Analysis of toxicity biomarkers for understanding copper and cadmium stress in freshwater algae

Anthropogenic inputs of metals into the aquatic environment are significant enough to affect the natural biogeochemical cycling of trace metals and to impact aquatic organisms. Trace metal bioavailability, and thus toxicity, are controlled largely by chemical speciation and therefore mediated by pH, natural ligands, and concentrations of major ions. Phytoplankton are some of the most sensitive species in freshwater environments and serve as useful sentinels for ecosystem toxicity. The freshwater green algae, Chlamydomonas reinhardtii, was examined as a model species in metal exposure bioassays experiments to assess toxicity, the effects of metals on gene expression, and in the development and application of a biotic ligand model (BLM).;A goal of this work was to evaluate novel, potentially more sensitive endpoints for toxicity that can be related to cell-associated metal, both surface-bound and internalized. Glutathione concentrations and ratios (GSH/GSSG) and also the formation of reactive oxygen species (ROS) were examined as endpoints for copper (Cu) and cadmium (Cd) metal toxicity and also for furthering our understanding of metal-specific mechanisms of toxicity in freshwater algae.;Glutathione was a more sensitive endpoint for acute Cu and Cd toxicity (< 6 h exposure) compared to traditional growth-based endpoints. For 24 h exposures, the sensitivity of glutathione and growth-based endpoints was similar for Cu toxicity and growth-based endpoints were more sensitive for Cd toxicity. Surface-adsorbed metal related to toxicity endpoints was the most sensitive indicator of Cu toxicity, but internalized metal was the most sensitive indicator for Cd toxicity. The stark contrast in glutathione pools measured in algal cells following Cd exposure compared to Cu-exposed cells may have been driven by the differential effect of the metals on glutathione reductase. The measured activity of the glutathione reductase enzyme was inhibited in response to Cu stress, but stimulated at the enzymatic and translational levels by Cd.;Parameters necessary for a species-specific biotic ligand model for C. reinhardtii were determined. These data, coupled with the toxicity relationships developed in this study, will be used to evaluate the BLM model by comparing modeled and measured toxicity in bioassays conducted with a suite of contrasting field samples.