Metabolism Interactions Promote the Overall Functioning of the Episymbiotic Chemosynthetic Community of Shinkaia crosnieri of Cold Seeps

To live or die: "Fine-tuning" adaptation revealed by systemic analyses in symbiotic bathymodiolin mussels from diverse deep-sea extreme ecosystems

Hydrothermal vents (HVs) and cold seeps (CSs) are typical deep-sea extreme ecosystems with their own geochemical characteristics to supply the unique living conditions for local communities. Once HVs or CSs stop emission, the dramatic environmental change would pose survival risks to deep-sea organisms. Up to now, limited knowledge has been available to understand the biological responses and adaptive strategy to the extreme environments and their transition from active to extinct stage, mainly due to the technical difficulties and lack of representative organisms. In this study, bathymodiolin mussels, the dominant and successful species surviving in diverse deep-sea extreme ecosystems, were collected from active and extinct HVs (Southwest Indian Ocean) or CSs (South China Sea) via two individual cruises. The transcriptomic analysis and determination of multiple biological indexes in stress defense and metabolic systems were conducted in both gills and digestive glands of mussels, together with the metagenomic analysis of symbionts in mussels. The results revealed the ecosystem- and tissue-specific transcriptional regulation in mussels, addressing the autologous adaptations in antioxidant defense, energy utilization and key compounds (i.e. sulfur) metabolism. In detail, the successful antioxidant defense contributed to conquering the oxidative stress induced during the unavoidable metabolism of xenobiotics commonly existing in the extreme ecosystems; changes in metabolic rate functioned to handle toxic matters in different surroundings; upregulated gene expression of sulfide:quinone oxidoreductase indicated an active sulfide detoxification in mussels from HVs and active stage of HVs & CSs. Coordinately, a heterologous adaptation, characterized by the functional compensation between symbionts and mussels in energy utilization, sulfur and carbon metabolism, was also evidenced by the bacterial metagenomic analysis. Taken together, a new insight was proposed that symbiotic bathymodiolin mussels would develop a "finetuning" strategy combining the autologous and heterologous regulations to fulfill the efficient and effective adaptations for successful survival.

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.

Variation in epibiotic bacteria on two squat lobster species of Munidopsidae

The relationships between epibiotic bacteria on deep-sea hosts and host lifestyle factors are of particular interest in the field of deep-sea chemoautotrophic environmental adaptations. The squat lobsters Shinkaia crosnieri and Munidopsis verrilli are both dominant species in cold-seep ecosystems, and they have different distributions and feeding behaviors. These species may have evolved to have distinct epibiotic microbiota. Here, we compared the epibiotic bacterial communities on the M. verrilli carapace (MV_carapace), S. crosnieri carapace (SC_carapace), and S. crosnieri ventral plumose setae (SC_setae). The epibiotic bacteria on SC_setae were dense and diverse and had a multi-layer configuration, while those on MV_carapace and SC_carapace were sparse and had a monolayer configuration. Chemoautotrophic bacteria had the highest relative abundance in all epibiotic bacterial communities. The relative abundance of amplicon sequence variant 3 (ASV3; unknown species in order Thiotrichales), which is associated with sulfide oxidation, was significantly higher in SC_setae than MV_carapace and SC_carapace. Thiotrichales species seemed to be specifically enriched on SC_setae, potentially due to the synthetic substrate supply, adhesion preference, and host behaviors. We hypothesize that the S. crosnieri episymbionts use chemical fluxes near cold seeps more efficiently, thereby supporting the host's nutrient strategies, resulting in a different distribution of the two species of squat lobster.