Molecular composition of dissolved organic matter in saline lakes of the Qing-Tibetan Plateau

Increased knowledege of the molecular composition of dissolved organic matter (DOM) is of great importance for an improved understanding of carbon cycling in lakes. However, such information remains limited for saline lakes. Here, DOM molecular compositions and microbial community structures were investigated in saline lakes with a full range of salinity from freshwater to hypersaline. An integrated approach, including Fourier transform-ion cyclotron resonance mass spectrometry and Illumina sequencing was employed. The results showed that in the studied saline lakes, aliphatic compounds, highly unsaturated and oxygen poor/rich compounds were the dominant DOM molecules. With increasing salinity, the molecular weight and carbon contents of the DOM molecules decreased, while their oxygen contents and carbon oxidation states increased. DOM molecular composition was significantly correlated with multiple environmental variables (i.e., chlorophyll a, salinity, pH) and with microbial components. The synergy of environmental and microbial factors exhibited great importance in explaining the variation of DOM molecular composition. Comparatively, microbial factors alone gave a much better explanation than environmental factors alone of the variation of DOM molecular composition, emphasizing the tight connection between DOM molecular composition and microbial communities. Taken together, these findings help improve our understanding of the molecular composition of DOM and its influencing factors in saline lakes.