Chromium(III) and (VI) tolerance and bioaccumulation in yeast: a survey of cellular chromium content in selected strains of representative genera
Introduction
Chromium uptake and bioremediation by yeast are gaining much attention since these eukaryotic microorganisms have proved to be useful in biotechnological practice. Yeast has been applied in the management of Cr-containing waste as well as in nutritional supplementation of this trace metal. Environmental risk caused by Cr contamination is due to a variety of industrial applications of chromium, which lead finally to heavy pollution of soils, ground and surface waters, and the atmosphere [1]. The chemistry of environmentally-released Cr compounds is very complex [2], [3]. Among many oxidation states of this element, ranging from 2− to 6+, the most common and stable are Cr(VI) and Cr(III). Both forms are excessively released into the environment; for example Cr(III) prevails in effluents from tanneries and pigment-producing plants whereas the sources of Cr(VI) are, among others: metallurgy, mining, fossil fuel combustion, wood preservation and cooling installation effluents [2].
Cr(VI) compounds are known to be extremely toxic to living organisms, causing allergies, eczema, irritations, and respiratory track disorders [2], [4]; they are also strongly mutagenic and cancerogenic [5], [6]. This toxic action is due to the negatively charged hexavalent Cr ion complexes which can easily cross cellular membranes by means of sulfate ionic channels [7], and then undergo immediate reduction reactions leading to formation of various reactive intermediates [8], [9], [10]. These intermediates are themselves harmful to cell organelles, proteins and nucleic acids [5], [11], [12].
Cr(III) has also been shown to negatively affect cellular structures [5], [7], [13]; however, its toxicity observed in vivo is much lower as compared to Cr(VI). This fact can be accounted for by the presence of positively charged complexes that are the predominant form of trivalent Cr, which are much less soluble and less suitable for transport inside cells. Still, the organically-bound Cr(III) derivatives might also be transported across cell membranes by some as yet unknown mechanism, as pointed out by Raspor et al. [13] and Srivastava et al. [14].
On the other hand, trivalent Cr has been found to be an essential trace element involved in protein structure stabilisation and lipid and glucose metabolism [15], [16]. Dietary Cr requirements for humans have been determined as 25–50 μg per day and the most convenient natural source of the metal seems to be the non-toxic and stable organically-bound Cr present in chromium-enriched biomass.
As a useful means for bioremediation of environmental chromium contamination, yeasts were used to treat Cr-containing effluents in order to remove toxic compounds from waters and soils [17], [18], [19], [20], [21], [22], [23]. They were also found to be very suitable organisms capable of conducting a bioprocess aimed at obtaining chromium-enriched biomass used for balanced nutrition of mammals and humans [13], [24]. In particular, yeasts which are found effective in accumulation of aggressive Cr compounds and able to bioconvert them into stable, non-toxic and bioavailable forms might be employed for successful environmental control.
The aim of this study is to screen a variety of yeast genera in terms of Cr(III) and Cr(VI) uptake capacity using cell cultures cultivated at optimal conditions, in the presence of growth-inhibitory chromium concentrations.
Section snippets
Yeast strains, growth conditions and viability studies
Fifty-one yeast strains that were used in the study are listed in Table 1. The strains represent various systematic groups such as Saccharomyces, Zygosaccharomyces, Pichia, Candida, Debaryomyces, Schwanniomyces, Cryptococcus, Kluyveromyces, Hansenula and several others. They were obtained from the yeast strain collections: American Type Culture Collection (ATCC), Halle University Collection, Germany (H), Collection of the Institute of Biochemistry and Physiology of Microorganisms, Pushchino,
Results
The chromium concentration required to inhibit growth 40–60% varied broadly and was dependent on the strain of yeast tested (see Fig. 1 for an example). For the case of Cr(III), the concentration ranged from 0.25–5 mM, and for Cr(VI), from 0.1–0.5 mM. The screening of the yeast cell sensitivity to chromium(III) and chromium(VI) and of the uptake of the metal after a 3-day incubation is given in Table 1. Cellular levels of Cr accumulated by the yeast cultures showed some dependence on the initial
Discussion
Yeasts are a very diverse group of eukaryotic microorganisms [26], [27] and the studies of their interaction with chromium and other heavy metals may reveal different resistance and bioremediation strategies. The aim of this study was to compare a number of yeasts of various genera in terms of their sensitivity to, and the uptake potential for Cr(III) and Cr(VI), as revealed by living biomass cultivated at optimal conditions in liquid rich growth media. Our data indicate profound differences
Acknowledgements
This work was supported by the Polish Research Committee Grant Project no. 6 P04G 08418 and by the Foundation of Polish–German Cooperation, Stiftung für Deutsch-Polnische Zusammenarbeit, Grant no. 4375/98/LN.
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