UC Riverside

Contaminants of emerging concern (CECs) are ubiquitous in agroecosystems and aquatic environments. Transformations of CECs occur via biotic and abiotic pathways, resulting in the co-existence of CECs with many transformation products (TPs). Small changes in a chemical’s structure, such as the addition or loss of a methyl group caused by methylation or demethylation, may bring significant alterations to its physicochemical properties, and further environmental behaviors. However, changes induced by methylation and demethylation in bioaccumulation, persistence in the environment, and toxicological effects of CECs are inadequately understood. Information about the occurrence of methylation and demethylation in higher plants and aquatic organisms is limited. In the first study of this dissertation research, the influence of methylation and demethylation on the uptake of four CECs (acetaminophen, diazepam, methylparaben and naproxen) and their methylated or demethylated TPs was characterized using two plant models - A. thaliana cell culture and hydroponically grown wheat seedlings. Results showed that methylation generally increased a chemical’s hydrophobicity, leading to increased uptake and accumulation in both plant models, as well as greater translocation in wheat seedlings. The second study considered the occurrence of methylation and demethylation in plants after uptake, and results showed that demethylation was generally more extensive than methylation. The rate of demethylation or methylation was dependent on the bond strength of R-CH3, with demethylation of methylparaben and methylation of acetaminophen more pronounced than the other compounds. In the third study, changes in the bioaccumulation of and acute toxicity to an aquatic invertebrate, Daphnia magna, were further characterized as the result of methylation or demethylation. Methylation of CECs generally enhanced their acute toxicity, which was attributed to increased hydrophobicity. Greater bioaccumulation of methylated counterparts was concurrently observed to support this conclusion. Demethylation occurred in D. magna at different rates for different CECs and their TPs, indicating differences in the level of activity of the involved enzymes. Results from this dissertation research underline the environmental significance of simple, commonly occurring transformation reactions such as methylation and demethylation, and highlight the need to consider TPs for a more holistic understanding of the environmental fate and risks of CECs.