Molecular traces reveal divergent sulfur organic matter evolution in hypoxic Yangtze Estuary

In the turbid waters where the Yangtze River meets the East China Sea, one of the ocean’s least understood chemical currencies is being transformed in ways scientists are only now beginning to decode. Dissolved organic sulfur, or DOS, is a vast reservoir of sulfur bound within organic molecules, and it plays a pivotal role in linking the global carbon and sulfur cycles. A new study published in the journal Biogeochemistry reveals that the fate of this overlooked pool of molecules depends critically on the path that river water takes after it leaves the estuary, and even more strikingly, on whether that water can breathe. The research, led by Huiying Ma, Ruochun Zhang, Si-Liang Li and colleagues at Tianjin University, together with Ding He’s group at The Hong Kong University of Science and Technology, provides the first direct molecular evidence that oxygen-starved waters actively forge new sulfur-containing organic compounds, a process known as sulfurization that may have profound consequences for how carbon is stored or released in coastal seas.

The team’s central question was deceptively simple: does dissolved organic sulfur evolve the same way regardless of where river plume water travels? The Yangtze River, Asia’s largest, discharges an enormous plume of fresh, nutrient-rich water into the East China Sea. This plume does not spread uniformly. It bifurcates into two distinctive branches. One extends northward into the open shelf, a pathway the researchers designated YEN, for Yangtze River Estuary Northward. The other hugs the coastline southward, a more confined route labeled YES, for Yangtze River Estuary Southward. Because both branches originate from the same river, they provide a natural experiment: two water masses with shared origins but different journeys, different salinity regimes, different light exposures, and critically, different oxygen histories.

To track the chemical evolution of dissolved organic sulfur along these two routes, the researchers combined conventional concentration measurements with one of the most powerful analytical tools available to modern organic geochemistry: ultrahigh-resolution mass spectrometry, specifically Fourier transform ion cyclotron resonance mass spectrometry, capable of resolving thousands of individual molecular formulas within a single seawater sample. Because dissolved organic matter in seawater is extraordinarily dilute and chemically complex, the team first concentrated the organic material using solid-phase extraction cartridges, which adsorb molecules onto a polymer resin before they are eluted and analyzed. This approach allowed them to characterize not just how much dissolved organic sulfur was present, but what kinds of sulfur-bearing molecules it comprised.

The concentration data told the first part of the story. As expected, dissolved organic sulfur concentrations declined as the plume water mixed with open-ocean seawater, a pattern consistent with dilution and removal processes. Yet the decline was not uniform between the two pathways. Water traveling the northward offshore branch consistently carried higher dissolved organic sulfur concentrations than water moving along the southward coastal route at comparable salinities. This divergence hinted that the two plume branches were not simply mixing passively with the ocean; they were undergoing fundamentally different chemical processing.

The molecular-level analysis sharpened that picture considerably. In the high-salinity zone of the northward branch, dissolved organic sulfur molecules displayed significantly lower aromaticity, meaning they contained fewer ring-structured, light-absorbing carbon frameworks, and higher oxygen-to-carbon ratios compared with molecules from the southward coastal branch. These signatures point to two distinct processing regimes. The offshore branch experiences greater exposure to sunlight, where photochemical reactions tend to break down aromatic structures and oxidize organic molecules, adding oxygen atoms and stripping away the complex ring systems that characterize terrestrial plant-derived material. The confined coastal branch, by contrast, remains turbid and light-limited, so microbial processing dominates, leaving a different molecular imprint. The result is two chemically distinguishable dissolved organic sulfur pools emerging from a single river source, shaped by the interplay of photochemistry and microbial metabolism along divergent hydrographic trajectories.

The most consequential discovery, however, lay beneath the surface. In the subsurface waters of the northward branch, where oxygen levels drop into hypoxic territory, the researchers documented a marked increase in two things simultaneously: the overall molecular diversity of the dissolved organic matter pool, and the relative abundance of sulfonated molecular formulas, molecules in which sulfur is bonded to carbon within sulfonate groups. This co-occurrence provides direct molecular evidence that hypoxia is not merely a passive backdrop but an active facilitator of sulfurization, the chemical incorporation of sulfur into organic molecules.

The mechanism likely centers on hydrogen sulfide and other reduced sulfur species that accumulate when oxygen depletion allows anaerobic microbial respiration to use sulfate as an electron acceptor. Under oxic conditions, such reduced sulfur species are rapidly oxidized or scavenged by iron minerals. But when oxygen vanishes, these reactive sulfur intermediates persist and can react with dissolved organic matter, attaching sulfur atoms to carbon skeletons through abiotic reactions such as electrophilic addition and nucleophilic substitution, or through microbial pathways that produce sulfur-containing metabolites. The resulting sulfurized molecules are often more chemically resistant to degradation, which means hypoxia may effectively lock carbon into more persistent forms, altering how long organic carbon remains sequestered in the ocean before returning to the atmosphere as carbon dioxide.

This finding carries implications well beyond the Yangtze. Hypoxic zones, often called dead zones, are expanding worldwide as coastal waters are fertilized by agricultural runoff and warming waters hold less dissolved oxygen. The East China Sea hosts one of the largest seasonal hypoxic zones on the planet, fed in part by the Yangtze’s nutrient discharge. If oxygen depletion systematically transforms dissolved organic matter into sulfurized, potentially refractory molecules, then the global rise of coastal hypoxia could be reshaping the marine carbon cycle in a way that current models, which largely treat dissolved organic matter as a uniform pool, fail to capture. Sulfurized organic matter may also influence trace metal binding, microbial food web dynamics, and the production and cycling of climatically active sulfur gases such as dimethyl sulfide, which contributes to cloud-forming aerosols.

The study also underscores the importance of plume trajectory as a control on biogeochemical processing. Estuarine and shelf models frequently treat a river plume as a single mixing entity, but the Yangtze data demonstrate that the same source water can yield chemically distinct dissolved organic sulfur pools depending on which pathway it follows. The northward offshore route, with its photic exposure and subsurface hypoxia, promotes oxidative transformation at the surface and sulfurization at depth. The southward coastal route, confined and turbid, produces a molecularly different signature dominated by microbial reworking. Understanding where water goes, and for how long it remains in low-oxygen conditions, may therefore be as important as knowing how much organic matter the river delivers.

Technically, the study’s strength lies in pairing bulk measurements with molecular formula assignments from ultrahigh-resolution mass spectra. Each detected formula, containing carbon, hydrogen, oxygen, nitrogen, and sulfur, can be classified by its elemental ratios and structural indices such as the aromaticity index, allowing researchers to distinguish, for example, a highly unsaturated phenolic compound from a saturated sulfonated fatty acid derivative. By tracking how the relative abundances of these molecular classes shift along salinity and oxygen gradients in each plume branch, the team constructed what amounts to a molecular movie of dissolved organic sulfur evolution, revealing transformations that bulk concentration measurements alone could never resolve.

The authors, whose affiliations span Tianjin University, The Hong Kong University of Science and Technology, The Chinese University of Hong Kong, and the Sanya Joint Laboratory of Marine Science Research, frame their results as establishing plume trajectory and oxygen availability as key regulators of dissolved organic sulfur composition and transformation pathways. They propose their findings as a mechanistic framework for sulfur-carbon coupling in large river estuaries and marginal seas, one that can be tested and refined in other major river-ocean systems, from the Mississippi to the Amazon, where plumes, stratification, and hypoxia intersect.

As coastal oceans continue to warm and oxygen depletion intensifies, the chemical ghost story written in the Yangtze plume may prove to be a preview of a more widespread phenomenon: an ocean in which the molecules carrying carbon and sulfur through the marine food web are increasingly forged in the dark, oxygen-poor depths of expanding dead zones. Deciphering those molecular signatures, this study suggests, will be essential to forecasting the future of the coastal carbon sink and the global sulfur cycle alike.