An investigation on the disruptive effect of pollution in cold- and warm- adapted clam populations

The purpose of this study was to examine the relationship between mitochondrial activity and gonad lipid stores in clams exposed to anthropogenic pollution at cold- and warm-water sites. The balance between energy expenses and energy reserves was measured by mitochondrial electron transport (MET) activity and lipid content in the gonad. The activity of malate dehydrogenase (MDH) was measured as an intermediary between energy production and the production of lipids in gonadal tissues. The results revealed that intertidal clam populations at warm-water sites under no source of pollution had less heavy metal content (Ag, As, Cr, Hg and Ni), lower MDH activity and temperature-dependent MET than clams from cold-water sites. However, MDH activity measured at 6 oC was higher at the warm-water sites. Lipid peroxidation in the gonad was higher in clams from the cold-water sites. The impacts of pollution differed among the study sites, clams from cold-water sites having increased MDH activity, temperature-dependent MET activity, higher lipid content and DNA strand breaks; clams from the warm-water sites had increased temperature-dependent MDH activity and lower gonadal lipid reserves. A multiple regression analysis revealed that gonad lipid reserves were positively correlated with MDH activity and negatively correlated with its temperature-dependent activity, suggesting that increased temperature sensitivity was negatively related to gonad energy reserves. The data show that pollution increases temperature sensitivity at the MET level in clams in cold water, while temperature sensitivity in MDH activity was observed in clams from warm-water sites. Discriminate function analysis revealed that pollution stress shows a tendency to be closer to clams adapted to warmer temperatures. In conclusion, pollution could increase MDH activity in cold-adapted clams which can lead to increased lipid stores in the gonad, oxidative stress and genotoxicity while pollution seems to increase the temperature dependence in MET. In warm-adapted clams, temperature dependent MDH activity was higher by pollution with decreased lipid content in the gonad tissues which was independent of gonad maturation and size.