Mercury pretreatment selects an enhanced cadmium-accumulating phenotype in Euglena gracilis

Pre-treatment of heterotrophic cultures of Euglena gracilis with 1.5 muM HgCl2 for at least 60 generations resulted in a cell population that showed both increased resistance to Cd2+ and ability to accumulate it, when compared to non-Hg2+-pretreated Euglena. These Hg2+-enhanced capacities were evident in cells cultured in the dark in a medium with lactate, but not in cells cultured with glutamate plus malate. After culturing with 0.1 mM CdCl2 through three consecutive transfers, the mercury-pretreated cells still grew and maintained high levels of glutathione-related metabolites, while the non-Hg2+-pretreated cells died. Cultures of Hg2+-pretreated cells, after transfer to media with or without cadmium, did not alter either their enhanced Cd2+ accumulation or their increased production of glutathione-related metabolites. These observations suggested that the Hg2+-pretreated population underwent a permanent change that improved its Cd2+ resistance. Several factors that contributed to the improved capacities included: (a) higher cellular malate, cysteine and glutathione levels induced by Hg2+ before and after Cd2+ exposure; and (b) increased storage of Cd2+ in mitochondria along with increased intramitochondrial citrate, cysteine, and glutathione levels. These characteristics suggested that this Cd2+ hyper-accumulating strain of E. gracilis might be a suitable candidate for Cd2+-bioremediation of polluted water systems.