power plant effluents (i.e., zinc, cadmium, acidic and alkaline pH, individually and paired) were investigated in 30-day exposures. Exposures were conducted in both laboratory and field-oriented artificial streams and then validated in the river receiving system of a power plant. Cellulolytic activity was reduced by laboratory and field exposures to cadmium and zinc at all levels tested from 0.012 to 0.10 mg cadmium/L and generally at 0.025 to 1.0 mg zinc/L. Clams detected acute lethal levels of metal and used valve closure as an avoidance mechanism for 14 days. Snails, however, did not effectively avoid exposures and were more sensitive to acute stress during all exposures. These behavioral responses were corroborated by both cellulolytic activity and metal accumulation. Measurements of cellulolytic activity for both test species in laboratory exposures differed from those in field artificial streams. Reduced enzyme activity in controls by day 30 was attributed to artificially induced stress associated with the laboratory environment. This factor precluded any analysis of laboratory responses for periods of exposure longer than 20 days as well as recovery analysis. Field oriented artificial streams provided a sufficient environment to adequately assess long-term stress and recovery as measured by cellulolytic activity and metal accumulation in both clams and snails. Enzyme activity responded to metal exposure with respect to both degree and duration of exposure. Cadmium and zinc combined exposures caused significantly reduced cellulolytic activity at the same concentration as those for cadmium alone. Reduced enzyme activity caused by cadmium and zinc addition at levels that were not detectable suggested that the cellulolytic index was sensitive to sublethal stressors. This was supported by metal uptake patterns in clams and snails. Cellulolytic activity responded to zinc addition at alkaline and acidic pH in a manner that supported pH optima for cellulases and bioavailability of metals. Effects seen in macroinvertebrate assemblages (diversity, richness, and similarity) were compared with cellulolytic activity of caged Corbicula from a site specific power plant discharge. Enzyme activity inhibition was the most sensitive indicator measured. Reductions in cellulolytic activity at stations monitored for total zine content were consistent with effects seen at comparable exposures to zine in field-located artificial streams. A zine concentration of 0.05 mg/L consistently caused the first significant reductions in cellulolytic activity. This concentration is comparable to the U.S. Environmental Protection Agency's Water Quality Criteria value (0.047 mg/L zinc) for protection of aquatic life.