mechanisms white rot fungi use to degrade pollutants - ACS Publications


trol the pH of its environment, usu ally by lowering it to about 4.5. It now appears that the fungus does this through proton excretion. In ad dition,...
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CRITICAL

Review

ES&T

MECHANISMS WHITE ROT FUNGI USE TO DEGRADE POLLUTANTS D A V I D

P.

B A R R

A N D

Utah State University,

78 A

Environ. Sci. Technol., Vol. 28, No. 2, 1994

S T E V E N

Logan, UT

D.

A U S T

84322-4705

0013-936X/94/0927-78A$04.50/0 © 1994 American Chemical Society

n recent years the possibility of using white rot fungi for bioremediation strategies has i n i t i a t e d c o n s i d e r a b l e research effort in academic, industrial, and government institutions. The interest in this subject arises from the ability of white rot fungi to degrade an extremely diverse range of very persistent or toxic environmental pollutants. This ability sets the use of white rot fungi apart from many of the existing methods of bioremediation. Perhaps the easiest way to understand t h e n o n s p e c i f i c ability of these fungi to degrade pollutants is to c o n s i d e r t h e i r e c o l o g i c a l "niche." White rot fungi are those organisms that are able to degrade lignin, the structural polymer found in woody plants. Lignin is a very complex three-dimensional polymer c o n s i s t i n g of n o n r e p e a t i n g phenyl propanoid units linked by various carbon—carbon a n d ether bonds (Figure 1) (2). The stereo irregularity of lignin makes it very resistant to attack by enzymes. In addition, it is impossible for lignin to be absorbed and degraded by intracellular enzymes. The enzymatic degradation of lignin is further complicated by the chiral carbons in this polymer that exist in both the L and D configurations. T h u s , the white rot fungi have developed very nonspecific m e c h a n i s m s for degrading lignin. This r e v i e w discusses the degradation of pollutants by these fungi from a mechanistic standpoint.

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Advantages of white rot fungi Most of these m e c h a n i s m s depend on a group of heme-containing peroxidases secreted into the extracellular environment of these fungi. These enzymes, k n o w n as lignin peroxidases (LiP) and manganesedependent peroxidases (MnP), are produced by the fungi in response to low levels of key s o u r c e s of carbon, nitrogen, or sulfur nutrients {2, 3). These conditions are often referred to as ligninolytic. The purification and characterization of these enzymes have been extensively reviewed recently {4, 5). The fungi contain enzymes that produce extracellular hydrogen peroxide from molecular oxygen. It has been reported that the fungi produce oxidase enzymes that utilize glucose, glyoxal, methyl glyoxal, and other p r o d u c t s of cellulose and lignin degradation as substrates for t h e production of H 2 0 2 {6, 7). Interestingly, these fungi do not use lignin

as a carbon source for growth; instead, they degrade the lignin to obtain the cellulose that is toward the interior of the wood fiber (8). T h e same u n i q u e , n o n s p e c i f i c mechanisms that give these fungi the ability to degrade lignin also allow them to degrade a wide range of pollutants to carbon dioxide (see box) (9-2 7). Along w i t h their ability to degrade these chemicals, w h i t e rot fungi possess a number of advantages n o t a s s o c i a t e d w i t h o t h e r bioremediation systems. Because key components of the white rot fungi lignin-degrading system are

The very nonspecific nature of the mechanisms used by these fungi allows them to degrade even complex mixtures of pollutants all the way to carbon dioxide. extracellular, the fungi can degrade very insoluble chemicals such as lignin or many of the hazardous environmental pollutants. Many of the pollutants in a hazardous waste site are toxic to the organisms that may be employed to degrade them. For example, cyanide is known to be a potent inhibitor of respiratory oxidase enzymes; thus, uptake of cyanide by bacteria inhibits growth. Yet, in order to metabolize cyanide, bacteria must take u p the pollutant because the enzymes are located inside the cell. As a result, cyanide concentrations as low as 4 ppm could inhibit microbial growth in a municipal sewage treatment system {18). However, the extracellular system of the white rot fungi enables the fungi to t o l e r a t e c o n s i d e r a b l y higher concentrations of a toxic pollutant such as cyanide. For example, cyanide was found to be quite

toxic to spores of the white rot fungus Phanerochaete chrysosporium (50% inhibition of glucose metabolism occurred at 2.6 ppm). This toxicity was due to the absence of LiP, which rapidly metabolize cyanide. However, ligninolytic, 6-day-old cultures could tolerate considerably higher cyanide concentrations (i.e., 50% inhibition of glucose metabolism occurred at 182 ppm), further supporting the idea that the LiP protect this fungus from cyanide (9). Because of this increased tolerance, the inhibition of cyanide mineralization by P. chrysosporium was not evident until concentrations reached 260 ppm cyanide. It was also found that complexing cyanide with metals enhanced its mineralization by the fungus [19). The enhancement of mineralization by iron, for example, follows the observation that potassium ferricyanide is membrane impermeable and presents very little toxicity to the fungus. This result is relevant because cyanide in soil may indeed be c o m p l e x e d w i t h m e t a l s s u c h as iron. Interestingly, w e found that the mineralization rate of cyanide was linear w h e n 3000 p p m were added to the cultures of P. chrysosporium in soil (29). The very nonspecific nature of the mechanisms used by these fungi allows them to degrade even complex mixtures of pollutants, such as creosote and Aroclor, all the way to carbon dioxide {20, 21). In contrast, a consortium of bacteria may be needed to successfully and completely degrade these same mixtures. For example, Abramowicz et al. {22) have found that anaerobic bacteria in Hudson River sediment can effectively dehalogenate polyc h l o r i n a t e d b i p h e n y l s (PCBs) to their monochloro congeners. However, aerobic bacteria are then required to degrade these monochlor i n a t e d b i p h e n y l s to c a r b o n dioxide. Another advantage of white rot fungi is that they do not require preconditioning to a particular pollutant. Because the degrading system of white rot fungi is induced by nutrient deprivation, limiting the nutrient source (i.e., glucose or ammonia) can initiate degradation. Furthermore, repression of enzyme synthesis does not occur when the concentration of a chemical is reduced to a level that is ineffective for enzyme induction. Because the i n d u c t i o n of t h e degradative enzymes is not dependent on the presence of the chemical, the fungi can Environ. Sci. Technol., Vol. 28, No. 2, 1994 79 A

FIGURE 1

Representative structure for a part of the lignin polymer H 2 COH

I R

CH

H 2 COH

I

I

CH

CH

I

CH

H 2 COH

H 3 CO Ο

-CH

HoCOH

I HCOH

H

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H3CO

CH |

I /r^\ HO-