Toxicity of hexavalent chromium to the earthworm Eisenia andrei: linking effects at different levlels of biological organization

Living organisms respond to chemicals at all the levels of biological organization, from molecules to ecosystems. In general, biochemical and physiological responses (molecules, cells, tissues) are triggered at low concentrations and after short exposure times, while ecological responses (life cycle traits, populations, communities, ecosystems) can be evidenced only at higher concentrations and after longer exposures. On the other hand, responses at the higher levels of organization are considered more relevant from an environmental point of view, as they often result in irreversible severe damage to ecological systems. It is important to clarify the relationship between low-level and high-level responses to different toxicants, particularly considering the increasing application of biomarkers as early warning systems to assess environmental pollution. In accordance with this view, effects of exposure to hexavalent chromium, Cr(VI), on life cycle endpoints were compared to effects on a battery of eight biomarkers in the earthworm Eisenia andrei. In spite of the importance of Cr(VI) as an environmental contaminant, its toxicity to earthworms, a major component of soil fauna, has received little attention; in particular almost no data are available on biochemical and physiological responses. A natural clayey soil was spiked at six Cr(VI) concentrations, from 6.25 to 200 mg/kg. Adult worms were exposed in laboratory to the spiked soils and to a control treatment (unspiked soil). Two sets of experimental containers were prepared: the first to measure lysosomal membrane stability, lysosomal lipofuscins and neutral lipids accumulation, metallothionein content, Ca2+ ATPase, catalase, glutathione S-transferase and acyl CoA oxydase activities, after 10 d of exposure; the second to asses survival, growth and chromium bioaccumulation after 28 d, and reproduction after 56 d of exposure. All the biological effects were significantly affected by Cr(VI) exposure and showed a clear concentration-effect relationship, except for the content of metallothioneins. Biomarkers showed different trends at increasing Cr(VI) concentrations, that could be classified as typical “increasing responses” (neutral lipids), “decreasing responses” (lysosomal membrane stability, Ca2+-ATPase and acyl Co-A oxydase activities) and “bell-shaped responses” (catalase and glutathione S-transferase activities). Biomarkers were more sensitive to Cr(VI) than life-cycle endpoints; among these, reproduction was the most responsive.