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Bioremoval of humic acid from water by white rot fungi: exploring the removal mechanisms

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ABSTRACT

Twelve white rot fungi (WRF) strains were screened on agar plates for their ability to bleach humic acid (HA). Four fungal strains were selected and tested in liquid media for removal of HA. Bioremediation was investigated by HA color removal and changes in the concentration and molecular size distribution of HA by size exclusion chromatography. Trametes versicolor and Phanerochaete chrysosporium showed the highest HA removal efficiency, reaching about 80%. Laccase and manganese peroxidase were measured as extracellular enzymes and their relation to the HA removal by WRF was investigated. Results indicated that nitrogen limitation could enhance the WRF extracellular enzyme activity, but did not necessarily increase the HA removal by WRF. The mechanism of bioremediation by WRF was shown to involve biosorption of HA by fungal biomass and degradation of HA to smaller molecules. Also, contradicting previous reports, it was shown that the decolorization of HA by WRF could not necessarily be interpreted as degradation of HA. Biosorption experiments revealed that HA removal by fungal biomass is dependent not only on the amount of biomass as the sorbent, but also on the fungal species. The involvement of cytochrome P450 (CYP) enzymes was confirmed by comparing the HA removal capability of fungi with and without the presence of a CYP inhibitor. The ability of purified laccase from WRF to solely degrade HA was proven and the importance of mediators was also demonstrated.

No MeSH data available.


Degradation of HA by laccase, with and without the presence of mediators, expressed as SEC results (area under curve %) of HA and FA (Control: humic acid without enzyme and mediators)
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Fig8: Degradation of HA by laccase, with and without the presence of mediators, expressed as SEC results (area under curve %) of HA and FA (Control: humic acid without enzyme and mediators)

Mentions: It is clear from Fig. 8 that laccase can degrade HA. Although, when comparing the results after 3 and 24 h, it becomes apparent that the degradation is very slow when there is no mediator present. In the absence of mediators, only 20% reduction in the concentration of HA was observed after 24 h. ABTS proved to be the best mediator among the tested mediators. In the presence of ABTS, more than 60% of the humic acid was degraded after 24 h. The changes in the FA concentration was not as significant as in HA concentration. Although, in ABTS samples, a 40% increase in the FA concentration was observed. This simultaneous decrease in HA and increase in FA concentrations clearly shows the conversion of HA to FA by laccase in the presence of ABTS as the mediator. When using VA as the mediator, 20% of HA was degraded after 24 h, without significant change in FA concentration. Therefore it can be concluded that different mediators have different effects on the mechanism of HA degradation by laccase. The analysis of the MW distribution of HA confirms that the measurement of average MW is not necessarily representing the polymerization or depolymerization of HA. The concentration of HA was reduced to 36% of its initial value after 24 h of treatment with laccase in the presence of ABTS. This reduction is clearly due to the degradation of HA to non-aromatic compounds or conversion to FA-like substances, since there was no fungal biomass present, hence no biosorption could occur. However, the average MW of HA after 24 h was increased to 2 kDa, which implies polymerization of HA by laccase. The composition of the remaining HA in the media consisted of (after normalization to 100%) 90% large, 7% medium and 3% small HA molecules. In comparison with the initial composition of HA, shows a shift towards the larger molecules. The reason for this false implication is that when comparing the MW distribution and average MW of HA before and after the treatment, the non-aromatic products of the degradation of HA were not considered, since they cannot be detected via the UV detector during the SEC analysis.Fig. 8


Bioremoval of humic acid from water by white rot fungi: exploring the removal mechanisms
Degradation of HA by laccase, with and without the presence of mediators, expressed as SEC results (area under curve %) of HA and FA (Control: humic acid without enzyme and mediators)
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5120169&req=5

Fig8: Degradation of HA by laccase, with and without the presence of mediators, expressed as SEC results (area under curve %) of HA and FA (Control: humic acid without enzyme and mediators)
Mentions: It is clear from Fig. 8 that laccase can degrade HA. Although, when comparing the results after 3 and 24 h, it becomes apparent that the degradation is very slow when there is no mediator present. In the absence of mediators, only 20% reduction in the concentration of HA was observed after 24 h. ABTS proved to be the best mediator among the tested mediators. In the presence of ABTS, more than 60% of the humic acid was degraded after 24 h. The changes in the FA concentration was not as significant as in HA concentration. Although, in ABTS samples, a 40% increase in the FA concentration was observed. This simultaneous decrease in HA and increase in FA concentrations clearly shows the conversion of HA to FA by laccase in the presence of ABTS as the mediator. When using VA as the mediator, 20% of HA was degraded after 24 h, without significant change in FA concentration. Therefore it can be concluded that different mediators have different effects on the mechanism of HA degradation by laccase. The analysis of the MW distribution of HA confirms that the measurement of average MW is not necessarily representing the polymerization or depolymerization of HA. The concentration of HA was reduced to 36% of its initial value after 24 h of treatment with laccase in the presence of ABTS. This reduction is clearly due to the degradation of HA to non-aromatic compounds or conversion to FA-like substances, since there was no fungal biomass present, hence no biosorption could occur. However, the average MW of HA after 24 h was increased to 2 kDa, which implies polymerization of HA by laccase. The composition of the remaining HA in the media consisted of (after normalization to 100%) 90% large, 7% medium and 3% small HA molecules. In comparison with the initial composition of HA, shows a shift towards the larger molecules. The reason for this false implication is that when comparing the MW distribution and average MW of HA before and after the treatment, the non-aromatic products of the degradation of HA were not considered, since they cannot be detected via the UV detector during the SEC analysis.Fig. 8

View Article: PubMed Central - PubMed

ABSTRACT

Twelve white rot fungi (WRF) strains were screened on agar plates for their ability to bleach humic acid (HA). Four fungal strains were selected and tested in liquid media for removal of HA. Bioremediation was investigated by HA color removal and changes in the concentration and molecular size distribution of HA by size exclusion chromatography. Trametes versicolor and Phanerochaete chrysosporium showed the highest HA removal efficiency, reaching about 80%. Laccase and manganese peroxidase were measured as extracellular enzymes and their relation to the HA removal by WRF was investigated. Results indicated that nitrogen limitation could enhance the WRF extracellular enzyme activity, but did not necessarily increase the HA removal by WRF. The mechanism of bioremediation by WRF was shown to involve biosorption of HA by fungal biomass and degradation of HA to smaller molecules. Also, contradicting previous reports, it was shown that the decolorization of HA by WRF could not necessarily be interpreted as degradation of HA. Biosorption experiments revealed that HA removal by fungal biomass is dependent not only on the amount of biomass as the sorbent, but also on the fungal species. The involvement of cytochrome P450 (CYP) enzymes was confirmed by comparing the HA removal capability of fungi with and without the presence of a CYP inhibitor. The ability of purified laccase from WRF to solely degrade HA was proven and the importance of mediators was also demonstrated.

No MeSH data available.