Enhancing Molecular Change: Iron-Mediated Photochemistry Increases DOM Transformation in Tropical Peatland Canals.

Drainage and deforestation of tropical peatlands release large amounts of dissolved organic matter (DOM) and iron (Fe) in sunlit drainage canals. Rapid sunlight-driven photochemical degradation of this DOM has been shown to reintroduce millennia-old peat carbon into the active carbon cycle through greenhouse gas production. However, the impact of photochemistry on the downstream fate of DOM in drainage canals remains unknown because DOM transformation during tropical peat photodegradation has never been measured. We investigated the potential of Fe to impact the quality of photodegradation products (photoproducts) formed during sunlight exposure of DOM from unshaded tropical peatland canals in Indonesia. After exposure of canal waters to sunlight in controlled experiments, DOM was isolated by solid-phase extraction and analyzed by high-resolution liquid chromatography-tandem mass spectrometry to identify photoproduced formulas and their chemical classes. Over 2400 formulas and 380 molecular structure candidates were found using this approach, including shikimates/phenylpropanoids and terpenoid-like compounds. We found that canal waters with high dissolved Fe concentrations had the highest chemical diversity of photoproducts, even compared to canal waters exposed to larger doses of light. Our results highlight the importance of Fe and related photo-Fenton reactions in impacting the nature of photoproducts, with potential consequences for downstream DOM lability and mineralization in tropical peatland canals.