Removal, accumulation and resistance to chromium in heterotrophic Euglena gracilis

► The protist Euglena gracilis has the ability to accumulate large amounts of chromium, particularly Cr(III). ► The Cr(III) toxicity depends on the medium composition, becoming high in the absence of carboxylic acids. ► There are increased activities of both particulate and soluble chromate reductase activities in response to the oxidative stress induced by Cr(VI) or Cr(III). ► Large secretion of malate, as well as enhanced intracellular contents of ascorbate and thiol-molecules are also part of the chromium resistance mechanisms in this protist. 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.