Extra-cellular chromate-reducing activity of the yeast cultures

Abstract

This paper reports on the experimental data supporting an essential role of extracellular reduction in chromate detoxification by baker's and non-conventional yeasts. A decrease of chromate content in the yeast culture coincides with an increase of Cr(III) content in extracellular liquid. At these conditions, cell-bound chromium level was insignificant and a dominant part of extra-cellular Cr(III) species was detected in the reaction with chromazurol S only after mineralization of the cell-free samples. This phenomenon of chromium "disappearance" can be explained by the formation of Cr(III) stable complexes with extra-cellular yeast-secreted components which are "inaccessible" in the reaction with chromazurol S without mineralization. It was shown that increasing sucrose concentration in a growth medium resulted in an increase of chromate reduction. A strong inhibition of chromate reduction by 0.25 mM sodium azide, a respiration inhibitor and a protonophore, testifies that extra-cellular chromate detoxification depends on energetic status of the yeast cells. It was shown that Cr(III)-biochelates produced in extra-cellular medium are of a different chemical nature and can be separated into at least two components by ion-exchange chromatography on anionit Dowex 1x10. A total yield of the isolated Cr(III)-biocomplexes is approximately 65 % (from initial level of chromate) with a relative molar ratio 8:5.

Metallophilic fungi research: an alternative for its use in the bioremediation of hexavalent chromium

“…The Cr(VI) is actively transported through the biological membrane of prokaryotic and eukaryotic microorganisms (Alluri et al 2007), and once inside the cell, Cr(VI) is reduced to Cr(III), most likely through the formation of instable intermediary forms of Cr(V) and Cr(IV) by nonenzymatic (indirect) or enzymatic reactions (direct) (Ksheminska et al 2006); the latter is still uncertain for eukaryotic microorganisms (Gadd 2010).…”

“…For eukaryotic cells, knowledge regarding enzymes for chromate reduction remains very limited (Ksheminska et al 2006). In bacteria, the existence of ChrA proteins as part of a chromate transporter (CHR) superfamily has been informed and is related to the transport of sulphate and chromate (Nies et al 1998).…”

Mechanisms of Chromate Detoxification in Yeasts

“…However, for the chromate-resistant strains of Candida maltosa a NAD-dependent chromate reducing activity was observed [14]. Recently we have found that some reducing substances, secreted extracellularly by yeast played a significant role in Cr(VI)-detoxification [10,11]. As a product of chromate reduction, trivalent Cr(III) formed complexes with some specific components of culture liquid which were not adsorbed by the cells.…”

The chromate resistance phenotype of some yeast mutants correlates with a lower level of Cr(V)-species generated in the extra-cellular medium

“…Taking into account significant kinetic inertness of chromium(III) salts in the formation of even thermodynamically stable complexes, it is promising to produce such complexes by using microbial cells capable of reducing chromium(VI) (chromate, dichromate) to Cr(III)-biocomplexes (Puzon et al 2005;Ksheminska et al 2006Ksheminska et al , 2008. Although the genetic and biochemical aspects of this process have been insufficiently studied, it is known that chromate anions are transported into the cells through sulfatespecific permease(s) coded, in the case of baker's yeast, by the genes SUL1 and SUL2 (Cherest et al 1997) and can be reduced, as a powerful oxidative agent, to Cr(III) by cellular reducing systems which can include enzymatic and non-enzymatic pathways.…”

“…The bacterial reduction of chromate is well-established. It runs through the different enzymatic pathways which function under aerobic and anaerobic conditions with the use of hydrogenase, cytochrome cdependent electron transfer chains, nitrate reductase, flavin reductase, ferrireductase, some flavoproteins, and NADH and NAD(P)H-dependent reductases (Ksheminska et al 2006). For eukaryotic microbial cells and, primarily, yeasts, the data on the chromatereducing systems are more ambiguous.…”