Photochemical fate of sulfadimethoxine in aquaculture waters.

Sulfadimethoxine (SDM) is an antibiotic often used in combination with ormetoprim to prevent the spread of disease in freshwater aquaculture. It is known to undergo photochemical degradation in natural sunlit surface waters, but the role of dissolved organic matter (DOM) in this process is poorly understood. Our results show that water from a eutrophic catfish pond at the Mississippi State University Delta Research and Extension Center facility in Stoneville, MS facilitates the rapid phototransformation of SDM. In contrast, water from a nearby stream (Deer Creek) whose DOM is derived from allochthonous precursors does not enhance SDM photodegradation. We attribute these disparate results to DOM composition, whereby dissolved organic matter originating from highly eutrophic water bodies is a better SDM photosensitizer. Experiments conducted concurrently using respective autochthonous (Pony Lake, Antarctica) and allochthonous (Suwannee River) derived fulvic acids corroborate these findings. Scavenging experiments and experiments conducted anoxically show that the main indirect photodegradation pathway occurs by triplet excited-state DOM oxidation. Finally, transformation products assayed by mass spectrometry reveal the same major SDM photoproducts in the presence and absence of dissolved organic matter.