Metal Ions And The Possible Role Of Siderophores In Biodegradation Of Dyes

Kirk et al. (1986) developed culture media for Phanerochaete chrysosporium containing certain supplemental nutrient mixtures of metal salts that enhance the level of lignin peroxidase activity produced in cultures of this fungus. Pointing et al. (2000) studied the ability of P. chrysosporium (IMI 284010), Pycnoporus sanguineus (HKUCC 4065), Trametes versicolor (HKUCC 4063), and an unidentified fungal strain (HKUCC 4062) to decolorize several azo, triphenylmethane, heterocyclic, and polymeric dyes. All four strains were described as sub-tropical basidiomyceteous fungi. P. chrysosporium, T. versicolor, and the unidentified strain completely decolorized in liquid cultures the azo dyes Amaranth, Amido Black, Aniline Blue, Congo Red, Methyl Orange, Orange G, Ponceau 2R, and Trypan Blue during a 14-day incubation period. P. sanguineus decolorized Amido Black, Aniline Blue, Methyl Orange, Orange G, and Trypan

Blue. This fungus did not decolorize Amaranth, Congo Red, and Ponceau 2R. These investigators also studied the ability of these fungi to decolorize several triphenylmethane, heterocyclic, and polymeric dyes. Several dyes were decolorized. However, in general, the azo dyes appeared to be more resistant to degradation. Also studied were the inhibitory effects of three metal ions [Cd(II), Cu(II), and Zn(II)] on dye degradation. These studies were performed using Poly R 478. It was shown that T. versicolor (HKUCC) and the unidentified strain (HKUCC 4062) decolorized this dye in the presence of 0.1 mM Cu(II), and Zn(II). Cd(II) appeared inhibitory to T. versicolor at this concentration. Interestingly, the unidentified strain was able to mediate decolorization of Poly R 478 at a Cd(II) concentration of 0.25 mM. In contrast, when P. chrysosporium was studied, decolorization was inhibited by metal ion concentrations of 0.1 mM. Although Poly R 478 is not an azo dye, this study suggests that the presence of metal ions in dye containing wastewater is one of the factors that must be taken into consideration in developing bioreactors based on the use of basidiomycetaceous fungi.

Fungi often grow in conditions where iron concentrations are vanishingly low. Iron is essential for metabolism (i.e., electron transport system). Thus fungi have evolved ways of sequestering this essential element. To do this, fungi secrete siderophores having very high affinities for iron and certain other polyvalent cations and it is in the form of these metal siderophore complexes that metals are absobed by fungi (Fekete et al. 1989). In addition to their role in sequestering metals, siderophores may have another important role as some siderophore-metal complexes appear to exhibit phenoloxidase-like activity. Some siderophore iron complexes are reduced from the ferric (III) to the ferrous(II) oxidation state. When lignin is the electron donor, measurable lignin degradation takes place. Minussi et al. (2001) have studied this phenomenon using several dyes and have presented circumstantial evidence that siderophore-metal complexes might be involved in the decolorization of Reactive Blue 19, Reactive Red 195, Reactive Yellow 145, and Reactive Black 5 by wood rotting fungi. Although these studies are inconclusive, the involvment of metal-siderophore complexes in the decolorization of azo dyes is an area that deserves further scrutiny.

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