Zhen Zhou

Ethane and propane are short‐chain alkanes that are ubiquitous in marine environments, while the biological formation of ethane and propane remains largely unconstrained in coastal subsurface sediments. Here, we detected nanomolar concentrations of ethane and propane along 2.7–5.7 m sediment cores with different geochemical gradients at coastal sites of the East China Sea. We further conducted sediment incubation experiments and demonstrated that diethyl sulfide, bromoethane, ethanethiol, ethanol, and acetate were potential precursors for ethane biosynthesis, while propane could be produced from propanol and propanethiol. Ethane production was influenced by the change in temperature, substrate concentration, and H ₂ partial pressure. Particularly, the addition of 80% H ₂ in the headspace resulted in a 75‐fold increase in ethane production, suggesting the involvement of H ₂ in ethanogenesis. The addition of methanogenesis inhibitor, 2‐bromoethanesulfonate, completely terminated the activity of methanogens and also inhibited ethane production. Taxonomic profiling of the ethane‐producing enrichment suggested that methanogens, primarily from the orders Methanomicrobiales , Methanosarcinales , and Methanococcales , might be involved in the formation of trace amounts of ethane, which was probably achieved via the proposed intermediate of CH ₃ CH ₂ ‐S‐CoM in a similar pathway for methane production. These results reveal the potential role of methanogens in ethane production and provide insights into the biogeochemical cycling of hydrocarbons in coastal sediments.

Carbon mobilization and sulfur transformation play a significant role in deep carbon and sulfur cycling. However, sulfur biogeochemistry and its coupling with carbon and iron cycling remain poorly constrained in hydrothermal sediments. We investigated the effect of temperature on carbon‐sulfur‐iron diagenesis in subsurface sediments (≤370 m depth) of the Guaymas Basin, Gulf of California. Sediments have a low average carbon‐to‐sulfur ratio (∼1.6), especially at hydrothermal active sites (1.3–0.67). These values are well below the typical value of 2.8 for marine sediment and could reflect carbon release and relative sulfur enrichment. Elevated temperatures accelerate the thermal breakdown of sedimentary organic matter, leading to hydrocarbon expulsion, increased dissolved organic carbon release, and ultimately carbon loss. Elemental sulfur content correlates positively with reactive iron, indicating iron oxides facilitate elemental sulfur accumulation. These results highlight the temperature effect on carbon storage and iron redox chemistry control on sulfur dynamics in hydrothermal systems.

Methylotrophic methanogenesis has recently been recognized as a key process driving cryptic methane cycling within sulfate‐reducing sediments. Here we conducted biogeochemical analyses of methanogenic substrates, activity, and communities in two sediment cores (4–5 m) from the East China Sea, to constrain the dynamics and controls of methylotrophic methanogenesis in coastal sediments. We detected micromolar concentrations of methane in the presence of sulfate and high methane concentrations (up to 4.2 mM) below the sulfate–methane transition zone (~ 150–170 cm). Stable isotope composition of methane was strongly depleted (−77‰ to −91‰), indicating its biological production. Methanogenic substrates including H ₂ /CO ₂ , acetate, and methylated compounds were detected in the porewaters and/or sediments. Radiotracer experiments indicated methane production from various substrates, and the presence of sulfate did not inhibit methanogenesis at either site. At the coastal site with the dominance of marine organic matter (total organic carbon: 0.6%; C/N ratio: ~ 6.4; δ ¹³ C‐total organic carbon: −22‰), methane was primarily produced from hydrogenotrophic methanogenesis, consistent with the progressive enrichment of ¹³ C in dissolved inorganic carbon with depth below the sulfate–methane transition zone. However, methylotrophic methanogenesis from methanol and trimethylamine contributed significantly to methane production (up to 30.2%) at the estuarine site (total organic carbon: 0.5%; C/N ratio: 7.4, δ ¹³ C‐total organic carbon: −23‰) with elevated terrestrial organic matter input, also reflected from the predominance of long chain odd carbon n‐alkanes. These findings suggested that organic carbon source and composition, instead of sulfate, control methanogenic activity, providing evidence that high terrestrial organic inputs could significantly enhance methylotrophic methanogenesis in coastal sediments.

Cold seeps are unique deep‐sea ecosystems that play an important role in marine biogeochemical cycles. However, the dynamics and regulation of sulfur isotope biogeochemistry in seep sediments remain unconstrained. We investigated the geochemical characteristics of active seep sites, including the content and stable isotope composition of sulfur species and sulfate reduction (SR) rates. The S‐shaped downcore distribution of sulfate suggested non‐steady state diagenesis induced by lateral advection of venting fluids. ³⁴ S‐enriched pyrite (−7.3‰) was found in surface sediment, suggesting the low sulfur isotope fractionation, calculated between 30 and 35‰. High estimates of cell‐specific sulfate reduction rates, 18–69 fmol cell ⁻¹ day ⁻¹ , together with data collected from previous studies, confirmed that elevated SR activity at seeps contributed to smaller sulfur isotope fractionation (11.5–35‰) than found typically in shelf sediments (44.5–66.5‰), leading to ³⁴ S‐enriched pyrite (∼–10‰). These results provided insights into sulfur isotope biogeochemistry in cold seep sediments.

Methanol metabolism is important in marine carbon cycling, while the control of metabolic mode and activity remains poorly constrained, particularly in coastal waters. Here, we investigated the biogeochemical cycling of methanol across the estuary–coast–shelf continuum of the East China Sea. Methanol concentrations varied from < 12.3 to 617.3 nmol L ⁻¹ , decreasing from coast to shelf. The total uptake rates of methanol in the estuary (143.4 ± 29.6 nmol L ⁻¹ d ⁻¹ ) and coast (92.3 ± 86.9 nmol L ⁻¹ d ⁻¹ ) were much higher than those in the shelf (20.4 ± 33.8 nmol L ⁻¹ d ⁻¹ ). While methanol was primarily used as an energy source in the coast and shelf (oxidation/total uptake: > 72.2%), much more methanol was channeled to the biomass (assimilation/total uptake: 31.9–49.7%) in the estuarine waters. As supported by incubation experiments with the addition of nutrients and other C1 compounds, enhanced methanol assimilation suggests the use of methanol as a supplementary source of carbon, while reduced methanol oxidation indicated the competitive utilization of other C1 compounds as an energy source. The estuary and coastal waters acted as net sources of methanol to the atmosphere, while ventilation accounted for < 5.7% of total loss, suggesting that microbial consumption was the dominant pathway of methanol removal. In the shelf, atmospheric deposition replenished surface methanol pools and contributed up to 17.5% ± 16.9% of microbial consumption. These results provided insights into the control of methanol metabolism and assessed the relative importance of different methanol sources and sinks along the estuary–coast–shelf continuum.

Microbial acetate metabolism is an important part of marine carbon cycling. We present a comprehensive study to constrain microbial acetate metabolism and its regulation in surface seawater of the northwest Pacific Ocean. We found that acetate oxidation (rate constant k: 0.016–0.506 day⁻¹) accounted for 77.6%–99.4% of the total microbial acetate uptake, suggesting that acetate was predominantly used as a microbial energy source. Acetate also served as a significant biomass carbon source, as reflected by the elevated contribution of acetate assimilation to bacterial carbon production. Acetate turnover was largely influenced by water mass mixing and nutrient conditions. Atmospheric deposition was a source of acetate in surface water and this process can also impact the microbial acetate uptake. Microbial utilization of acetate could account for up to 25.9% of the bacterial carbon demand, suggesting the significant role of acetate metabolism in microbial carbon cycling in the open ocean.

Methanol metabolism can play an important role in marine carbon cycling. We made contemporaneous measurements of methanol concentration and consumption rates in the northwest Pacific Ocean to constrain the pathways and dynamics of methanol cycling. Methanol was detected in relatively low concentrations (89.7% of methanol was utilized as an energy source. Surface water acted as a net methanol sink at most sites, with an average flux of 9 μmol L⁻¹ day⁻¹. Atmospheric deposition accounted for 22.7% of microbial methanol consumption in the mixed layer, illustrating that the atmosphere is less important than internal processes for driving methanol cycling in these pelagic waters.