Interaction of Copper or Chromium with Yeasts: Potential Application on Polluted Environmental Clean Up

Abstract

Heavy metal pollution is one the most serious environmental problems. Mining and industrial activity are the main sources of heavy metal contamination. Heavy metals, such as copper (Cu) and chromium (Cr), play an important role as trace elements in biochemical reactions but these heavy metal ions are toxic at higher concentrations, producing serious public health problems. Traditional technologies for the removal of these heavy metals are very expensive and may have several disadvantages. New techniques are required for reducing their concentrations to environmentally acceptable levels at low costs. Microorganisms are potent bioremediators. To date, studies on heavy metal resistant have mainly concerted in bacteria. Nevertheless, one of the most ubiquitous biomass types utilized on a large scale by man for centuries are yeasts, moreover this microorganism is an inexpensive, readily available source of biomass. Furthermore, yeast cells are able to accumulate a broad range of heavy metals to varying degrees under a wide range of external condition. Recent researches have made emphasis to the yeasts isolated from contaminated area. This allows isolating indigenous yeasts able to live in these unfavorable environments. Candida fukuyamaensis and Rhodotorula mucilaginosa were isolated from a copper filter at a mine plant. These yeasts show multiple heavy metal tolerance. They are able to remove Cu(II) of the culture medium by a process of bioaccumulation in the cytoplasm. The resistance mechanism in the yeasts is through sequestration in cytoplasmic of copper ions rather than avoidance or compartmentalization. However, the copper accumulation profiles of both strains are different: whereas the metal is later released by C. fukuyamaensis. R. mucilaginosa has the capacity to keep copper inside the cell. The toxicity of copper is evidenced for the morphological changes and the increased oxidative stress response by both strains. However the copper bioaccumulation by R. mucilaginosa is increased by adaptation of the strain under controlled aeration On the other hand, Lecythophora sp. NGV-1, Candida sp. NGV-9 and Aureobasidium pullulans VR-8 isolated from tannery effluents and from a nickel-copper mine, show Cr(VI) tolerance . In these cases, the yeast Cr(VI) tolerant is due to the reduction of Cr(VI) to Cr(III) and no bioaccumulation. In vivo Cr(VI) toxicity is 1,000-fold more cytotoxic than Cr(III). Apart from its toxicity, Cr(VI) is highly soluble and thus mobile and biologically available in the ecosystems. In contrast, Cr(III) has a high affinity for organic compounds resulting in the formation of complexes that precipitate as amorphous hydroxides. The reduction of Cr(VI) to Cr(III) is an interesting approach for environmental clean up. The ability of metal processing by yeasts can be used to concentrate, remove and recover metals from streams and could enhance the efficiency of wastewater treatment processes. This capacity depends of the specific interaction of strain and heavy metal. Different mechanisms of heavy metal resistance may affect different bioremediation strategies.