The removal, uptake and toxicity of chromium in Euglena gracilis cultured in absence and presence of malate with Cr(VI) or Cr(III) was evaluated. The malate extrusion and the extra- and intracellular Cr(VI) reduction capacity were determined and the contents of molecules with thiol group and ascorbate were also evaluated. Absence of malate in the medium decreased cell growth, increased Cr(III) toxicity, induced faster Cr(VI) disappearance from medium, and increased intracellular and intramitochondrial chromium accumulation. Both chromium species induced soluble and particulate ascorbate-dependent chromate reductase activities. Cells also secreted large amounts of malate and increased intracellular contents of thiol-molecules to bind extracellular and intracellular Cr(III), respectively. The former process was supported by significant increase in malate-producing enzyme activities and the assessment of the Cr-complexes indicated the in situ formation with thiol-molecules. The present results establish new paradigms regarding chromium stress on algae-like microorganisms: (i) Cr(III) may be more toxic than Cr(VI), depending on the culture (or environmental) conditions; (ii) several simultaneous mechanisms are turned on to inactivate chromium species and their toxic effects. These mechanisms, now well understood may further optimize, by genetically modifying E. gracilis, and facilitate the development of strategies for using this protist as potential bio-remediator of chromium-polluted water systems. (C) 2011 Elsevier B.V. All rights reserved.