Uptake and transformation of phenols by duckweed (Lemna gibba)

Phenol and its derivatives are common constituents of numerous natural and synthetic chemicals including pesticides. As a consequence of anthropological activities, a portion of them may reach various water bodies as contaminants which will have a great impact on the aquatic environment including biota. Aquatic plants are known to be one of the major habitants and food for many species and hence the distribution and metabolism of chemicals in aquatic plants is very important in order to understand their effects on the aquatic ecosystem. Duckweed is a representative species of aquatic plant, which is widely distributed in freshwater systems. Although the fate of phenol and its derivatives within duckweed has been extensively investigated, there are few mechanistic studies on the uptake and degradation behavior in relation to their physico-chemical properties. Recently, Tront et al. and Reinhold et al. studied the uptake of various halogenated phenols into Lemna minor and suggested that the extensive metabolic transformation of phenols via phase II reaction has an important role in increasing their uptake by shifting a steady state equilibrium reached between the contaminants in cytosol and medium to a transient state. Tront et al. explained that the fraction in the protonated form of chemicals at a cytosolic pH which was calculated from the corresponding pKa values had a meaningful correlation with the metabolic transformation rate. In their theory, contaminants that were ionized in the cytosol would exhibit limited partitioning into internal membranes, thereby suppressing the substrate accesses to relevant enzymes. On the other hand, Reinhold et al. reported that the chemical status of phenols within the substrate–enzyme complex is very important when evaluating their transformation rates such as deprotonation of the hydroxy group at the reaction site expressed by their pKa values and Hammett’s constant (s). However, they could not show any direct evidence of a relationship between the uptake and transformation of chemicals because the latter was not actually measured in their studies. The objective of the present experiment was to examine the uptake and metabolism of phenol and its derivatives by duckweed (Lemna gibba) and to investigate the relationship between the uptake/metabolism behavior of the phenols and various chemical indexes. We also identified some of the phase II metabolites of phenol derivatives produced in duckweed. Uptake and transformation of phenols by duckweed (Lemna gibba)