Photodegradation of ormetoprim in aquaculture and stream-derived dissolved organic matter.

Ormetoprim (OMP) is an antibiotic approved for use in the United States to prevent the spread of disease in freshwater aquaculture. It has been shown in the previous literature to be photochemically stable to direct photolysis, but the role of photosensitization processes in the presence of dissolved organic matter (DOM) on the rate of degradation is not well understood. The present results show that water and DOM (specifically the fulvic acid fraction) isolated from a eutrophic aquaculture catfish pond and a nearby stream (Deer Creek) at the Mississippi State University Delta Research and Extension Center facility in Stoneville, MS, significantly increased the phototransformation of OMP relative to direct photolysis. Similar results were reported for reference fulvic acids obtained from the International Humic Substances Society. Results from a combination of scavenging experiments and experiments conducted under anoxic conditions indicate the indirect photodegradation pathway occurs by hydroxyl radical, singlet oxygen attack, and reaction with triplet excited-state DOM.