Hypoxia diversifies molecular composition of dissolved organic matter and enhances preservation of terrestrial organic carbon in the Yangtze River Estuary

Penghui Li; Wenzhao Liang; Yuping Zhou; Yuanbi Yi; Chen He; Quan Shi; Ding He

doi:10.1016/j.scitotenv.2023.167661

Hypoxia diversifies molecular composition of dissolved organic matter and enhances preservation of terrestrial organic carbon in the Yangtze River Estuary

Sci Total Environ. 2024 Jan 1:906:167661. doi: 10.1016/j.scitotenv.2023.167661. Epub 2023 Oct 7.

Authors

Penghui Li¹, Wenzhao Liang², Yuping Zhou³, Yuanbi Yi², Chen He⁴, Quan Shi⁴, Ding He⁵

Affiliations

¹ School of Marine Sciences, Sun Yat-sen University, Zhuhai 519082, China; Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China; Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Zhuhai 519082, China.
² Department of Ocean Science, Center for Ocean Research in Hong Kong and Macau, The Hong Kong University of Science and Technology, Hong Kong, China.
³ School of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao, China.
⁴ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Changping District, Beijing 102249, China.
⁵ Department of Ocean Science, Center for Ocean Research in Hong Kong and Macau, The Hong Kong University of Science and Technology, Hong Kong, China; State Key Laboratory of Marine Pollution, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. Electronic address: dinghe@ust.hk.

PMID: 37813254
DOI: 10.1016/j.scitotenv.2023.167661

Abstract

Dissolved organic matter (DOM) is an essential component of the global carbon cycle, and estuaries link the rivers and the oceans, thus playing important roles in land-ocean DOM transformation and transport. However, the effects of hypoxia on DOM transport and fate in estuaries and coastal oceans remains poorly understood. To address this gap, we characterized the molecular composition of DOM in bottom water (BW) and sediment porewater (PW) at hypoxic and non-hypoxic sites in the Yangtze River Estuary (YRE) using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. Our results showed significant differences in DOM molecular composition between hypoxic and non-hypoxic areas for both BW and PW. Specifically, DOM in hypoxic sites was more recalcitrant than that in non-hypoxic areas for both BW and PW, with lower H/C, and higher O/C, double bond equivalent, and modified aromaticity index. The presence of higher polyphenols, and black carbon in hypoxic areas suggested that hypoxic conditions could facilitate the preservation of terrestrial organic matter. Furthermore, we identified a much higher number of hypoxia-unique formulas than ocean-non-hypoxia-unique formulas, indicating that hypoxia could diversify the DOM pool. Within hypoxia-unique formulas for PW, both biologically labile (unsaturated aliphatic compounds and peptides) and recalcitrant formulas (carboxyl-rich alicyclic molecules) were found, suggesting that hypoxia could facilitate the preservation of labile formulas and the production of recalcitrant formulas. In addition, we formulated that the sulfurization is more important in PW than BW in hypoxic areas based on the higher dissolved organic sulfur (DOS) abundance and larger number of hypoxia-only formulas in hypoxic PW, and also the precursor analysis results. Overall, our study provides insights into the effect of hypoxia on the molecular characteristics and preservation of DOM in estuaries and coastal oceans, highlighting the importance of considering hypoxia in understanding the biogeochemical processes of these ecosystems.

Keywords: Dissolved organic matter; Hypoxia; Molecular composition; Sulfurization; Yangtze River Estuary.