Role of humic substances on degradation of aromatic compounds by oxidoreductive enzymes

Abstract

To improve the efficiency of bioremediation strategies, we must understand the effects of humic substances (HS) on pollutant biodegradation. The possible roles of humic acid (HA) and fulvic acid (FA) are that they: 1) can deactivate enzyme responsible for biodegradation, 2) can act as competitive enzymatic substrates or 3) are enzymatically inert but sequester the pollutant substrate and protect it from enzymatic degradation. In part 1 experiment, degradation of pentachlorophenol (PCP) by purified laccase isolated from Trametes versicolor in combination with one of these HS, Aldrich humic acid (AHA), Leonardite humic acid (LHA), Suwannee River fulvic acid (SRFA), and Waskish peat fulvic acid (WFA) at 28°C and pH 5.0 was studied. In part 2, degradation of phenanthrene by crude fungal ligninolytic enzymes from Agrocybe sp. CU 43 in addition of HS and dissolved organic matter (DOM) from rice paddy field soil were used to test those three hypotheses. Both experiments showed that HS and DOM exhibited inhibitory effects for enzymatic degradation of the pollutants. The enzymatic degradation rate of the pollutants was slower with the increase concentration of HS and DOM. However, HS and DOM could not deactivate enzymes. HS and DOM showed the capabilities to associate with PCP and phenanthrene, protecting the pollutants from enzymatic degradation; consequently the pollutants’ bioavailability was decreased. The inhibitory effect of HS and DOM by competitive or linear mixed types for the pollutant degradation suggest that HS and DOM could, moreover, be substrates for the enzymes. Lower degradation rate constants (ḱ) were found in HS with higher inhibitor binding constants (Ki) and binding constant (Kdom). Terrestrial HA which occupied higher % aromaticity and molecular weight exhibited stronger Ki and Kdom than aquatic FA. This is the first time to clarify the mechanisms by which HS and DOM alter enzymatic degradation rate of the model pollutants. We proposed the model of which once aromatic pollutants enter the environment containing HS and DOM, they could firstly be sorbed to HS and DOM, decreasing biodegradation rate. Then, unbound pollutants would be degraded by enzyme, while HS and DOM could act as substrates for enzymes. Amount and nature of HS and DOM could be important factors in the sorption phenomena and inhibitory effect. Higher aromaticity and molecular weight HS showed the higher propensity in sorption and stronger inhibitor, providing longer time for bioremediation.