Deciphering the composition and degradation of dissolved organic matter in a large subtropical river using optical indices and high-resolution FT-ICR-MS.

Dissolved organic matter (DOM) is very important in aquatic environments, yet it is challenging to characterize DOM as a highly complex mixture of thousands of molecules, and the knowledge of the effects of different degradation processes on different molecules remains limited. This study examined the distribution and degradation of DOM in a large subtropical river using optical techniques and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). At the molecular level, DOM was mainly composed of CHO and lignin-like compounds, which was related to the dominance of forestland in the watershed and resulted in a low biological lability index (MLB). The modified aromaticity index (AI), unsaturation degree (DBE), and humic content (HIX) decreased while MLB, H/C, absorption spectral slope (S), and biological index (BIX) increased in the estuarine zone due to the increasing autochthonous contribution. Photo- and microbial degradation resulted in a similar decrease in the bulk dissolved organic carbon, while they showed opposite effects on the DOM composition. Photo-degradation removed all fluorescent components and decreased molecular weight, HIX, AI, DBE, %CHO, %lignin-like, %tannin-like, and %condensed aromatic-like compounds. In contrast, bio-degradation preferentially consumed lipid-like, protein-like, and carbohydrate-like compounds, with increases in %lignin-like, %tannin-like, %condensed aromatic-like compounds, and humic-like fluorescent components. Overall, the application of ultra-high resolution mass spectrometry provided valuable insights into the composition and behavior of DOM at the molecular level and revealed the contrasting effects of photo- and microbial degradation on different compounds. These results have implications for better understanding the composition and transformation of aquatic DOM.