Rural drainage ditches are widely distributed and function as important sinks for heavy metals and antibiotics, whereas, effects of the combined pollution on electron behaviors in nitrogen (N) transformation in ditch wetlands remain unclear.

Researchers from Wuhan Botanical Garden conducted an 84-day study to investigate the combined effects of cadmium and doxycycline on N removal performance and transformation processes (nitrification, denitrification, anammox, and nitrous oxide (N₂O) production) and the underlying mechanisms in constructed ditch wetlands.

This study indicated that combined cadmium and doxycycline pollution decreased unamended denitrification and anaerobic ammonia oxidation rates but increased the N₂O production rate. Microbial metabolic activities, especially glucokinase and glyceraldehyde-3-phosphate dehydrogenase were inhibited by combined pollution. Long-term exposure to combined pollutants destroyed the electron transport system activity (ETSA), attenuated nitrate and nitrite reductase activities, and inhibited the denitrifying gene expression. ETSA and nicotinamide adenine dinucleotide affected unamended denitrification, anaerobic ammonia oxidation, and N₂O production rates both directly and indirectly by affecting nitrate reductase and nitrite reductase activities.

This study is an important advancement in the study of ecotoxicology and provides new insights into the mechanisms by which combined pollution alters N transformation from the view of microbial electron behaviors (electron production, transfer, and consumption).

This work was supported by the National Natural Science Foundation of China. Associate professor MA Lin and Professor LIU Wenzhi are the first author and the corresponding author, respectively. The relevant research results have been published in Environmental Chemistry Letters entitled “Effect of combined cadmium and doxycycline pollution on the nitrogen cycle in constructed ditch wetlands”.