Biological removal of phenols

OMWW phenolic compounds, and in particular the lower molecular weight ones, are toxic molecules. Fiorentino et al. (2003) characterized phenolic products such as 1,2-dihydroxybenzene (catechol), derivatives of benzoic acid, phenylacetic acid, phenylethanol and cinnamic acid. The OMWW were fractioned by ultra-filtration and reverse osmosis techniques and tested for toxicity on aquatic organisms from different trophic levels: the alga Pseudokirchneriella subcapitata; the rotifer Brachionus calyciflorus; and two crustaceans, the cladoceran Daphnia magna and the anostracan Thamnocephalus platyurus. The fraction most toxic to the test organisms was that from reverse osmosis containing compounds of low molecular mass (<350 Da), and this was especially due to the presence of catechol and hydroxytyrosol, the most abundant components of the fraction. Sampedro et al. (2004) found that nine saprobic fungi were capable of completely removing monomeric toxic phenols from the DOR from the olive oil extraction process after 20 weeks of growth. Removal rate depended on the type of fungi and phenol. Results showed that most of the fungi tested eliminated o-diphenols and then non-o-diphenols. However, some fungi did not follow this trend. Phanerochaete chrysosporium first removed hydroxytyro-sol and tyrosol and later their glucosides; in contrast Paecylomyces farino-sus hydrolyzed hydroxytyrosol and tyrosol glucosides at the first stage, 2 weeks of growth, and then eliminated all monomeric phenols. The behaviour of this fungus seems of great interest for recovering phenolic antioxi-dants from the DOR. Similarly, differences in DOR decolorization capacity among the fungi tested were also observed. Coriolopsis rigida showed the highest capacity, followed by Phebia radiata, Pycnoporus cinnabarinus and Phanerochaete chrysosporium. Therefore, both decolorization and monomeric phenol elimination pointed out that saprobic fungi could be used to detoxify the DOR obtained from the two-phase system of the olive oil extraction process. The lignin-degrading ability of white rot fungi seems to be associated with the release of extracellular enzymes - which mainly include lignin-peroxidases, Mn-dependent peroxidases and laccase - and these enzymes could participate both in the removal of monomeric phenols and in the decolorization of olive residues. Perez et al. (1987) first described decolorization of OMWW by Phanerochaete chrysosporium, and suggested that the decolorization occurred through the breakdown of coloured phenolic polymers into monomers, which were subsequently mineralized, but this is not a well-explained process.

Many other researchers have studied the depolymerization and dephe-nolization of the olive residues by saprobic fungi and a highly significant correlation was found between decolorization and laccase production; however, the same authors do not find similar correlations in other experiments. Laccase alone is able to remove monomeric phenols from OMWW, but the decolorization and dephenolization of OMWW by fungi seems to be a sequential process: monomeric phenols are first oxidized and polymerized and then the depolymerization and, therefore, decolorization occurred. Soil saprobic fungi are important and common components of the rhizo-sphere soil from which they obtain nutritional benefits in the form of inorganic compounds and exudates from the root. These soil fungi are important because they take part in the mobilization of nutrients and degradation of phytotoxic substances, they produce substances that promote or inhibit the growth of other rhizosphere microorganisms, they add great amounts of microbial biomass to the soil and they also contribute to the optimum use of nutrients by the plant.

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