Speciation and distribution of arsenic in cold seep sediments of the South China Sea.

Arsenic (As) is an abundant metalloid in marine environments, while the biogeochemical cycling of As in cold seeps remains poorly understood. We characterized the speciation of As and investigated controls of As distribution in cold seeps of South China Sea. High methane concentrations (0.2-5.5 mmol/L) and rapid sulfate depletion were observed in the seepage. Dissolved inorganic arsenic (DIAs) was enriched in the porewater ranging from 7.5 to 23.5 µg/L. As in the solid phase ranged from 2.9 to 22.6 µg/g, and sulfide mineral-bound As dominated the total arsenic (TAs) pool, followed by iron (manganese, aluminum) oxide-bound As. The significant correlations between porewater Fe2+ and DIAs reflect the controls of iron on DIAs release. Incubation experiments showed that adsorption to the solid phase and sulfate reduction activity affected the bioavailability and removal of DIAs, suggesting that multiple processes regulate the speciation and transformation of As in seep sediments.

The diversification and potential function of microbiome in sediment-water interface of methane seeps in South China Sea.

The sediment-water interfaces of cold seeps play important roles in nutrient transportation between seafloor and deep-water column. Microorganisms are the key actors of biogeochemical processes in this interface. However, the knowledge of the microbiome in this interface are limited. Here we studied the microbial diversity and potential metabolic functions by 16S rRNA gene amplicon sequencing at sediment-water interface of two active cold seeps in the northern slope of South China Sea, Lingshui and Site F cold seeps. The microbial diversity and potential functions in the two cold seeps are obviously different. The microbial diversity of Lingshui interface areas, is found to be relatively low. Microbes associated with methane consumption are enriched, possibly due to the large and continuous eruptions of methane fluids. Methane consumption is mainly mediated by aerobic oxidation and denitrifying anaerobic methane oxidation (DAMO). The microbial diversity in Site F is higher than Lingshui. Fluids from seepage of Site F are mitigated by methanotrophic bacteria at the cyclical oxic-hypoxic fluctuating interface where intense redox cycling of carbon, sulfur, and nitrogen compounds occurs. The primary modes of microbial methane consumption are aerobic methane oxidation, along with DAMO, sulfate-dependent anaerobic methane oxidation (SAMO). To sum up, anaerobic oxidation of methane (AOM) may be underestimated in cold seep interface microenvironments. Our findings highlight the significance of AOM and interdependence between microorganisms and their environments in the interface microenvironments, providing insights into the biogeochemical processes that govern these unique ecological systems.

Lipidomic diversity and proxy implications of archaea from cold seep sediments of the South China Sea.

Cold seeps on the continental margins are characterized by intense microbial activities that consume a large portion of methane by anaerobic methanotrophic archaea (ANME) through anaerobic oxidation of methane (AOM). Although ANMEs are known to contain unique ether lipids that may have an important function in marine carbon cycling, their full lipidomic profiles and functional distribution in particular cold-seep settings are still poorly characterized. Here, we combined the 16S rRNA gene sequencing and lipidomic approaches to analyze archaeal communities and their lipids in cold seep sediments with distinct methane supplies from the South China Sea. The archaeal community was dominated by ANME-1 in the moderate seepage area with strong methane emission. Low seepage area presented higher archaeal diversity covering Lokiarchaeia, Bathyarchaeia, and Thermoplasmata. A total of 55 core lipids (CLs) and intact polar lipids (IPLs) of archaea were identified, which included glycerol dialkyl glycerol tetraethers (GDGTs), hydroxy-GDGTs (OH-GDGTs), archaeol (AR), hydroxyarchaeol (OH-AR), and dihydroxyarchaeol (2OH-AR). Diverse polar headgroups constituted the archaeal IPLs. High concentrations of dissolved inorganic carbon (DIC) with depleted δ13CDIC and high methane index (MI) values based on both CLs (MICL) and IPLs (MIIPL) indicate that ANMEs were active in the moderate seepage area. The ANME-2 and ANME-3 clades were characterized by enhanced glycosidic and phosphoric diether lipids production, indicating their potential role in coupling carbon and phosphurus cycling in cold seep ecosystems. ANME-1, though representing a smaller proportion of total archaea than ANME-2 and ANME-3 in the low seepage area, showed a positive correlation with MIIPL, indicating a different mechanism contributing to the IPL-GDGT pool. This also suggests that MIIPL could be a sensitive index to trace AOM activities performed by ANME-1. Overall, our study expands the understanding of the archaeal lipid composition in the cold seep and improves the application of MI using intact polar lipids that potentially link to extent ANME activities.

Active pathways of anaerobic methane oxidization in deep-sea cold seeps of the South China Sea.

IMPORTANCE: Cold seeps occur in continental margins worldwide and are deep-sea oases. Anaerobic oxidation of methane is an important microbial process in the cold seeps and plays an important role in regulating methane content. This study elucidates the diversity and potential activities of major microbial groups in dependent anaerobic methane oxidation and sulfate-dependent anaerobic methane oxidation processes and provides direct evidence for the occurrence of nitrate-/nitrite-dependent anaerobic methane oxidation (Nr-/N-DAMO) as a previously overlooked microbial methane sink in the hydrate-bearing sediments of the South China Sea. This study provides direct evidence for occurrence of Nr-/N-DAMO as an important methane sink in the deep-sea cold seeps.

Bacterial symbiont diversity in Arctic seep Oligobrachia siboglinids.

BACKGROUND: High latitude seeps are dominated by Oligobrachia siboglinid worms. Since these worms are often the sole chemosymbiotrophic taxon present (they host chemosynthetic bacteria within the trophosome organ in their trunk region), a key question in the study of high latitude seep ecology has been whether they harbor methanotrophic symbionts. This debate has manifested due to the mismatch between stable carbon isotope signatures of the worms (lower than -50‰ and usually indicative of methanotrophic symbioses) and the lack of molecular or microscopic evidence for methanotrophic symbionts. Two hypotheses have circulated to explain this paradox: (1) the uptake of sediment carbon compounds with depleted δC13 values from the seep environment, and (2) a small, but significant and difficult to detect population of methanotrophic symbionts. We conducted 16S rRNA amplicon sequencing of the V3-V4 regions on two species of northern seep Oligobrachia (Oligobrachia webbi and Oligobrachia sp. CPL-clade), from four different high latitude sites, to investigate the latter hypothesis. We also visually checked the worms' symbiotic bacteria within the symbiont-hosting organ, the trophosome, through transmission electron microscopy. RESULTS: The vast majority of the obtained reads corresponded to sulfide-oxidizers and only a very small proportion of the reads pertained to methane-oxidizers, which suggests a lack of methanotrophic symbionts. A number of sulfur oxidizing bacterial strains were recovered from the different worms, however, host individuals tended to possess a single strain, or sometimes two closely-related strains. However, strains did not correspond specifically with either of the two Oligobrachia species we investigated. Water depth could play a role in determining local sediment bacterial communities that were opportunistically taken up by the worms. Bacteria were abundant in non-trophosome (and thereby symbiont-free) tissue and are likely epibiotic or tube bacterial communities. CONCLUSIONS: The absence of methanotrophic bacterial sequences in the trophosome of Arctic and north Atlantic seep Oligobrachia likely indicates a lack of methanotrophic symbionts in these worms, which suggests that nutrition is sulfur-based. This is turn implies that sediment carbon uptake is responsible for the low δ13C values of these animals. Furthermore, endosymbiotic partners could be locally determined, and possibly only represent a fraction of all bacterial sequences obtained from tissues of these (and other) species of frenulates.

Mercury Isotopes in Deep-Sea Epibenthic Biota Suggest Limited Hg Transfer from Photosynthetic to Chemosynthetic Food Webs.

Deep oceans receive mercury (Hg) from upper oceans, sediment diagenesis, and submarine volcanism; meanwhile, sinking particles shuttle Hg to marine sediments. Recent studies showed that Hg in the trench fauna mostly originated from monomethylmercury (MMHg) of the upper marine photosynthetic food webs. Yet, Hg sources in the deep-sea chemosynthetic food webs are still uncertain. Here, we report Hg concentrations and stable isotopic compositions of indigenous biota living at hydrothermal fields of the Indian Ocean Ridge and a cold seep of the South China Sea along with hydrothermal sulfide deposits. We find that Hg is highly enriched in hydrothermal sulfides, which correlated with varying Hg concentrations in inhabited biota. Both the hydrothermal and cold seep biota have small fractions (<10%) of Hg as MMHg and slightly positive Δ199Hg values. These Δ199Hg values are slightly higher than those in near-field sulfides but are 1 order of magnitude lower than the trench counterparts. We suggest that deep-sea chemosynthetic food webs mainly assimilate Hg from ambient seawater/sediments and hydrothermal fluids formed by percolated seawater through magmatic/mantle rocks. The MMHg transfer from photosynthetic to chemosynthetic food webs is likely limited. The contrasting Hg sources between chemosynthetic and trench food webs highlight Hg isotopes as promising tools to trace the deep-sea Hg biogeochemical cycle.

Elevated nutrients and surface chlorophyll-α associated with natural methane seeps in the Haima cold seep area of the Qiongdongnan Basin, northern South China Sea.

Cold seeps are a significant source of methane to the ocean. However, nutrients and Chl-α in the euphotic layer overlying cold seeps have been poorly studied. Variations in Chl-α, nutrients, environmental parameters, and CH4 concentrations in the Haima cold seeps were analyzed. Results show that the overlying water exhibits a typical low nutrient and low Chl-α marine environment. Phosphate and Chl-α were significantly elevated, and the average SCM in cold seeps was much higher than that in control stations. Spearman correlation analysis indicated Chl-α in cold seep was positively correlated with salinity and negatively with nutrient and CH4 concentrations. It implied that the CH4 concentrations may promote the increase of Chl-α, and may be linked to CH4 plumes, bringing cold, nutrient-rich waters to the thermocline. However, due to the CH4 plumes hardly to track, more sampling is needed to determine the effects on Chl-α and phytoplankton in the euphotic layer.

Six new species of free-living nematodes (Nematoda: Enoplida) from deep-sea cold seeps on Hikurangi Margin, New Zealand.

Little is known about the taxonomy of deep-sea nematode species inhabiting cold seep habitats. An opportunity to characterize the nematode species communities of New Zealand cold seeps was provided by a 2019 research voyage to New Zealand's Hikurangi Margin, during which macrofauna cores were obtained at two seeps at approximately 1,250 and 2,000 m water depth. Here, six new species of the orderEnoplida are described. Metacylicolaimus catherinae sp. nov. represents the first record of the genus for the New Zealand Exclusive Economic Zone and for the deep sea globally. Halalaimus talaurinus sp. nov., Thalassoalaimus duoporus sp. nov. and Crenopharynx crassipapilla sp. nov. are only the second species of their respective genera to be described/recorded from New Zealand waters, and Oncholaimus adustus sp. nov. is the eighth species of the genus to be recorded from the region. Rhabdodemania zealandiaensis sp. nov. was among the most abundant and widespread species found at the Hikurangi Margin seep sites. A few specimens had been found in a previous ecological study of meiofaunal nematode communities on Chatham Rise, a submarine ridge south of Hikurangi Margin. It is possible that this species has a preference for seep environments due to elevated food availability, however it does not seem to be exclusively found in seeps. We find no evidence for an affinity between nematode seep communities in New Zealand and elsewhere, which is consistent with the high variability in nematode community observed to date among regions. Ongoing work on the ecology and distribution of nematode communities at the Hikurangi Margin seep sites will help determine spatial patterns in abundance and species distributions in more detail, including the identification of any species/taxa with affinities with seeps.

Molecular evidence for the production of labile, sulfur-bearing dissolved organic matter in the seep sediments of the South China Sea.

Cold seeps with methane-rich fluids leaking out of the seafloor usually support massive biomass of chemosynthetic organisms and associated fauna. A substantial amount of methane is converted to dissolved inorganic carbon by microbial metabolism, and this process also releases dissolved organic matter (DOM) into pore water. Here, pore water samples from "Haima cold seeps" sediments and the non-seep reference sediments in the northern South China Sea were analyzed for optical properties and molecular compositions of pore water DOM. Our results showed that the relative abundance of protein-like DOM, H/Cwa and molecular lability boundary percentage (MLBL%) in the seep sediments were significantly higher than those in the reference sediments, indicating that more labile DOM related to unsaturated aliphatic compounds is produced in the seep sediments. Spearman's correlation of the fluoresce and molecular data suggested that the humic-like components (C1 and C2) mainly constituted the refractory compounds (CRAM, highly unsaturated and aromatics compounds). In contrast, the protein-like component (C3) had high H/C ratios featuring high degree of DOM lability. The amount of S-containing formulas (CHOS and CHONS) was greatly elevated in the seep sediments, likely caused by abiotic and biotic sulfurization of DOM in the sulfidic environment. Although the abiotic sulfurization was proposed to have a stabilizing effect on organic matter, our results implied that the biotic sulfurization in the cold seep sediments would increase DOM lability. Overall, the labile DOM accumulated in the seep sediments is closely linked to methane oxidation, which not only support heterotrophic communities and but also likely have an impact on carbon and sulfur cycling in the sediments and the ocean.

A New Method for Training CycleGAN to Enhance Images of Cold Seeps in the Qiongdongnan Sea.

Clear underwater images can help researchers detect cold seeps, gas hydrates, and biological resources. However, the quality of these images suffers from nonuniform lighting, a limited range of visibility, and unwanted signals. CycleGAN has been broadly studied in regard to underwater image enhancement, but it is difficult to apply the model for the further detection of Haima cold seeps in the South China Sea because the model can be difficult to train if the dataset used is not appropriate. In this article, we devise a new method of building a dataset using MSRCR and choose the best images based on the widely used UIQM scheme to build the dataset. The experimental results show that a good CycleGAN could be trained with the dataset using the proposed method. The model has good potential for applications in detecting the Haima cold seeps and can be applied to other cold seeps, such as the cold seeps in the North Sea. We conclude that the method used for building the dataset can be applied to train CycleGAN when enhancing images from cold seeps.

Community assemblages and species coexistence of prokaryotes controlled by local environmental heterogeneity in a cold seep water column.

The distribution and heterogeneity characteristics of microbial communities in cold seep water columns are significant factors governing the efficiency of methane filtering and carbon turnover. However, this process is poorly understood. The diversity of vertically stratified microbial communities and the factors controlling the community assemblage process in the water column above the Haima cold seep were investigated in this study. The prokaryotic community diversities varied distinctly with vertical changes in hydrochemistry. Cyanobacteria dominated the light-transmitting layers and Proteobacteria dominated the deeper layers. With respect to microbial community assemblages and co-occurrence networks, stochastic processes were particularly important in shaping prokaryotic communities. In the shallow (≥85 m) and mesopelagic water columns (600-800 m), microbial community characteristics were affected by deterministic processes, reduced network connectivity, and modularity. Microbial community diversities and assemblage processes along a vertical profile were influenced by the vertical variations in pH, temperature, DIC, and nutrients. Stochastic processes may have facilitated the formation of complex co-occurrence networks. Briefly, the distribution of local environmental heterogeneity along the vertical dimension could drive unique microbial community assemblage and species coexistence patterns. This study provides new perspectives on how microorganisms adapt to the environment and build communities, and how species coexist in shared habitats.

First record of microplastics in the Gigantidas platifrons (Mytilidae: Bathymodiolus) and Shinkaia crosnieri (Munidopsidae: Shinkaia) from cold-seep in the South China Sea.

The deep sea is considered the final sink for microplastics (MPs) pollution in marine environments. Despite MPs being consumed by many organisms, their presence in cold-seep organisms remains unclear. At cold seep site F in the South China Sea, which lies 1155 m deep, MPs were investigated in Gigantidas platifrons and Shinkaia crosnieri, both ecologically important species. Microplastics ingestion rates were 80.98 % and 81.25 % in G. platifrons and S. crosnieri, respectively. The average MPs abundance for G. platifrons and S. crosnieri was 2.80 ± 0.69 MPs/ind (2.20 ± 2.75 MPs/g) and 2.30 ± 0.27 MPs/ind (4.74 ± 3.08 MPs/g), with no significant difference between species. There were predominantly fibrous MPs ingested by cold-seep organisms, sized 500-1000 µm, blue in color, and polyethylene terephthalate and cellophane in composition. The results confirm the presence of MPs in cold-seep organisms in the South China Sea, providing further evidence that MP pollution exists in the deep sea.

Macrofaunal Distribution, Diversity, and Its Ecological Interaction at the Cold Seep Site of Krishna-Godavari Basin, East Coast of India.

Cold seeps are characterized by typical endemic communities with associated microorganisms that depend on sulfide, methane, reduced nitrogenous compounds, and metals as electron donors for their survival through chemosynthesis. The discovery of an active cold seep site in January 2018 in the Krishna-Godavari (K-G) basin of Bay of Bengal was followed by a transit cruise in March 2018 to investigate the distribution and diversity of macrofauna. Further, the ambient sediment and pore water biochemistry were estimated to understand its relationship with macrofauna and the microbial associates of the sediment. Samples were collected at a water depth of around 1750 m at 3 stations: SP1, SP2, and SP3, using the box corer. The benthic fauna at the sites consisted mainly of Bivalvia, shrimps of Caridea family, Gastropoda species, Malacostraca species, Polychaeta, and few species of Echinoidea, Ophiuroidea, and Echiura. A total of 2313 macrofaunal individuals belonging to 8 classes, 18 families, and 20 species were identified from all the three stations. The communities were diverse at these sites with an average Shannon diversity index of 1.64 and are closely related to the lineages previously studied in ecologically similar environments. Most of the macrofauna were found to be filter feeders preferring a low organic carbon environment. Relict vesicomyid clams at the present study site suggest the succession from vesicomyids to the present composition of bivalve mussels and siboglinid worms. The microbial associates in the sediment significantly correlated with methane and hydrogen sulfide concentrations. The study suggests that the K-G basin cold seep serves as a conducive environment for the flourishing of benthic communities and therefore can support a rich biodiversity.

Methane supply drives prokaryotic community assembly and networks at cold seeps of the South China Sea.

Marine cold seeps are unique chemosynthetic habitats fuelled by deeply sourced hydrocarbon-rich fluids discharged at the seafloor. Through oxidizing methane and other hydrocarbons, microorganisms inhabiting cold seeps supply subsurface-derived energy to higher trophic levels, sustaining highly productive oases of life in the deep sea. Despite the central role of microbiota in mediating biogeochemical cycles, the factors that govern the assembly and network of prokaryotic communities in cold seeps remain poorly understood. Here we analysed the geochemical and microbiological profiles of 11 different sediment cores from two spatially distant cold seeps of the South China Sea. We show that prokaryotic communities belonging to the same methane-supply regimes (high-methane-supply, low-methane-supply and non-seep control sediments) had a highly similar community structure, regardless of geographical location, seep-associated biota (mussel, clam, microbial mat) and sediment depth. Methane supply appeared to drive the niche partitioning of anaerobic methanotrophic archaea (ANME) at the regional scale, with ANME-1 accounting for >60% sequence abundance of ANME in the high-methane-supply sediments, while ANME-2 dominated (>90%) the low-methane-supply sediments. Increasing methane supply enhanced the contribution of environmental selection but lessened the contributions of dispersal limitation and drift to overall community assembly. High methane supply, moreover, promoted a more tightly connected, less stable prokaryotic network dominated by positive correlations. Together, these results provide a potentially new framework for understanding the niches and network interplay of prokaryotic communities across different methane seepage regimes in cold-seep sediments.

Deep-Sea Natural Products from Extreme Environments: Cold Seeps and Hydrothermal Vents.

The deep sea has been proven to be a great treasure for structurally unique and biologically active natural products in the last two decades. Cold seeps and hydrothermal vents, as typical representatives of deep-sea extreme environments, have attracted more and more attention. This review mainly summarizes the natural products of marine animals, marine fungi, and marine bacteria derived from deep-sea cold seeps and hydrothermal vents as well as their biological activities. In general, there were 182 compounds reported, citing 132 references and covering the literature from the first report in 1984 up to March 2022. The sources of the compounds are represented by the genera Aspergillus sp., Penicillium sp., Streptomyces sp., and so on. It is worth mentioning that 90 of the 182 compounds are new and that almost 60% of the reported structures exhibited diverse bioactivities, which became attractive targets for relevant organic synthetic and biosynthetic studies.

Genomic, biochemical, and phylogenetic evaluation of bacteria isolated from deep-sea sediment harboring methane hydrates.

Over half of the organic carbon on Earth's surface is trapped in marine sediment as methane hydrates. Ocean warming causes hydrate dissociation and methane leakage to the water column, rendering the characterization of microbes from hydrate depositions a pressing matter. Through genomic, phylogenetic, and biochemical assays, we characterize the first microorganisms isolated from the Rio Grande Cone (Brazil), reservoir responsible for massive methane releases to the water column. From sediment harboring rich benthic communities, we obtained 43 strains of Brevibacillus sp., Paenibacillus sp. and groups of Bacillus sp. Methane-enriched samples yielded strains of the Pseudomonas fluorescens complex, exhibiting fluorescent siderophore production and broad multi-carbon catabolism. Genomic characterization of a novel Pseudomonas sp. strain indicated 32 genes not identified in the closest related type-species, including proteins involved with mercury resistance. Our results provide phylogenetic and genomic insights on the first bacterial isolates retrieved from a poorly explored region of the South Atlantic Ocean.

Interpreting Molecular and Isotopic Biosignatures in Methane-Derived Authigenic Carbonates in the Light of a Potential Carbon Cycle in the Icy Moons.

Finding evidence of life beyond Earth is the aim of future space missions to icy moons. Icy worlds with an ocean underlying the icy crust and in contact with a rocky subsurface have great astrobiological interest due to the potential for water-rock interactions that may provide a source of nutrients necessary to sustain life. Such water-rock interactions in icy moons can be indirectly investigated using analogous environments on the deep seafloor on Earth. Here, we investigate the presence of molecular and isotopic biomarkers in two submarine cold seep systems with intense rock-fluid interactions and carbon sink as carbonates with the aim of gaining understanding of potential carbon cycles in the icy worlds' oceans. Authigenic carbonates associated to cold seeps (a chimney from the Gulf of Cádiz and a clathrite from the Pacific Hydrate Ridge) were investigated for their mineralogical composition and lipid biomarker distribution. Molecular and compound-specific isotopic composition of lipid biomarkers allowed us to infer different carbonate origins in both carbonate scenarios: biogenic methane (clathrite) versus thermogenic methane together with allochthonous carbon (chimney). In the Pacific cold seep, carbonate precipitation of the clathrite was deduced to result from the anaerobic oxidation of methane by syntrophic action of methanotrophic archaea with sulfate-reducing bacteria. The distinct carbon sources (thermogenic methane, pelagic biomass, etc.) and sinks (gas clathrates, clathrite, chimney carbonates) were discussed in the light of potentially similar carbon cycling pathways in analogous icy-moon oceans. We show how the isotopic analysis of carbon may be crucial for detecting biosignatures in icy-world carbon sinks. These considerations may affect the strategy of searching for biosignatures in future space missions to the icy worlds.

Insights into carbon-fixation pathways through metagonomics in the sediments of deep-sea cold seeps.

Carbon fixation by chemoautotrophic microorganisms in the dark ocean has a major impact on global carbon cycling and ecological relationships in the ocean's interior. At present, six pathways of autotrophic carbon fixation have been found: the Calvin cycle, the reductive Acetyl-CoA or Wood-Ljungdahl pathway (rAcCoA), the reductive tricarboxylic acid cycle (rTCA), the 3-hydroxypropionate bicycle (3HP), the 3-hydroxypropionate/4-hydroxybutyrate cycle (3HP/4HB), and the dicarboxylate/4-hydroxybutyrate cycle (DC/4HB). Although our knowledge about carbon fixation pathways in the ocean has increased significantly, carbon fixation pathways in the cold seeps are still unknown. In this study, we collected sediment samples from two cold seeps and one trough in the south China sea (SCS), and investigated with metagenomic and metagenome assembled genomes (MAGs). We found that six autotrophic carbon fixation pathways present in the cold seeps and trough with rTCA cycle was the most common pathway, whose genes were particularly high in the cold seeps and increased with sediment depths; the rAcCoA cycle mainly occurred in the cold seep regions, and the abundance of module genes increased with sediment depths. We also elucidated members of chemoautotrophic microorganisms involved in these six carbon-fixation pathways. The rAcCoA, rTCA and DC/4-HB cycles required significantly less energy probably play an important role in the deep-sea environments, especially in the cold seeps. This study provided metabolic insights into the carbon fixation pathways in the cold seeps, and laid the foundation for future detailed study on processes and rates of carbon fixation in the deep-sea ecosystems.

Biogeographic distributions of microbial communities associated with anaerobic methane oxidation in the surface sediments of deep-sea cold seeps in the South China Sea.

Cold seeps are oasis for the microbes in the deep-sea ecosystems, and various cold seeps are located along the northern slope of the South China Sea (SCS). However, by far most microbial ecological studies were limited to specific cold seep in the SCS, and lack of comparison between different regions. Here, the surface sediments (0-4 cm) from the Site F/Haima cold seeps and the Xisha trough in the SCS were used to elucidate the biogeography of microbial communities, with particular interest in the typical functional groups involved in the anaerobic oxidation of methane (AOM) process. Distinct microbial clusters corresponding to the three sampling regions were formed, and significantly higher gene abundance of functional groups were present in the cold seeps than the trough. This biogeographical distribution could be explained by the geochemical characteristics of sediments, such as total nitrogen (TN), total phosphorus (TP), nitrate (NO3 -), total sulfur (TS) and carbon to nitrogen ratios (C/N). Phylogenetic analysis demonstrated that mcrA and pmoA genotypes were closely affiliated with those from wetland and mangroves, where denitrifying anaerobic methane oxidation (DAMO) process frequently occurred; and highly diversified dsrB genotypes were revealed as well. In addition, significantly higher relative abundance of NC10 group was found in the Xisha trough, suggesting that nitrite-dependent DAMO (N-DAMO) process was more important in the hydrate-bearing trough, although its potential ecological contribution to AOM deserves further investigation. Our study also further demonstrated the necessity of combining functional genes and 16S rRNA gene to obtain a comprehensive picture of the population shifts of natural microbial communities among different oceanic regions.

Laminatubus (Serpulidae, Annelida) from eastern Pacific hydrothermal vents and methane seeps, with description of two new species.

The bathyal serpulid Laminatubus alvini ten Hove Zibrowius, 1986 was described from the periphery of hydrothermal vents of the Galapagos Rift and has been recorded from other vent communities of the East Pacific Rise (EPR). Here we assessed the biodiversity of serpulids collected from eastern Pacific hydrothermal vents and methane seeps using DNA sequences and morphology. Laminatubus alvini showed little genetic variation over a wide geographic range from the Alarcon Rise vents in southern Gulf of California (~23°N), to at least a point at 38°S on the EPR. Specimens from several methane seeps off Costa Rica and the Gulf of California (Mexico) differed markedly from those of Laminatubus alvini on DNA sequence data and in having seven thoracic chaetigers and lacking Spirobranchus-type special collar chaetae, thus fitting the diagnosis of Neovermilia. However, phylogenetic analysis of molecular data showed that L. alvini and the seep specimens form a well-supported clade. Moreover, among the seep specimens there was minimally a ~7% distance in mitochondrial cytochrome b sequences between a shallow-water (1000 m) seep clade restricted to Costa Rica and a deep-water clade (1800 m) from Costa Rica to Gulf of California. We describe the seep taxa here as morphologically indistinguishable L. paulbrooksi n. sp. and L. joycebrooksae n. sp.