South China Sea Sediments Research Articles

Inventorization and ecological risk assessment of tetrabromobisphenol A and hexabromocyclododecane in sediments from Guangdong coastal area of South China Sea

Brominated flame retardants (BFRs) exhibit excellent flame retardant properties and are widely used in various industries. Among the common BFRs, tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCDs) pose substantial ecological and human health risks due to their extensive application and long-range transport. This study established 131 sample collection sites along the coast of the South China Sea (SCS) in Guangdong Province to assess the concentration, distribution, inventory, and ecological risk of TBBPA and HBCDs in surface sediments. The concentrations of TBBPA in SCS sediments ranged from < limit of detection (LOD) to 80 μg/kg dry weight (dw), and those of HBCDs from < LOD to 18 μg/kg dw. The diastereoisomers of HBCDs (α-, β-, and γ-HBCD) in the sediment samples accounted for 36 %, 13 %, and 51 %, respectively. Human activities, particularly those associated with nearby electronic waste disassembly and textile and garment industries, considerably influenced the dispersion of TBBPA and HBCDs. The inventories of TBBPA and HBCDs in Guangdong Province's SCS were estimated to be 3.2 × 105 kg and 7.2 × 104 kg, respectively. The average risk quotient values ranged from <0.01 to 0.016, indicating a low to negligible environmental risk. This study provides deeper insights into the distribution and scientific significance of HBCDs and TBBPA in SCS sediment samples, elucidates the current state of BFR contamination, and offers recommendations for future research on environmental safety and human health in the region.

A shift in the South China Sea marine radiocarbon reservoir age associated with the opening of the Balabac Strait

Accurate chronologies are crucial when comparing paleo-climate records, especially in the case of terrestrial and marine comparisons. It is essential to understand temporal variability of the radiocarbon reservoir effect to produce an accurate chronology. Here we establish a high-resolution Bayesian chronology for a northern South China Sea (SCS) sediment core. By applying a piece-wise linear regression method, we observe a distinct shift in the marine radiocarbon reservoir age at ~16.9 ka BP from our core. This shift hampers the accuracy of chronology, even though within higher dating resolution. Together with chronologies within different resolution, we show that the piece-wise linear regression approach is potential in producing accurate chronology for marine sediment. Based on comparisons with other sediment records from the region we show that the shift in radiocarbon reservoir age was pervasive throughout the SCS basin, coeval with the opening of the Balabac Strait, as global sea level rose. Hence, we highlight that the opening of the Balabac strait had a basin-wide effect on surface ocean circulation in the SCS.

Sub-arc Mantle Heterogeneity of the Northern Luzon Volcanic Arc: Mineral and Whole Rock Compositional Variability in Mantle Xenoliths from Lutao Island

Abstract Mantle xenoliths hosted in volcanic rocks from the island of Lutao offer a glimpse into the nature of the mantle beneath the northern Luzon volcanic arc. The xenoliths are spinel-bearing and composed mostly of harzburgite with one lherzolite and one olivine orthopyroxenite. The olivine (Fo92.5–88.9), orthopyroxene (Mg# = 94.6–89.2), and clinopyroxene (Wo49.1–38.1En57.0–45.4Fs3.0–11.0) compositions are similar to those of abyssal peridotites. The spinel compositions are variable and can be principally divided into high-Al (Cr# &amp;lt; 45) and low-Al (Cr# &amp;gt; 45) groupings. The whole rock compositions are similar to abyssal peridotite (Al2O3 = 0.95–2.07 wt %; Mg# = 88.5–90.9) and have U-shaped chondrite normalized rare earth element patterns. The Sr-Nd isotopes of the xenoliths are broadly chondritic (87Sr/86Sri = 0.704400–0.707908; εNd(t) = 0.0 − +1.5). The two-pyroxene equilibrium temperatures range from 900 to 1200 °C with the majority of temperature estimates &amp;gt;1000 °C. The olivine-orthopyroxene-spinel oxygen barometry estimates yielded ΔFMQ values from 0 to +2 and correspond to moderately oxidizing to oxidizing conditions. The xenoliths are likely derived from the Philippine Sea Plate lithospheric mantle that was modified by melt extraction and/or fluid enrichment processes. Trace element and isotopic mixing modeling indicate that 1–2% contamination by subducted South China Sea sediment can explain the Sr-Nd isotopic enrichment and Th and U elemental variability within the xenoliths assuming an initial composition similar to enriched depleted mid-ocean ridge mantle (E-DMM). The anomalously high two-pyroxene equilibrium temperatures of the Lutao xenoliths relative to other regions of the northern Luzon volcanic arc (Iraya &amp;lt;1000 °C) indicate that they were affected by a high-temperature event that was likely a consequence of recent intra-arc rifting that occurred after collision (&amp;lt;6 Ma) between the Luzon arc and the Eurasian margin.

Experimental investigation and modeling on the dissociation kinetics of methane hydrate in clayey silt cores in depressurization

Natural gas hydrate (NGH) in clayey silts might be the majority of its global reserve. A thorough understanding on the dissociation kinetics of NGH in clayey silts is crucial for its safe and efficient exploitation. In this study, the dissociation characteristics of methane hydrate in quartz silts, montmorillonites and South China Sea (SCS) sediments made cores in multi-step depressurization was experimentally studied and modeled. In quartz silt core, methane hydrate thoroughly dissociated once the pressured is lowered to the equilibrium pressure in bulk. In montmorillonite and SCS sediment cores, methane hydrate dissociation initiated at higher pressure than quartz silt. Capillary and clay induced water activity depressions are suggested to be the major causes. Stepwise methane hydrate dissociation was discovered in the montmorillonite and SCS sediment cores in multi-stage depressurization. In one step depressurization, hydrate in the fine grain cores with larger surface area dissociated faster. In montmorillonite and SCS sediment cores, lower water content leads to faster dissociation at the same pressure, which may be attributed to the higher driven force for dissociation (fugacity difference fe-f). A kinetic model for NGH dissociation in clayey silts was established by introducing fitted equivalent diameter and surface area correction factor of hydrate particles to the Kim-Bishnoi model. In the model, the hydrate equilibrium conditions in clayey silts were predicted by the water activity measurement (WAM) technique. With a maximum deviation of 9.81% in gas production, the model prediction and experimental results are in good agreement. The experimental findings and model reported in this work may be helpful to understand the NGH phase transition mechanism in clayey silt sediments.

Magnetic Fingerprints for the Paleoenvironmental Evolutions Since the Last Deglaciation: Evidence From the Northwestern South China Sea Sediments

AbstractRapid global changes since the last deglaciation can be well documented in marginal sea sediments, while isolating individual paleoenvironmental signals is still challenging. Here, we identified magnetic minerals and unmixed their variations in sediments from the northwestern South China Sea to decipher environmental variations since the last deglaciation. The variation in the hematite to goethite ratio indicates a relatively drier mid‐Holocene and a wetter early and late Holocene in the Red River catchment. The position shift and intensity variation of Western Pacific Subtropical High may account for the Holocene precipitation changes. A higher chemical weathering intensity accompanied by a relatively more fine‐grained magnetite input during the last deglaciation suggests decoupling of the Asian Summer Monsoon and chemical weathering in the catchment, which might have been caused by the shelf exposure during the deglaciation. In addition, the relative abundance of biogenic magnetite increased with the sea level and possible deep‐water temperature rise. Therefore, magnetic minerals such as hematite, goethite, detrital and biogenic magnetite are markedly potential fingerprints for continent‐ocean environmental changes.

Spatial distribution and modelling of 239+240Pu in the sediments and seawater columns of the South China Sea and Indian Ocean

In order to investigate the 239+240Pu potential influence in the ocean, and develop a new method for rapidly monitoring radioactive pollution, the 239+240Pu spatial distribution in the South China Sea (SCS) and the Indian Ocean (IND) sediments is analyzed by SF-ICP-MS (ELEMENT 2). The inventory-weighted mean activities of 239+240Pu were 0.413 ± 0.333 mBq/g, 0.128 ± 0.044 mBq/g, and 0.483 ± 0.606 mBq/g in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 239+240Pu activity spatial distribution in the SCS sediments was influenced by the current, the vertical distribution of Pu in seawater, and the transport of particulate matter. The 239+240Pu activity spatial distribution in the IND sediments could be impacted by Antarctic Intermediate Water. The average of 240Pu/239Pu atomic ratios were 0.258 ± 0.034, 0.219 ± 0.031, and 0.212 ± 0.028 in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 240Pu/239Pu atomic ratios in the SCS and IND indicate that Pu from the Pacific Proving Ground (PPG) is transported to the IND via the SCS internal current and transverse ocean currents within Indonesia. In addition, a seawater advection-dispersion equation (S-ADE) model is established based on the actual physical processes of radionuclides in the seawater column and well fitting results were obtained (R2 = 0.49 to 0.99). The 239+240Pu data and the geographic information from the sample site were used to correct the Pu distribution in the seawater. The calculated 239+240Pu mean concentrations in the surface seawater were 2.465 mBq/m3 and 2.205 mBq/m3 for the SCS and the eastern IND seawater, respectively, and the result is consistent with the previous measurements. Then, the 239+240Pu stored in the study area of SCS and eastern IND was estimated to be approximately 1.0–1.4% of the global ocean based on the model. This study provides a useful model for guiding and designing future monitoring of pollution by anthropogenic Pu and other isotopes.

Enhanced weathering during glacial periods and its dynamic response to climate and sea-level changes inferred from the northern South China Sea sediments

Secondary weathering of exposed continental shelf sediment at low-latitudes may play a significant role in atmospheric CO2 consumption during glacial periods, which is negative feedback for climate stability. However, more lines of evidences are required to verify the link of glacial weathering with climate. This study presents a comprehensive analysis of geochemical proxies that indicate the silicate weathering intensity in the northern South China Sea (SCS) over orbital timescales since ∼244 ka. Provenance discrimination results, based on the Sr-Nd isotopes values and the patterns of rare earth elements (REEs), suggest that the terrigenous sediment was primarily derived from Taiwan Island. The silicate weathering records revealed two different weathering regimes during glacial periods in the northern SCS, corresponding to changes in monsoon climate and sea-level. During the early period of Marine Isotope Stage 6 (MIS 6), the increase in weathering intensity was in response to climate variability, and the rapid delivery of terrigenous sediment through contour currents allowed for the preservation of climate signals in the continental margin. In contrast, the significant increase in weathering intensity during the glacial periods at late MIS 2 and late MIS 6, was a result of the sedimentary recycling process dominated by sea-level rises. In this case, the marginal sea weathering records did not truly reflect climate changes, due to secondary weathering processes on the exposed continental shelf. Our new geochemical results emphasize the dynamic coupling effects of climate and sea-level changes in transmitting and preserving silicate weathering signals at low-latitude during glacial periods. When using marginal sea sediments for paleoclimatic and paleo-weathering reconstruction, one should be cautious of the complex influence of sediment source-to-sink processes on sediment composition, and a holistic approach may provide more robust constraints.

Complete genome sequence of Streptomyces sp. HNA39, a new cyclizidine producer isolated from a South China Sea sediment.

Streptomyces sp. HNA39 is a promising candidate for the production of antineoplastic metabolites screened from a collection of 448 actinomycetes derived from coastal sediments. The complete genome sequence of HNA39 comprises a 7,351,753-bp linear chromosome with a GC content of 71.94%. Whole genome analysis reveals the presence of 29 putative biosynthetic gene clusters (BGCs) encoding secondary metabolites. Among them, a type I PKS BGC shows an 82% similarity with the cyclizidine (CLD) BGC identified from Streptomyces NCIB 11649. LC-MS profiles further supported the production of new CLD congeners. Bafilomycins were also found produced in abundance, corresponding to another type I PKS BGC highly homologous to that of bafilomycin B1 from S. lohii. CLDs are indolizidine alkaloids consisting a fused five- and six-membered ring system with an intriguing cyclopropane terminal linked by a trans-dienic chain. The cyclization mechanism of the cylopropyl ring, one of its pharmacophores, is still unknown. Genome sequencing of the new CLD producer and subsequent comparative analysis of their gene clusters would further our understanding of the chemistry behind cyclopropane formation.

Mariluticola halotolerans gen. nov., sp. nov., a novel member of the family Devosiaceae isolated from South China Sea sediment.

A novel member of class Alphaproteobacteria was isolated from marine sediment of the South China Sea. Cells of strain LMO-2T were Gram-stain negative, greyish in colour, motile, with a single lateral flagellum and short rod in shape with a slight curve. Strain LMO-2T was positive for oxidase and negative for catalase. The bacterium grew aerobically at 10-40 °C (optimum, 25-30 °C), pH 5.5-10.0 (optimum, pH 7.0) and 0-9 % NaCl (w/v; optimum, 2-3 %). Phylogenetic analysis of the 16S rRNA gene sequence and phylogenomic analysis of the whole genome sequence indicated that strain LMO-2T represents a new genus and a new species within the family Devosiaceae, class Alphaproteobacteria, phylum Pseudomonadota. Comparisons of the 16S rRNA gene sequences of strain LMO-2T showed 94.8 % similarity to its closest relative. The genome size is ~3.45 Mbp with a DNA G+C content of 58.17 mol%. The strain possesses potential capability for the degradation of complex organic matter, i.e. fatty acid and benzoate. The predominant cellular fatty acids (>10 %) were C16 : 0 and C18 : 1 ω7c 11-methyl. The sole respiratory quinone was ubiquinone-10. The major identified polar lipids were diphosphatidylglycerol, phosphatidylglycerol and phospholipid. Based on the polyphasic taxonomic data, strain LMO-2T represents a novel genus and a novel species for which the name Mariluticola halotolerans gen. nov., sp. nov., was proposed in the family Devosiaceae. The type strain is LMO-2T (=CGMCC 1.19273T=JCM 34934T).

Classification of Marine Sediment in the Northern Slope of the South China Sea Based on Improved U-Net and K-Means Clustering Analysis

The classification of marine sediment based on acoustic data is crucial for various applications such as marine resource exploitation, marine engineering construction, and marine ecological environment maintenance. It serves as a valuable alternative to limited geological sampling. However, the accuracy of sediment classification is limited due to constraints in acoustic data detection methods, data quality, and classification techniques. To address this issue, this study proposes an automatic classification method for marine sediment using an improved U-convolutional neural network and K-means clustering algorithm. In the coding part, a spatial pyramid pool layer is introduced to fuse low-dimensional feature data of different scales with the features of each level of the corresponding coding layer. This fusion method enhances the accuracy of the constructed relationship between the physical property parameters of the seabed bottom. The K-means clustering algorithm is optimized through selecting the point at the density center as the initial clustering center during the initial clustering center selection stage. This approach solves the sensitivity problem of the initial clustering center of K-means, improves the edge extraction effect of sediment types, and enhances the classification accuracy of sediment types. To validate the proposed method, an application test is conducted in the Northern Slope area of the South China Sea. The mean grain size of sediments in the study area is predicted using the improved U-Net neural network and the seafloor reflection intensity of the sub-bottom profile. Compared to the standard U-Net network results, the mean grain size prediction results show an increase of 4.9% and 2.8%, respectively. The sediment with the predicted mean grain size is then classified using the K-means clustering algorithm, resulting in the division of five sediment types: gravelly sand, sand, silty sand, sandy silt, and clayey silt. These classifications align well with the South China Sea sediment type map. The findings of this study not only provide an important supplement to existing marine sediment classification methods but also contribute significantly to understanding the sedimentary environment and processes in the Northern Slope of the South China Sea.

Sr-Nd isotopic fingerprints of Red River sediments and its implication for provenance discrimination in the South China Sea

The Red River has a sediment discharge of 130 Mt/yr and is one of the most important sources of fluvial materials to the South China Sea (SCS). However, due to the paucity of Sr-Nd isotopic data of the Red River delta sediments, the Sr-Nd isotopic fingerprints of the Red River sediment have not been well defined, which hinders the provenance discrimination of modern sediment in the SCS. This study presents the grain size, element, and Sr-Nd isotope data of Red River delta sediments to examine the controlling factors of sediment Sr-Nd isotopic compositions and better characterize the sediment discharge of the modern Red River to the SCS. Compiled rock Sr-Nd isotopic data in the Red River catchment indicate that catchment bedrock lithology exerts a dominant control on distinct sediment Sr-Nd isotopic features among its main branches, Song Da, Thao River, and Song Lo. The Sr-Nd isotopic compositions of Red River delta sediments are concentrated and between these upstream branches, indicating a result of the mixing process. A Bayesian mixing model shows that the Song Lo, Song Da and Thao River make an approximately equal contribution to the modern Red River delta. However, in delta sediments, the correlation between the 87Sr/86Sr and mean grain size suggests a hydrodynamic sorting effect on sediment 87Sr/86Sr ratios, which should be a concern in the calculation of mixing models. Combining newly measured and published data in the Red River delta, we show that the Sr-Nd isotopic fingerprints of Red River sediment are 0.7304 ± 0.0027 (1σ, n = 32) and − 12.0 ± 0.4 (1σ, n = 32) respectively. Published Sr-Nd isotopic compositions of delta sediments in the Pearl River and Mekong River are also compiled to make a comparison with the Red River. The Mekong River sediment has less radiogenic 87Sr/86Sr of 0.7212 ± 0.0008 (1σ, n = 7) and higher εNd values of −10.1 ± 0.7 (1σ, n = 8), and Pearl River sediment shows higher 87Sr/86Sr ratios of 0.7378 (n = 1) and εNd values of −11.8 ± 0.4 (1σ, n = 6), which is similar to those of Red River sediment. This study provides new insight into the reliable assessment of sediment Sr-Nd isotopic fingerprints for large rivers in SE Asia, which is favorable for the provenance discrimination of modern SCS sediment.

Distribution pattern of N-damo bacteria along an anthropogenic nitrogen input gradient from the coastal mangrove wetland to the South China sea sediments

Microbial nitrite-dependent anaerobic methane oxidation (n-damo) process is important for mitigating methane emission and anthropogenic nitrogen inputs in the marine environment. However, the distribution pattern of n-damo bacteria along an anthropogenic N-input gradient from the coastal wetland to the pristine South China Sea is poorly understood. This study investigated the diversity and abundance of n-damo bacteria in samples collected along a N-input gradient from Mai Po (MP) mangrove wetland sediments of the Pearl River Estuary (PRE) to the deep ocean sediments of the South China Sea (SCS). Retrieved 16S rDNA sequences showed a shift of n-damo community composition of complex structures with both freshwater and marine n-damo lineages in MP intertidal sediments to marine dominated characteristic in SCS sediments. The observed variation of Shannon and Chao1 indexes of n-damo bacteria shared a similar trend of a decrease at first followed by an increase along the targeting gradient with previously investigated methanogens, anaerobic methanotrophic archaea, ammonia-oxidizing archaea and ammonia-oxidizing bacteria, but had a reverse pattern with anammox bacteria. The community structure of pmoA gene sequences contained freshwater lineages only in SCS continental shelf sediments closer to the PRE, and turned to group with other marine samples in deeper and pristine sediments. Results suggested that n-damo bacteria might be a major contributor to anaerobic denitrification in the SCS sediments because their abundances were much higher than previously studied anammox bacteria in the same sample set. The distribution pattern of n-damo bacterial diversity, richness and abundance along the anthropogenic N-input gradient implies that they could be used as a bio-indicator for monitoring the anthropogenic/terrestrial inputs in marine environments.

Ecological Distribution and Diversity of Key Functional Genes for Denitrification in Surface Sediments of the Northern South China Sea: Implications for Potential N2O Emissions

Denitrification is an important pathway for nitrogen sink and N2O emissions, but little is known about the ecological distribution of key functional genes of denitrification and their potential N2O emissions in marine sediments. In this study, we analyzed the abundance, ecological distribution, and diversity of key functional genes (nir and nosZ) for denitrification in the northern South China Sea (SCS) surface sediments. Our results showed that the gene abundances varied from 105 to 108 and from 106 to 107 copies·g-1 for the nirS and nirK, respectively. The nosZ II/nosZ I gene abundance ratios were 1.28–9.88 in shallow-sea and deep-sea sediments, suggesting that the nosZ II gene should play a dominant role in N2O reduction in the northern SCS sediments. Moreover, the significantly higher abundance ratios of nir/nosZ in deep-sea surface sediments implied that there might be stronger N2O emissions potential in deep-sea sediments than in shallow-sea sediments. The ecological distribution profiles of the nirS, nosZ I, and nosZ II gene communities varied with water depth, and denitrification genes in shallow-sea and deep-sea sediments differed in their sensitivity to environmental factors. Water temperature was the major factor affecting both the abundance and the community distribution of the nirS gene in deep-sea sediments. Nitrate was the major factor shaping the community of nosZ I and nosZ II genes in shallow-sea sediments. Our study provides a pattern of ecological distribution and diversity for the nir and nosZ genes and emphasizes the role of these key functional genes in potential N2O emissions of the northern SCS surface sediments.

Experimental investigation and modeling on the dissociation conditions of methane hydrate in clayey silt cores

AbstractAs the majority of global natural gas hydrate reserve, the dissociation conditions of hydrate in clayey silts are of great significance for its efficient production. In this work, the dissociation conditions of methane hydrate in clayey silt cores were experimentally measured by step‐heating method at the temperature range of 280.76–289.55 K and pressure range of 8.11–15.03 MPa, respectively. Various cores including quartz powder, montmorillonite, and South China Sea sediments at the water content range of 20%–33% were used for investigation. The results showed that the dissociation temperatures of methane hydrate in clayey silt cores depressed compared to bulk hydrate. The grain size, salinity, and lithology of clayey silt cores significantly affect the dissociation conditions of hydrate. In comparison to grain size, salinity, and lithology had a more significant influence on the equilibrium temperature depression. The dissociation temperature depression of methane hydrate was considered as a consequence of the water activity depression which is caused by the effect of capillary, salt, or clay. A water activity meter was used to measure the water activity in clayey silt cores. The influence of salt component and mineral characteristics on the water activity was investigated. By combining the measured water activity data with the Chen‐Guo model, a novel water activity measurement (WAM) method for the hydrate dissociation conditions prediction was proposed. With the maximum deviation less than 12%, the predicted results are in good agreement with the experimental data. It demonstrated that the WAM method could effectively predict the dissociation conditions of methane hydrate in clayey silts with convenience and accuracy.

Age-Heterogeneity in Marine Sediments Revealed by Three-Dimensional High-Resolution Radiocarbon Measurements

Marine sedimentary archives are routinely used to reconstruct past environmental changes. In many cases, bioturbation and sedimentary mixing affect the proxy time-series and the age-depth relationship. While idealized models of bioturbation exist, they usually assume homogeneous mixing, thus that a single sample is representative for the sediment layer it is sampled from. However, it is largely unknown to which extent this assumption holds for sediments used for paleoclimate reconstructions. To shed light on 1) the age-depth relationship and its full uncertainty, 2) the magnitude of mixing processes affecting the downcore proxy variations, and 3) the representativity of the discrete sample for the sediment layer, we designed and performed a case study on South China Sea sediment material which was collected using a box corer and which covers the last glacial cycle. Using the radiocarbon content of foraminiferal tests as a tracer of time, we characterize the spatial age-heterogeneity of sediments in a three-dimensional setup. In total, 118 radiocarbon measurements were performed on defined small- and large-volume bulk samples ( ∼ 200 specimens each) to investigate the horizontal heterogeneity of the sediment. Additionally, replicated measurements on small numbers of specimens (10 × 5 specimens) were performed to assess the heterogeneity within a sample volume. Visual assessment of X-ray images and a quantitative assessment of the mixing strength show typical mixing from bioturbation corresponding to around 10 cm mixing depth. Notably, our 3D radiocarbon distribution reveals that the horizontal heterogeneity (up to 1,250 years), contributing to the age uncertainty, is several times larger than the typically assumed radiocarbon based age-model error (single errors up to 250 years). Furthermore, the assumption of a perfectly bioturbated layer with no mixing underneath is not met. Our analysis further demonstrates that the age-heterogeneity might be a function of sample size; smaller samples might contain single features from the incomplete mixing and are thus less representative than larger samples. We provide suggestions for future studies, optimal sampling strategies for quantitative paleoclimate reconstructions and realistic uncertainty in age models, as well as discuss possible implications for the interpretation of paleoclimate records.

Devosia salina sp. nov., isolated from South China Sea sediment.

An aerobic, Gram-stain-negative, rod-shaped and motile strain, designated SCS-3T, was isolated from deep-sea sediment of the South China Sea. Phylogenetic analysis based on the 16S rRNA gene sequence similarities revealed that strain SCS-3T represented a novel species of the genus Devosia, with closely related strains 'Devosia sediminis' MSA67T (98.61 %), Devosia riboflavina IFO13584T (98.22 %) and Devosia indica IO390501T (97.72 %). The G+C content of the genomic DNA is 63.44 mol%. The digital DNA-DNA hybridization values with 'D. sediminis' MSA67T, D. riboflavina IFO13584T and D. indica IO390501T were 24.50, 21.8 and 24.80 %, respectively. The major polar lipids of strain SCS-3T were diphosphatidylglycerol, phosphatidylglycerol and three unidentified glycolipids. Ubiquinone-10 was the sole isoprenoid quinone, and C16 : 0, C18 : 1 ω7c 11-methyl and summed feature 8 (C18 : 1 ω7c and/or C18 : 1 ω6c) were the major fatty acids. Based on polyphasic taxonomic data, strain SCS-3T represents a novel species of the genus Devosia, for which the name Devosia salina sp. nov. is proposed. The type strain is SCS-3T (=JCM 34403T=GDMCC 1.2221T).

Plutonium isotopes in the northwestern South China Sea: Level, distribution, source and deposition

The spatial distribution of plutonium isotopes (239Pu, 240Pu) in the surface sediments collected from the northwestern South China Sea (SCS) in 2018 was investigated. The 239,240Pu concentrations in surface sediments vary from 0.048 to 0.960 mBq/g (with mean of 0.282 ± 0.242 mBq/g) depending on the geographical feature of the sampling location such as the river estuary, continental shelf, slope and deep basin. Higher 240Pu/239Pu atomic ratios (0.24–0.31) in the surface sediment of the SCS compared to the global fallout value of 0.18 were observed, this is attributed to the input of close-in fallout of the Pacific Proving Ground (PPG) transported by the North Equatorial Current and Kuroshio Current to the northern SCS. The contribution of the PPG derived plutonium in the SCS sediment was estimated to be 39%–78% using a simple two-end member mixing model based on the measured 240Pu/239Pu atomic ratios in the sediment. Besides the soluble 239,240Pu level in seawater, load of suspended particulate matter from the river runoff and biological debris, hydrographic and hydrodynamic conditions are key parameters influencing the deposition process of plutonium to the sediment.

High-Ca vent fluids discharged from the Lutao arc volcanic hydrothermal system are associated with albitization and recycling of marine carbonate

The chemical and isotopic characteristics of calcium (Ca) in subduction zones are closely related to the budget of Ca and carbon cycles. Here we investigate the ultra-high Ca concentrations that characterize the hydrothermal fluids discharged from two types of vents, named the Zhudanqu brine vent (ZDQ) and the Huwaichi vapor spring (HWC), in the Lutao hydrothermal system at the north Luzon arc. The Ca concentrations of up to 159 mM and Ca/Cl ratio of up to 0.26 in the ZDQ vent fluids are possibly the highest ever reported for Ca enrichment in global seawater-circulated hydrothermal/geothermal systems. The differences in chemical compositions between the ZDQ and the HWC vent fluids are primary controlled by subcritical phase separation. The brine phase constitutes the ZDQ vent fluids, while the HWC vent fluids represent mixtures of the vapor phase and seawater. Both the vapor and the brine phases exhibit similar δ44/40Ca values (0.72 ± 0.05‰), suggesting no significant Ca isotope fractionation has occurred during phase separation.The hydrothermal endmember before phase separation (the “Lutao endmember”) presents depletions of 213 ± 15 mM of Na, 24.4 ± 0.4 mM of SO42−, and 10.2 mM of K, and enrichment of 130.2 ± 5.5 mM of Ca with respect to the percolated seawater. The total gained Ca is 154.6 ± 5.9 mM with a δ44/40Ca value of 0.67‰ – 0.77‰ (0.72 ± 0.05‰), considering anhydrite precipitation during hydrothermal circulation. The Holocene raised coral reef is unlikely to contribute substantial Ca into the Lutao system. Much of the gained Ca (111.6 ± 7.5 mM) is produced by high-degree albitization of the Lutao host rock, which is promoted by the low water/rock ratio (~ 2), slightly alkaline conditions, and relatively lower temperature of the Lutao system with respect to most mid-ocean ridge hydrothermal systems. Ca derived from this process inherits the Ca isotopes of plagioclase in the Lutao host rocks (δ44/40Ca = 0.82 ± 0.06‰). According to mass and isotopic balances, the recycled marine carbonate is proposed to contribute 43 ± 13.4 mM Ca with a δ44/40Ca value of 0.46−0.63+0.35‰ into the Lutao system. Such isotopically lighter Ca is derived from either pore fluids expulsed from underlying Philippine Sea sediments, or more probably, carbonate-bearing subduction fluids from the subducting South China Sea sediments and slab. The carbonate solubility in the subduction fluids could maintain at 600 mM near the reaction zone. The carbonate-rich fluids were subsequently migrated into the Lutao reaction zone and released an additional 43 ± 13.4 mM Ca via dolomitization. A small amount (~ 9%) addition of carbonate-rich fluids would not significantly change the budgets of Na, Mg, and Cl but could generate substantial Ca enrichment and Ca isotopic variations.

Dimethylsulfoniopropionate Biosynthetic Bacteria in the Subseafloor Sediments of the South China Sea.

Dimethylsulfoniopropionate (DMSP) is one of Earth’s most abundant organosulfur molecules, and bacteria in marine sediments have been considered significant producers. However, the vertical profiles of DMSP content and DMSP-producing bacteria in subseafloor sediment have not been described. Here, we used culture-dependent and -independent methods to investigate microbial DMSP production and cycling potential in South China Sea (SCS) sediment. The DMSP content of SCS sediment decreased from 11.25 to 20.90 nmol g–1 in the surface to 0.56–2.08 nmol g–1 in the bottom layers of 8-m-deep subseafloor sediment cores (n = 10). Very few eukaryotic plastid sequences were detected in the sediment, supporting bacteria and not algae as important sediment DMSP producers. Known bacterial DMSP biosynthesis genes (dsyB and mmtN) were only predicted to be in 0.0007–0.0195% of sediment bacteria, but novel DMSP-producing isolates with potentially unknown DMSP synthesis genes and/or pathways were identified in these sediments, including Marinobacter (Gammaproteobacteria) and Erythrobacter (Alphaproteobacteria) sp. The abundance of bacteria with the potential to produce DMSP decreased with sediment depth and was extremely low at 690 cm. Furthermore, distinct DMSP-producing bacterial groups existed in surface and subseafloor sediment samples, and their abundance increased when samples were incubated under conditions known to enrich for DMSP-producing bacteria. Bacterial DMSP catabolic genes were also most abundant in the surface oxic sediments with high DMSP concentrations. This study extends the current knowledge of bacterial DMSP biosynthesis in marine sediments and implies that DMSP biosynthesis is not only confined to the surface oxic sediment zones. It highlights the importance of future work to uncover the DMSP biosynthesis genes/pathways in novel DMSP-producing bacteria.

Diversity and Distribution of Uncultured and Cultured Gaiellales and Rubrobacterales in South China Sea Sediments.

Actinobacteria are ubiquitous in marine ecosystems, and they are regarded as an important, underexplored, potential pharmaceutical resource. The orders Gaiellales and Rubrobacterales are deep taxonomic lineages of the phylum Actinobacteria, both are represented by a single genus and contain only a few species. Although they have been detected frequently by high-throughput sequencing, their functions and characteristics in marine habitats remain unknown due to the lack of indigenous phenotypes. Here, we investigated the status of the orders in South China Sea (SCS) sediments using culture-independent and culture-dependent methods. Gaiellales is the second-most abundant order of Actinobacteria and was widely distributed in SCS sediments at water depths of 42–4,280 m, and four novel marine representatives in this group were successfully cultured. Rubrobacterales was present at low abundance in energy-limited marine habitats. An isolation strategy for Rubrobacterales from marine samples was proposed, and a total of 138 mesophilic Rubrobacterales strains were isolated under conditions of light and culture time combined with high-salinity or low-nutrient media. Marine representatives recovered in this study formed branches with a complex evolutionary history in the phylogenetic tree. Overall, the data indicate that both Gaiellales and Rubrobacterales can adapt to and survive in extreme deep-sea environments. This study lays the groundwork for further analysis of the distribution and diversity of the orders Gaiellales and Rubrobacterales in the ocean and provides a specific culture strategy for each group. The results open a window for further research on the ecological roles of the two orders in marine ecosystems.