Insight into the characterization of dissolved organic matter in shallow lakes with different trophic states and their net photo-generation capacity of reactive oxygen species

Abstract

Reactive oxygen species (ROS) are ubiquitous in the aquatic environment, and they are closely related to several biogeochemical processes. Dissolved organic matter (DOM) is one of the main photosensitizers involved in the formation of ROS and it also serves as a sink for ROS by involving in scavenging, quenching, and antioxidant reactions. The net effect of these processes depends on the concentration, source, and composition of the DOM. Current studies have mainly focused on the steady-state concentration of reactive oxygen species ([ROS]_ss) produced by the total DOM in lakes with different trophic states and ignored the net photo-generation capacity of ROS ([ROS]_DOM, the net steady concentration of ROS generated per unit mass of DOM), leading to a vague understanding of the photochemical properties of DOM in aquatic systems, especially in shallow lakes with different trophic states. In this study, the optical composition of DOM was determined with optical characterization, such as specific UV–Vis and excitation-emission matrices with fluorescence regional integration (FRI-EEMs), and its molecular characteristics were analyzed by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results revealed that DOM in lakes with different trophic states had mixed endogenous and exogenous characteristics, accompanied by an increasing trend in endogenous characteristics with the increasing trophic state of lakes. Spectroscopic probes were used to detect the steady-state concentration of ROS and further calculate the [ROS]_DOM, such as [³DOM*]_DOM, [^•OH]_DOM, [¹O₂]_DOM and [O₂^.-]_DOM. The results indicated that the [ROS]_DOM in lakes with light-eutrophic states was significantly higher than that in lakes with moderate-eutrophic and hyper-eutrophic states, which indicated that the DOM in lower trophic state lakes has a higher net photo-generation capacity of ROS. Pearson analysis results showed that [³DOM*]_DOM, [^•OH]_DOM, [¹O₂]_DOM and [O₂^.-]_DOM had a significant positive correlation with lignin/CRAMs-like, aromatic, and tannin compounds, as well as the fluorescence components, fulvic- and humic-like substances and the UV–Vis indicator: SUVA₂₅₄ revealed that DOM with higher humification and aromaticity had a higher net photo-generation capacity of ROS in different trophic state lakes. In addition, the molecular uniqueness of the DOM was dominated by lignin/CRAMs-like and aromatic compounds, which were positively correlated with [ROS]_DOM, in the following order: [³DOM*]_DOM > [^•OH]_DOM > [¹O₂]_DOM > [O₂^.-]_DOM. This study emphasizes the importance of focusing on the source, composition, and net photo-generation capacity of ROS by DOM, which would help evaluate the photochemical potential and other behaviors of DOM in lakes with different trophic states and provide guidance for the risk assessment of DOM input from different sources.