Seafloor methane emission on the Makran continental margin.

Seafloor methane emission is widespread on both active and passive continental margins, which may exerts significant impact on global climate change, ocean acidification, cold seep ecosystem, and global carbon cycle. However, due to the limitation of the thick water body, systematic knowledge of detection, quantification and activity of the submarine methane seepage is still unreachable, which greatly limits the assessment of the environmental impact. In 2018, a comprehensive geological survey, including multibeam mapping, seafloor observation, and seismic reflection profiling, was conducted using R/V "Haiyangdizhi 10" on the Makran continental margin. Sixty-five gas flares, which indicated seafloor methane seepage, were detected in a total survey area of 32,000 km2. The total methane flux of the surveyed area is estimated to be 4.7-5.9 × 103 Mg/yr, accounting for 0.013-0.016% of the global seafloor methane emission. In addition, three gas seeps, which were active in 2007, were inactive during our survey in 2018. It is inferred that the intermittent activity might be related to the periodic pressure release and accumulation in the system. All the flares vanish in the water column, which indicates that all the methane gas was oxidized and/or dissolved by seawater. No methane was observed entering the atmosphere in gas phase. In this study, we present new data sets of methane seeps on the Makran continental margin, which are useful to better understand the behavior of the submarine methane seepage.

Frenulate siboglinids at high Arctic methane seeps and insight into high latitude frenulate distribution.

Frenulate species were identified from a high Arctic methane seep area on Vestnesa Ridge, western Svalbard margin (79°N, Fram Strait) based on mitochondrial cytochrome oxidase subunit I (mtCOI). Two species were found: Oligobrachia haakonmosbiensis, and a new, distinct, and undescribed Oligobrachia species. The new species adds to the cryptic Oligobrachia species complex found at high latitude methane seeps in the north Atlantic and the Arctic. However, this species displays a curled tube morphology and light brown coloration that could serve to distinguish it from other members of the complex. A number of single tentacle individuals were recovered which were initially thought to be members of the only unitentaculate genus, Siboglinum. However, sequencing revealed them to be the new species and the single tentacle morphology, in addition to thin, colorless, and ringless tubes indicate that they are juveniles. This is the first known report of juveniles of northern Oligobrachia. Since the juveniles all appeared to be at about the same developmental stage, it is possible that reproduction is either synchronized within the species, or that despite continuous reproduction, settlement, and growth in the sediment only takes place at specific periods. The new find of the well-known species O. haakonmosbiensis extends its range from the Norwegian Sea to high latitudes of the Arctic in the Fram Strait. We suggest bottom currents serve as the main distribution mechanism for high latitude Oligobrachia species and that water depth constitutes a major dispersal barrier. This explains the lack of overlap between the distributions of northern Oligobrachia species despite exposure to similar current regimes. Our results point toward a single speciation event within the Oligobrachia clade, and we suggest that this occurred in the late Neogene, when topographical changes occurred and exchanges between Arctic and North Atlantic water masses and subsequent thermohaline circulation intensified.

Ubiquitous Presence and Novel Diversity of Anaerobic Alkane Degraders in Cold Marine Sediments.

Alkanes are major constituents of crude oil and are released to the marine environment by natural seepage and from anthropogenic sources. Due to their chemical inertness, their removal from anoxic marine sediments is primarily controlled by the activity of anaerobic alkane-degrading microorganisms. To facilitate comprehensive cultivation-independent surveys of the diversity and distribution of anaerobic alkane degraders, we designed novel PCR primers that cover all known diversity of the 1-methylalkyl succinate synthase gene (masD/assA), which catalyzes the initial activation of alkanes. We studied masD/assA gene diversity in pristine and seepage-impacted Danish coastal sediments, as well as in sediments and alkane-degrading enrichment cultures from the Middle Valley (MV) hydrothermal vent system in the Pacific Northwest. MasD/assA genes were ubiquitously present, and the primers captured the diversity of both known and previously undiscovered masD/assA gene diversity. Seepage sediments were dominated by a single masD/assA gene cluster, which is presumably indicative of a substrate-adapted community, while pristine sediments harbored a diverse range of masD/assA phylotypes including those present in seepage sediments. This rare biosphere of anaerobic alkane degraders will likely increase in abundance in the event of seepage or accidental oil spillage. Nanomolar concentrations of short-chain alkanes (SCA) were detected in pristine and seepage sediments. Interestingly, anaerobic alkane degraders closely related to strain BuS5, the only SCA degrader in pure culture, were found in mesophilic MV enrichments, but not in cold sediments from Danish waters. We propose that the new masD/assA gene lineages in these sediments represent novel phylotypes that are either fueled by naturally occurring low levels of SCA or that metabolize medium- to long-chain alkanes. Our study highlights that masD/assA genes are a relevant diagnostic marker to identify seepage and microseepage, e.g., during prospecting for oil and gas, and may act as an indicator of anthropogenic oil spills in marine sediments.