Bioremediation is considered the use of biological agents to transform pollutants into innocuous products. The main biological agents of bioremediation are microorganisms and plants (phytoremediation) and their enzymes. A large number of bacteria and fungi are able to transform, partly or completely, many xenobiotic organic compounds. The use of microorganisms to remediate a polluted environment may, however, present some limitations, such as adsorption of microbial cells on solid particles, diffusion constraints, competition effects by indigenous microorganisms, and inhibition by toxic compounds. A possible strategy for overcoming these drawbacks is the use of enzymes, which could be obtained, in large quantities from microbial populations as well as from plants. As it is well known, enzymes can be active outside the parent cells. The extracellular activity of enzymes is expected to be increasingly exploited in future bioremediation technologies. To prevent losses of enzymatic activity under severe field condition, many investigation have been focused on developing methods to stabilize enzymes, preferably by immobilization on solid supports or by gel coating. The use of enzymes naturally stabilized in plant tissues also has been investigated. Basically, two classes of enzymes are examined for bioremediation purposes, i.e., oxidoreductases and hydrolases, including enzymes highly efficient in the transformation of several pollutants. The feasibility of enzymatic treatment has been demonstrated mainly in laboratory studies but some attempts at field scale were done as well. For example, several fungal phenoloxidases effectively oxidised xenobiotic phenols and anilines through polymerization or binding to humus. Tyrosinase was used as a free enzyme to dephenolize different wastewater samples. Both free and immobilized laccase and peroxidase were applied to detoxify soils polluted by 2,4 dichlorophenols. Further studies are necessary to identify additional enzymes that can be able to transform the increasing number of chemicals polluting the environment.
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