Element variation in a clam shell and its implications for cold seep irregular eruptions: Calyptogena sp. in the Haima cold seep.

Cold seep is characterized by methane-rich fluids released from subsurface reservoirs, and it sustains the chemosynthetic ecosystems on the seafloor. Previous studies suggest that the activity of cold seep could affect the seawater chemistry and ambient temperature. However, the short-term seep activity was hardly reconstructed due to the focus of studies on carbonate or sediment. Vent macrofauna provide such an opportunity by recording in shells the immediate environment in which they grow. The carbonate skeleton of organisms could theoretically preserve environmental variation. Therefore, high-resolution archives are urgently required to understand the influence of cold seep activity on biogeochemistry. In this study, SEM, EDS mapping, EBSD mapping, and LA-ICP-MS analyses were conducted on a clam (Calyptogena sp.) shell collected alive in 2018 from the Haima cold seep in South China Sea. The CaCO3, Na, Mg, Sr, and Ba contents and the Sr/Ca, Ba/Ca, and Na/Ca ratios in the hinge plate were measured on LA-ICP-MS by spot analysis and line scanning. The element contents in the hinge are as follows: Mg (38.5-109 µg/g), Na (3117-5246 µg/g), Sr (970-5371 µg/g), and Ba (2.9-11.5 µg/g). The results show that Sr, Re, and Ba content vary synchronously along the direction of growth, but Na has an opposite trend. The element analyses indicate that the eruption of the Haima cold seep was irregular, causing temperature, redox state, and pH changes in the cold seep ecosystem. These findings show that the irregular cold seep activities exert vital influences on the biogeochemistry of the cold seep ecosystem, which shed a light on cold seep biomonitoring.

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.

Host-Endosymbiont Genome Integration in a Deep-Sea Chemosymbiotic Clam.

Endosymbiosis with chemosynthetic bacteria has enabled many deep-sea invertebrates to thrive at hydrothermal vents and cold seeps, but most previous studies on this mutualism have focused on the bacteria only. Vesicomyid clams dominate global deep-sea chemosynthesis-based ecosystems. They differ from most deep-sea symbiotic animals in passing their symbionts from parent to offspring, enabling intricate coevolution between the host and the symbiont. Here, we sequenced the genomes of the clam Archivesica marissinica (Bivalvia: Vesicomyidae) and its bacterial symbiont to understand the genomic/metabolic integration behind this symbiosis. At 1.52 Gb, the clam genome encodes 28 genes horizontally transferred from bacteria, a large number of pseudogenes and transposable elements whose massive expansion corresponded to the timing of the rise and subsequent divergence of symbiont-bearing vesicomyids. The genome exhibits gene family expansion in cellular processes that likely facilitate chemoautotrophy, including gas delivery to support energy and carbon production, metabolite exchange with the symbiont, and regulation of the bacteriocyte population. Contraction in cellulase genes is likely adaptive to the shift from phytoplankton-derived to bacteria-based food. It also shows contraction in bacterial recognition gene families, indicative of suppressed immune response to the endosymbiont. The gammaproteobacterium endosymbiont has a reduced genome of 1.03 Mb but retains complete pathways for sulfur oxidation, carbon fixation, and biosynthesis of 20 common amino acids, indicating the host's high dependence on the symbiont for nutrition. Overall, the host-symbiont genomes show not only tight metabolic complementarity but also distinct signatures of coevolution allowing the vesicomyids to thrive in chemosynthesis-based ecosystems.