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Methane is a potent greenhouse gas; methane production and consumption within seafloor sediments has generated intense interest. Anaerobic oxidation of methane (AOM) and methanogenesis (MOG) primarily occur at the depth of the sulfate-methane transition zone or underlying sediment respectively. Methanogenesis can also occur in the sulfate-reducing sediments through the utilization of non-competitive methylated compounds; however, the occurrence and importance of this process are not fully understood. Here, we combined a variety of data, including geochemical measurements, rate measurements and molecular analyses to demonstrate the presence of a cryptic methane cycle in sulfate-reducing sediments from the continental shelf of the northern South China Sea. The abundance of methanogenic substrates as well as the high MOG rates from methylated compounds indicated that methylotrophic methanogenesis was the dominant methanogenic pathway; this conclusion was further supported by the presence of the methylotrophic genus Methanococcoides. High potential rates of AOM were observed in the sediments, indicating that methane produced in situ could be oxidized simultaneously by AOM, presumably by ANME-2a/b as indicated by 16S rRNA gene analysis. A significant correlation between the relative abundance of methanogens and methanotrophs was observed over sediment depth, indicating that methylotrophic methanogenesis could potentially fuel AOM in this environment. In addition, higher potential rates of AOM than sulfate reduction rates at in situ methane conditions were observed, making alternative electron acceptors important to support AOM in sulfate-reducing sediment. AOM rates were stimulated by the addition of Fe/Mn oxides, suggesting AOM could be partially coupled to metal oxide reduction. These results suggest that methyl-compounds driven methane production drives a cryptic methane cycling and fuels AOM coupled to the reduction of sulfate and other electron acceptors.

Volatile fatty acids (VFAs) and alcohols are key intermediates of anaerobic carbon metabolism, yet their biogeochemical cycling remains poorly constrained in hydrothermal systems. We investigated the abundance, stable carbon isotopic composition, and metabolic cycling of VFAs and alcohols to elucidate their generation and utilization pathways in hydrothermally influenced sediments (4 °C to 90 °C) from the Guaymas Basin. Acetate (up to 229 μM) and methanol (up to 37 μM) were abundant in porewaters. The δ13C values of acetate varied between −35.6‰ and −18.1‰. Carbon isotopic signatures, thermodynamic predictions, and experimental incubations suggested biological sources such as fermentation and acetogenesis for acetate. Acetate and methanol were predominantly consumed by nonmethanogenic processes (e.g., sulfate reduction), as reflected in high oxidation rates versus low methanogenesis rates, and further evidenced through inhibition experiments with molybdate. These results reveal an important role for VFAs and alcohols as energy sources for diverse chemoheterotrophs in organic-rich hydrothermally influenced sediments.

Isotopic compositions of coeval sulfide and sulfate have been widely employed to reconstruct the seawater chemistry evolution over geologic time; however, these signals can be modulated by other factors, such as the sedimentation rate. Here, we present a data set of pyrite sulfur isotopes (δ34Spy) derived from 60-m drilled core sediments deposited on the inner shelf of the East China Sea since 16.5 ka. The resulting δ34Spy values range from −38.2 to 15.0‰ (Vienna Canyon Diablo Troilite; V-CDT), representing a range of 53.2‰. Freshwater deposition before 12.3 ka produced a limited pyrite abundance and δ34Spy values that fall within the typical range of freshwater environments. After the marine incursion in the study area at 12.3 ka, variations in the δ34Spy values become significantly correlated with sedimentation rates (SRs; r = 0.78, p 0‰), suggesting that SRs exert controls on δ34Spy values. We propose that these sedimentary controls are expressed by modulating the connectivity between porewaters and the overlying seawater. Low SRs tend to form an open diagenetic system, in which the pyrites are able to preserve the large biological fractionations associated with microbial sulfate reduction (and disproportionation). In contrast, high SRs favour a restricted diagenetic system, in which the distillation of porewater sulfate produces pyrites enriched in 34S, masking the magnitude of biological fractionation during microbial sulfur cycling. These findings highlight the critical effect of the local sedimentation regime on pyrite sulfur isotopic compositions, especially within shallow depositional environments, over geologic time.

Low molecular weight (LMW) compounds are key intermediates in organic matter degradation and their metabolism supports a diverse microbial community in anoxic sediments. Much remains to be learned about the metabolic pathways and turnover rates of LMW compounds as well as the factors that influence their metabolism in the environment. We studied the microbial metabolism of acetate, methanol and methylamine in sediments from Northern Gulf of Mexico to elucidate their role as energy and carbon sources for microorganisms mediating terminal metabolic processes and to constrain the importance of C1 and C2 metabolism in the sedimentary carbon cycle. In terms of carbon flow, methanogenesis was a minor process in sulfate-rich sediments; acetate and methanol were oxidized primarily by non-methanogenic heterotrophs as an energy source. A large fraction of utilized acetate (20–76%) was assimilated into biomass, suggesting that acetate was a significant biomass-building source. In situ thermodynamic calculations suggested the utilization of acetate and methanol by sulfate-reducing bacteria was more favorable than the utilization by methanogens, consistent with the observed higher oxidation rates than methanogenesis rates as well as results from inhibitor experiments employing 2-bromoethanesulfonate and molybdate. Methylamine, by contrast, was utilized for methane production (up to 100%), but its utilization was not limited to methanogens. At an organic-poor deep-sea site, elevated acetate and methanol oxidation rates in the absence of sulfate-reducing and methanogenic activity indicated that metabolism of LMW compounds was coupled to other electron accepting processes, such as denitrification. Heterotrophic carbon assimilation was an important pathway for generating biomass at this site, as evidenced by comparable rates of acetate assimilation to bicarbonate incorporation. The oxidation and assimilation rates of LMW compounds generally decreased with sediment depth, following the trend of organic carbon degradation rates. Environmental factor, such as temperature but not pressure, affected the metabolism of LMW compounds; the oxidation of acetate, methanol and methylamine exhibited distinct responses to changes in temperature. In seep sediments, the oxidation of acetate and methanol accounted for 5% and 0.9%, respectively, of the total organic carbon oxidation. Collectively, this assessment of C1 and C2 LMW compound utilization shows that these compounds serve as energetic substrates to fuel methanogens, sulfate reducers and other microbes.

Dimethylated sulfur compounds play an important role in global sulfur cycle. We investigated the seasonality of dimethylsulfoniopropionate (DMSP), dimethylsulfoxide (DMSO), dimethylsulfide (DMS) and associated processes in two north China marginal seas during 2014 and 2016. High concentrations of DMS, DMSP and DMSO occurred in summer/spring, while the lowest were observed in winter. This clear seasonality was primarily driven by biomass abundance and phytoplankton communities, reflected in chlorophyll a concentrations and the composition/ratios of diatoms and dinoflagellates. The spring maximum was attributed to the annual occurrence of algal bloom. The sea-to-air fluxes of DMS also varied largely between seasons, with an average of 8.84, 11.87, 10.50 and 2.14 μmol m−2 day−1 in spring, summer, autumn and winter, respectively. Given the seasonal uncertainty of sea-to-air flux, the seasonality or situations where specific blooms occur regularly should be considered for accurate estimation of annual global DMS emission.

The spatial distributions, fluxes, and environmental effects of non-methane hydrocarbons (NMHCs) were investigated in the Yellow Sea (YS) and the East China Sea (ECS) in spring. The average concentrations of ethane, propane, i-/n-butane, ethylene, propylene and isoprene in the seawater were 18.1 ± 6.4, 15.4 ± 4.7, 6.8 ± 2.9, 6.4 ± 3.2, 67.1 ± 26.7, 20.5 ± 8.7 and 17.1 ± 11.1 pmol L−1, respectively. The alkenes in the surface seawater were more abundant than their saturated homologs and NMHCs concentrations (with the exception of isoprene) decreased with carbon number. The spatial variations of isoprene were consistent with the distributions of chlorophyll a (Chl-a) and Chaetoceros, Skeletonema, Nitzschia mainly contributed to the production of isoprene, while the others’ distributions might be related to their photochemical production. Observations in atmospheric NMHCs indicated alkanes in the marine atmosphere decreased from inshore to offshore due to influence of the continental emissions, while alkenes were largely derived from the oceanic source. In addition, no apparent diurnal discrepancy of atmospheric NMHCs (except for isoprene) were found between daytime and night. As the main sink of NMHCs in seawater, the average sea-to-air fluxes of ethane, propane, i-/n-butane, ethylene and propylene were 31.70, 29.75, 18.49, 15.89, 239.6, 67.94 and 52.41 nmol m−2 d−1, respectively. The average annual emissions of isoprene accounted for 0.1–1.3% of the global ocean emissions, which indicated that the coastal and shelf areas might be significant sources of isoprene. Furthermore, this study represents the first effort to estimate the environmental effects caused by NMHCs over the YS and the ECS and the results demonstrated contributions of alkanes to ozone and secondary organic aerosol (SOA) formation were lower than those of the alkenes and the largest contributor was isoprene.

International Ocean Discovery Program (IODP) Expedition 385 drilled organic-rich sediments with sill intrusions on the flanking regions and in the northern axial graben in Guaymas Basin, a young marginal rift basin in the Gulf of California. Guaymas Basin is characterized by a widely distributed, intense heat flow and widespread off-axis magmatism expressed by a dense network of sill intrusions across the flanking regions, which is in contrast to classical mid-ocean ridge spreading centers. The numerous off-axis sills provide multiple transient heat sources that mobilize buried sedimentary carbon, in part as methane and other hydrocarbons, and drive hydrothermal circulation. The resulting thermal and geochemical gradients shape abundance, composition, and activity of the deep subsurface biosphere of the basin. Drill sites extend over the flanking regions of Guaymas Basin, covering a distance of ~81 km from the from the northwest to the southeast. Adjacent Sites U1545 and U1546 recovered the oldest and thickest sediment successions (to ~540 meters below seafloor [mbsf]; equivalent to the core depth below seafloor, Method A [CSF-A] scale), one with a thin sill (a few meters in thickness) near the drilled bottom (Site U1545), and one with a massive, deeply buried sill (~356–430 mbsf) that chemically and physically affects the surrounding sediments (Site U1546). Sites U1547 and U1548, located in the central part of the northern Guaymas Basin segment, were drilled to investigate a 600 m wide circular mound (bathymetric high) and its periphery. The dome-like structure is outlined by a ring of active vent sites called Ringvent. It is underlain by a remarkably thick sill at shallow depth (Site U1547). Hydrothermal gradients steepen at the Ringvent periphery (Holes U1548A–U1548C), which in turn shifts the zones of authigenic carbonate precipitation and of highest microbial cell abundance toward shallower depths. The Ringvent sill was drilled several times and yielded remarkably diverse igneous rock textures, sediment–sill interfaces, and hydrothermal alteration, reflected by various secondary minerals in veins and vesicles. Thus, the Ringvent sill became the target of an integrated sampling and interdisciplinary research effort that included geological, geochemical, and microbiological specialties. The thermal, lithologic, geochemical, and microbiological contrasts between the two deep northwestern sites (U1545 and U1546) and the Ringvent sites (U1547 and U1548) form the scientific centerpiece of the expedition. These observations are supplemented by results from sites that represent attenuated cold seepage conditions in the central basin (Site U1549), complex and disturbed sediments overlying sills in the northern axial trough (Site U1550), terrigenous sedimentation events on the southeastern flanking regions (Site U1551), and hydrate occurrence in shallow sediments proximal to the Sonora margin (Site U1552). The scientific outcomes of Expedition 385 will (1) revise long-held assumptions about the role of sill emplacement in subsurface carbon mobilization versus carbon retention, (2) comprehensively examine the subsurface biosphere of Guaymas Basin and its responses and adaptations to hydrothermal conditions, (3) redefine hydrothermal controls of authigenic mineral formation in sediments, and (4) yield new insights into many geochemical and geophysical aspects of both architecture and sill–sediment interaction in a nascent spreading center. The generally high quality and high degree of completeness of the shipboard datasets present opportunities for interdisciplinary and multidisciplinary collaborations during shore-based studies. In comparison to Deep Sea Drilling Project Leg 64 to Guaymas Basin in 1979, sophisticated drilling strategies (for example, the advanced piston corer [APC] and half-length APC systems) and numerous analytical innovations have greatly improved sample recovery and scientific yield, particularly in the areas of organic geochemistry and microbiology. For example, microbial genomics did not exist 40 y ago. However, these technical refinements do not change the fact that Expedition 385 will in many respects build on the foundations laid by Leg 64 for understanding Guaymas Basin, regardless of whether adjustments are required in the near future.

Gypsum (CaSO4 center dot 2H(2)O) precipitated in nonevaporative marine environments is rarely reported, and the related diagenetic process and its implications for the sulfur cycle (e.g., sulfide reoxidation) are still not well known. We employ coupled sulfur and oxygen isotopes of authigenic gypsum, as well as sulfur isotopic compositions of pyrite, from core EC2005, which is drilled on the inner shelf of the East China Sea, to constrain the sulfur and oxygen sources for gypsum precipitation. The delta S-34(gypsum) values range from -24.8 to -12.2 parts per thousand, which are much more depleted in S-34 than modem seawater sulfate, suggesting additional S-34-depleted sulfur sources, i.e., coexisting pyrites produced by microbial sulfate reduction. According to a mixing model for paired sulfur isotopes of gypsum and pyrite, > 70% of gypsum sulfur is derived from pyrite reoxidation. The delta O-18(gypsum) values, ranging from 1.1 to 4.6 parts per thousand, suggest that the oxygen isotopes of porewater sulfate should fall between -2.4 and 1.1 parts per thousand, which are more depleted than seawater sulfate in O-18. Two mechanisms are proposed to explain the reoxidation of pyrite at different depths: in the lower section (10-4 m; Unit II), pyrite is oxidized by metal oxides under anaerobic conditions, producing O-18-depleted oxygen from seawater; while in the upper section (4-0 m; Unit I), pyrite reoxidation may occur under aerobic conditions, involving atmospheric oxygen. Either the aerobic or anaerobic mechanisms could cause drops in the porewater pH, leading to carbonate dissolution. These processes could therefore elevate the concentrations of sulfate and calcium ions in porewater, in which authigenic gypsum crystals are precipitated. In addition, both sulfur and oxygen isotopic signals of gypsum indicate a possible influence of methane on sulfate reduction in the mud sediments of core EC2005, which may be related to biogenic gas (dominated by CH4) in this region. Our new findings thus indicate that sulfide reoxidation in marine sediments plays an important role in the biogeochemical sulfur cycle on the continental margin.

Sulfur cycling in the biosphere is tightly interwoven with the cycling of carbon and nitrogen, through various biological and geochemical processes. Marine microorganisms, due to their high abundance, diverse metabolic activities, and tremendous adaptation potential, play an essential role in the functioning of global biogeochemical cycles and linking sulfur transformation to the cycling of carbon and nitrogen. Currently many coastal regions are severely stressed by hypoxic or anoxic conditions, leading to the accumulation of toxic sulfide. A number of recent studies have demonstrated that dissimilatory sulfur oxidation by heterotrophic bacteria can protect marine ecosystems from sulfide toxicity. Sulfur-oxidizing bacteria have evolved diverse phylogenetic and metabolic characteristics to fill an array of ecological niches in various marine habitats. Here, we review the recent findings on the microbial communities that are involved in the oxidation of inorganic sulfur compounds and address how the two elements of sulfur and carbon are interlinked and influence the ecology and biogeochemistry in the ocean. Delineating the metabolic enzymes and pathways of sulfur-oxidizing bacteria not only provides an insight into the microbial sulfur metabolism, but also helps us understand the effects of changing environmental conditions on marine sulfur cycling and reinforces the close connection between sulfur and carbon cycling in the ocean.

Microbial production of methane is an important terminal metabolic process during organic matter degradation in marine sediments. It is generally acknowledged that hydrogenotrophic and acetoclastic methanogenesis constitute the dominant pathways of methane production; the importance of methanogenesis from methylated compounds remains poorly understood. We conducted various biogeochemical and molecular genetic analyses to characterize substrate availability, rates of methanogenesis, and methanogen community composition, and further evaluated the contribution of different substrates and pathways for methane production in deltaic surface and subsurface sediments of the Western Mediterranean Sea. Major substrates representing three methanogenic pathways, including H2, acetate, and methanol, trimethylamine (TMA), and dimethylsulfide (DMS), were detected in the pore waters and sediments, and exhibited variability over depth and between sites. In accompanying incubation experiments, methanogenesis rates from various 14C labeled substrates varied as well, suggesting that environmental factors, such as sulfate concentration and organic matter quality, could significantly influence the relative importance of individual pathway. In particular, methylotrophic and hydrogenotrophic methanogenesis contributed to the presence of micromolar methane concentrations in the sulfate reduction zone, with methanogenesis from methanol accounting for up to 98% of the total methane production in the topmost surface sediment. In the sulfate-depleted zone, hydrogenotrophic methanogenesis was the dominant methanogenic pathway (67–98%), and enhanced methane production from acetate was observed in organic-rich sediment (up to 31%). Methyl coenzyme M reductase gene (mcrA) analysis revealed that the composition of methanogenic communities was generally consistent with the distribution of methanogenic activity from different substrates. This study provides the first quantitative assessment of methylotrophic methanogenesis in marine sediments and has important implications for marine methane cycling. The occurrence of methylotrophic methanogenesis in surface sediments could fuel the anaerobic oxidation of methane (AOM) in the shallow sulfate reduction zone. Release of methane produced from methylotrophic methanogenesis could be a source of methane efflux to the water column, thus influencing the benthic methane budgets.

Among the most extreme habitats on Earth, dark, deep, anoxic brines host unique microbial ecosystems that remain largely unexplored. As the terminal step of anaerobic degradation of organic matter, methanogenesis is a potentially significant but poorly constrained process in deep-sea hypersaline environments. We combined biogeochemical and phylogenetic analyses with incubation experiments to unravel the origin of methane in the hypersaline sediments of Orca Basin in the northern Gulf of Mexico. Substantial concentrations of methane, up to 3.4 mM, coexisted with high concentrations of sulfate from 16 to 43 mM in two sediment cores retrieved from the northern and southern parts of Orca Basin. The strong depletion of 13C in methane (−77‰ to −89‰) points towards a biological source. While low concentrations of competitive substrates limited the significance of hydrogenotrophic and acetoclastic methanogenesis, the presence of non-competitive methylated substrates (methanol, trimethylamine, dimethyl sulfide, dimethylsulfoniopropionate) supported the potential for methane generation through methylotrophic methanogenesis. Thermodynamic calculations demonstrated that hydrogenotrophic and acetoclastic methanogenesis were unlikely to occur under in situ conditions, while methylotrophic methanogenesis from a variety of substrates was highly favorable. Likewise, carbon isotope relationships between methylated substrates and methane suggested methylotrophic methanogenesis was the major source of methane. Stable and radio-isotope tracer experiments with 13C-labeled bicarbonate, acetate and methanol and 14C-labeled methylamine indicated that methylotrophic methanogenesis was the predominant methanogenic pathway. Based on 16S rRNA gene sequences, halophilic methylotrophic methanogens related to the genus Methanohalophilus dominated the benthic archaeal community in the northern basin and also occurred in the southern basin. High abundances of methanogen lipid biomarkers such as intact polar and polyunsaturated hydroxyarchaeols were detected in sediments from the northern basin, with lower abundances in the southern basin. Strong 13C-depletion of saturated and monounsaturated hydroxyarchaeol were consistent with methylotrophic methanogenesis as the major methanogenic pathway. Collectively, the availability of methylated substrates, thermodynamic calculations, experimentally determined methanogenic activity as well as lipid and gene biomarkers support the hypothesis that methylotrophic methanogenesis is the predominant pathway of methane formation in the presence of sulfate in Orca Basin sediments.

To better understand the depositional constraints on the sulfate reduction process, we present a set of sulfur isotope data for hand-picked pyrites (δ34Spyr) from the coarse fraction (63–250 μm) of bulk sediments, coupled with the contents of carbonate, total organic carbon (TOC) and nitrogen (TN), as well as δ13C values of the TOC, from sediments of core EC2005 on the inner shelf of the East China Sea (ECS) since 16.4 ka. Our results show that core sediments were deposited in a terrestrial environment before 13.1 ka (stage I) when pyrite rarely occurred. During stages II (13.1–12.3 ka) and III (12.3–12.1 ka) seawater began to influence the core location, forming a tidal flat environment where abundant pyrites are well preserved. Then the inner shelf of the ECS was fully flooded, and two intervals with high (stage IV:12.1–6.0 ka; stage VI: 5.2–1.5 ka) and low sedimentation rate (stage V: 6.0–5.2; stage VII: 1.5 ka-present) are identified respectively. When compared with previous δ34S values of chromium-reducible sulfur (δ34SCRS) derived from the bulk sediments, δ34Spyr values are usually higher, indicating the macroscopic pyrites form in the later stages of sedimentary diagenesis when ongoing sulfate reduction has distilled porewater sulfate to a high degree. Considering the wide occurrence of biogenic shallow gas (dominated by CH4) in these sediments, sulfate-driven anaerobic oxidation of methane (SO4-AOM) and organoclastic sulfate reduction (OSR) are expected to be two competitive pathways for sulfate reduction. We propose that the changes in sedimentation rate controls the pace of migration of the sulfate methane transition zone (SMTZ). When sedimentation rate is low, the SMTZ may be fixed in a certain depth (e.g., sediments deposited in stage III and V), where H2S is produced by SO4-AOM. In contrast, when sedimentation rate is very high (> 1 cm/yr), the SMTZ would be deep, and H2S is mostly produced by OSR. Further, our results imply that sulfur diagenesis in recent (since 1.5 ka) ECS mud sediments might be influenced by physical reworking (e.g., typhoon) and/or biological activity (bioturbation), resulting in partial reoxidation of pyrite crystals in fluid mud. Our findings therefore demonstrate that pyrite sulfur isotope compositions (bulk δ34SCRS & hand-picked δ34Spyr) are sensitive to local environmental evolution and the combined use of these two indicators can provide new insights into pathway of sulfate reduction in unsteady environments.

One carbon (C1) metabolism plays an important role in marine carbon cycling but the dynamics and modes of C1 transformations are not fully understood. We made contemporaneous measurements of methylamine and methanol metabolism to elucidate the role of C1 compounds as sources of carbon, energy and nitrogen. Methanol and methylamine were predominantly used as an energy source in offshore waters (oxidation rate constant: kmethanol : 0.02-0.10 day-1 ; kmethylamine : 0.01-0.18 day-1 ), but were also important sources of biomass carbon in coastal waters (assimilation rate constant: kmethanol : 0.04-0.10 day-1 ; kmethylamine : 0.01-0.05 day-1 ). The relative extent of assimilation versus oxidation for these substrates correlated positively with chlorophyll, nutrients and heterotrophic bacterial production. Methanol oxidation and assimilation were stimulated significantly by nutrient addition. In contrast, methylamine metabolism was inhibited by ammonium or nitrate, suggesting that methylamine served as a nitrogen source. A preliminary metagenomic survey revealed a diverse population of putative C1-utilizing microorganisms. These results show that the remineralization of methylamine could provide both C and N sources for microbes. Both methanol and methylamine contribute to microbial energetic and carbon substrate demands with a distinctly different signature in nearshore versus offshore environments.

Methylated amines and sulfides are ubiquitous organic nitrogen and sulfur compounds in the marine environment and could serve as important energy substrates to methanogens inhabiting anoxic sediments. However, their abundance and isotopic values remain largely unconstrained in marine sediments. In this study, we investigated the distribution of trimethylamine (TMA), dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in Aarhus Bay, Denmark and provided the first report for their stable carbon isotopic composition. Simultaneous measurement of those two compounds in small volumes of pore waters and sediments was accomplished with gas chromatography in combination with either a purge and trap system for quantification or a headspace method for carbon isotopic analysis. TMA in the solid phase (exchangeable pool, 0.3-6.6 mu mol kg(-1) wet sediment; base-extractable pool, 2-18 mu mol kg(-1)) was much more abundant than the dissolved pool (

Cold seeps are widespread chemosynthetic ecosystems in the deep-sea environment, and cold seep microbial communities of the South China Sea are poorly constrained. Here we report on the archaeal communities, particularly those involved in methane metabolization, in sediments of a newly discovered cold seep (named 'Haima') on the northwest slope of the South China Sea. Archaeal diversity, abundance and distribution were investigated in two piston cores collected from a seep area (QDN-14B) and a non-seep control site (QDN-31B). Geochemical investigation of the QDN-14B core identified an estimated sulfate-methane transition zone (Estimated SMTZ) at 300-400 cm below sea floor (cmbsf), where a high abundance of anaerobic methane-oxidizing archaea (ANME) occurred, as revealed by analysis of the 16S rRNA gene and the gene (mcrA) encoding the α-subunit of the key enzyme methyl-coenzyme M reductase. ANME-2a/b was predominant in the upper and middle layers of the estimated SMTZ, whereas ANME-1b outcompeted ANME-2 in the sulfate-depleted bottom layers of the estimated SMTZ and the methanogenic zone. Fine-scale phylogenetic analysis further divided the ANME-1b group into three subgroups with different distribution patterns: ANME-1bI, ANME-1bII and ANME-1bIII. Multivariate analyses indicated that dissolved inorganic carbon and sulfate may be important factors controlling the composition of the methane-metabolizing community. Our study on ANME niche separation and interactions with other archaeal groups improves our understanding of the metabolic diversity and flexibility of ANME, and the findings further suggest that ANME subgroups may have evolved diversified/specified metabolic capabilities other than syntrophic anaerobic oxidation of methane coupled with sulfate reduction in marine sediments.

Low-molecular-weight (LMW) alcohols are produced during the microbial degradation of organic matter from precursors such as lignin, pectin, and carbohydrates. The biogeochemical behavior of these alcohols in marine sediment is poorly constrained but potentially central to carbon cycling. Little is known about LMW alcohols in sediment pore waters because of their low concentrations and high water miscibility, both of which pose substantial analytical challenges. In this study, three alternative methods were adapted for the analysis of trace amounts of methanol and ethanol in small volumes of saline pore waters: direct aqueous injection (DAI), solid-phase microextraction (SPME), and purge and trap (P&T) in combination with gas chromatography (GC) coupled to either a flame ionization detector (FID) or a mass spectrometer (MS). Key modifications included the desalination of samples prior to DAI, the use of a threaded midget bubbler to purge small-volume samples under heated conditions and the addition of salt during P&T. All three methods were validated for LMW alcohol analysis, and the lowest detection limit (60 nM and 40 nM for methanol and ethanol, respectively) was achieved with the P&T technique. With these methods, ambient concentrations of volatile alcohols were determined for the first time in marine sediment pore waters of the Black Sea and the Gulf of Mexico. A strong correlation between the two compounds was observed and tentatively interpreted as being controlled by similar sources and sinks at the examined stations.

The concentrations of trichlorofluoromethane (CFC-11), methyl iodide (CH3I), dibromomethane (CH2Br2), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and bromoform (CHBr3) were determined in the marine atmosphere and surface seawater of the East China Sea (ECS) and the South Yellow Sea (SYS) between March and April 2017. Mean mixing ratios (range) of CFC-11, CH3I, CH2Br2, CHBrCl2, CHBr2Cl, and CHBr3 were 192.6 (179.4–226.9), 2.0 (0.7–4.5), 1.0 (0.3–8.0), 0.8 (0.2–5.0), 0.7 (0.2–3.0), and 5.6 (1.1–25.3) pptv, respectively. Anthropogenic activities contributed to elevated atmospheric concentrations of CFC-11, while a combination of biological processes in the seawater and terrestrial sources contributed to elevated atmospheric concentrations of CH3I and bromoalkanes. Mean (range) concentrations of CFC-11, CH3I, CH2Br2, CHBrCl2, CHBr2Cl, and CHBr3 in the surface seawater were 9.47 (4.43–21.28), 4.65 (3.70–6.05), 12.07 (9.63–31.07), 5.45 (1.06–30.04), 13.51 (4.36–54.29), and 15.80 (1.21–34.51) pmol L−1, respectively. Productions of CH3I and bromoalkanes in the surface seawater might be influenced by phytoplankton, picophytoplankton, heterotrophic bacteria, dissolved organic carbon, and nutrients. CFC-11 in the atmosphere and seawater showed little diurnal variation. Compared with weak variation in the seawater, CH3I in the atmosphere showed a significant diurnal variation with a range of 1.2–5.6 pptv. The variation of bromoalkanes concentrations in the atmosphere corresponded to the water concentrations, with the highest value occurring at mid-day. In the current study, the emissions of CH3I, CH2Br2, and CHBr3 from the ECS and the SYS accounted for about 1.1%, 1.8%, and 0.01% of the annual global ocean emissions, respectively, highlighting the importance of the coastal areas to the reactive bromine and iodine in the atmosphere.

Spatial distributions of dimethylsulfide (DMS) and total dimethylsulfoniopropionate (DMSP) were determined in the Yellow Sea (YS) and the East China Sea (ECS) during April–May, 2009. The average concentrations of DMS and DMSP in surface water in the study area were 4.91 (1.18–12.6) and 25.2 (5.47–73.7) nmol L− 1, respectively. Significant relationships were observed between the sulfur compounds and chlorophyll a concentrations, suggesting that phytoplankton biomass might play an important role in controlling DMS and DMSP distributions in the YS and the ECS. Zooplankton community and complex hydrographic conditions including the Yangtze River effluent and the Kuroshio water might also influence DMS and DMSP spatial variability. The interannual and seasonal variations of DMS distribution were investigated in the study area by comparing the present results with previous data from the past two decades. Collectively these data suggest that more DMS was released after the 2000s likely because of shifts of phytoplankton population caused by the anthropogenic eutrophication in the YS and the ECS. The average flux of DMS from the YS and the ECS to the atmosphere was estimated to be 13.7 μmol m− 2 d− 1, implying a significant relative contribution of continental shelf sea areas to global oceanic DMS emissions.

Dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) production by Scrippsiella trochoidea and Prorocentrum minimum was investigated to characterize the effects of physiological stage and salinity on DMS and DMSP pools of these two marine phytoplankton species. Axenic laboratory cultures of the two marine algae were tested for DMSP production and its conversion into DMS. The results demonstrated that both algal species could produce DMS, but the average concentration of DMS per cell in S. trochoidea (12.63 fmol/L) was about six times that in P. minimum (2.01 fmol/L). DMS and DMSP concentrations in algal cultures varied significantly at different growth stages, with high release during the late stationary growth phase and the senescent phase. DMS production induced by three salinities (22, 28, 34) showed that the DMS concentrations per cell in the two algal cultures increased with increasing salinity, which might result from intra-cellular DMSP up-regulation with the change of osmotic stress. Our study specifies the distinctive contributions of different physiological stages of marine phytoplankton on DMSP and DMS production, and clarifies the influence of salinity conditions on the release of DMS and DMSP. As S. trochoidea and P. minimum are harmful algal bloom species with high DMS production, they might play an additional significant role in the sulfur cycle when a red tide occurs.