Unraveling the Molecular Composition and Reactivity Differentiation of Algae- and Macrophyte-Derived Dissolved Organic Matter in Plateau Lakes: Insights from Optical Properties and High Resolution Mass Spectrometry Characterization

Most lacustrine dissolved organic matter (DOM) still lacks comprehensive environmental sources and molecular characterization, especially in plateau lakes. Herein, macrophytes and algae from contrasting lakes of the Yunnan-Guizhou Plateau, together with Suwannee River fulvic acid (SRFA), were used to characterize the total identified DOM (Bulk-DOM) and low-molecular-weight DOM (LMW-DOM, <200 Da). To address this, we combined spectroscopy with Fourier transform ion cyclotron resonance (FT-ICR) and Orbitrap mass spectrometry (MS). Algae-derived DOM (ADOM) exhibited endogenous DOM characteristics, while macrophyte-derived DOM (MDOM) showed the characteristics of endogenous and terrigenous DOM. ADOM contained numerous heteroatoms, with high proportions of proteins, carbohydrates, and lipids. The chemical structures of ADOM were more aliphatic and degradable than that of MDOM. Conversely, MDOM and SRFA had higher degree of humification and aromaticity and showed greater resistance to microbial degradation. The capability of Orbitrap MS to characterize P-containing molecules was superior to FT-ICR MS. Moreover, significant differences were found between FT-ICR and Orbitrap MS in weighted average carbon atom number, weighted average mass-to-charge ratio, carbohydrates, and P-containing compounds. LMW-DOM accounted for approximately 10% of Bulk-DOM. Compared to Bulk-DOM, LMW-DOM was more active than Bulk-DOM because of the reduced state and more N-containing compounds. This study provides a valuable perspective to reveal the molecular characteristics and behaviors of ADOM and MDOM, which has crucial implications for carbon cycling in aquatic ecosystems.