Compositional and Functional Shifts in the Epibiotic Bacterial Community of Shinkaia crosnieri Baba & Williams (a Squat Lobster from Hydrothermal Vents) during Methane-Fed Rearing

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.

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.

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

Remarkably diverse bacteria have been observed as biofilm aggregates on the surface of deep-sea invertebrates that support the growth of hosts through chemosynthetic carbon fixation. Growing evidence also indicates that community-wide interactions, and especially cooperation among symbionts, contribute to overall community productivity. Here, metagenome-guided metatranscriptomic and metabolic analyses were conducted to investigate the taxonomic composition, functions, and potential interactions of symbionts dwelling on the seta of Shinkaia crosnieri lobsters in a methane cold seep. Methylococcales and Thiotrichales dominated the community, followed by the Campylobacteriales, Nitrosococcales, Flavobacteriales, and Chitinophagales Metabolic interactions may be common among the episymbionts since many separate taxon genomes encoded complementary genes within metabolic pathways. Specifically, Thiotrichales could contribute to detoxification of hydroxylamine that is a metabolic by-product of Methylococcales. Further, Nitrosococcales may rely on methanol leaked from Methylococcales cells that efficiently oxidize methane. Elemental sulfur may also serve as a community good that enhances sulfur utilization that benefits the overall community, as evidenced by confocal Raman microscopy. Stable intermediates may connect symbiont metabolic activities in cyclical oxic-hypoxic fluctuating environments, which then enhance overall community functioning. This hypothesis was partially confirmed via in situ experiments. These results highlight the importance of microbe-microbe interactions in symbiosis and deep-sea adaptation.

IMPORTANCE Symbioses between chemosynthetic bacteria and marine invertebrates are common in deep-sea chemosynthetic ecosystems and are considered critical foundations for deep-sea colonization. Episymbiotic microorganisms tend to form condensed biofilms that may facilitate metabolite sharing among biofilm populations. However, the prevalence of metabolic interactions among deep-sea episymbionts and their contributions to deep-sea adaptations are not well understood due to sampling and cultivation difficulties associated with deep-sea environments. Here, we investigated metabolic interactions among the episymbionts of Shinkaia crosnieri, a dominant chemosynthetic ecosystem lobster species in the Northwest Pacific Ocean. Meta-omics characterizations were conducted alongside in situ experiments to validate interaction hypotheses. Furthermore, imaging analysis was conducted, including electron microscopy, fluorescent in situ hybridization (FISH), and confocal Raman microscopy (CRM), to provide direct evidence of metabolic interactions. The results support the Black Queen Hypothesis, wherein leaked public goods are shared among cohabitating microorganisms to enhance the overall adaptability of the community via cooperation.

Metatranscriptomics by In Situ RNA Stabilization Directly and Comprehensively Revealed Episymbiotic Microbial Communities of Deep-Sea Squat Lobsters

Shinkaia crosnieri is an invertebrate that inhabits an area around deep-sea hydrothermal vents in the Okinawa Trough in Japan by harboring episymbiotic microbes as the primary nutrition. To reveal physiology and phylogenetic composition of the active episymbiotic populations, metatranscriptomics is expected to be a powerful approach. However, this has been hindered by substantial perturbation (e.g., RNA degradation) during time-consuming retrieval from the deep sea. Here, we conducted direct metatranscriptomic analysis of S. crosnieri episymbionts by applying in situ RNA stabilization equipment. As expected, we obtained RNA expression profiles that were substantially different from those obtained by conventional metatranscriptomics (i.e., stabilization after retrieval). The episymbiotic community members were dominated by three orders, namely, Thiotrichales, Methylococcales, and Campylobacterales, and the Campylobacterales members were mostly dominated by the Sulfurovum genus. At a finer phylogenetic scale, the episymbiotic communities on different host individuals shared many species, indicating that the episymbionts on each host individual are not descendants of a few founder cells but are horizontally exchanged. Furthermore, our analysis revealed the key metabolisms of the community: two carbon fixation pathways, a formaldehyde assimilation pathway, and utilization of five electron donors (sulfide, thiosulfate, sulfur, methane, and ammonia) and two electron accepters (oxygen and nitrate/nitrite). Importantly, it was suggested that Thiotrichales episymbionts can utilize intercellular sulfur globules even when sulfur compounds are not usable, possibly also in a detached and free-living state.

IMPORTANCE Deep-sea hydrothermal vent ecosystems remain mysterious. To depict in detail the enigmatic life of chemosynthetic microbes, which are key primary producers in these ecosystems, metatranscriptomic analysis is expected to be a promising approach. However, this has been hindered by substantial perturbation (e.g., RNA degradation) during time-consuming retrieval from the deep sea. In this study, we conducted direct metatranscriptome analysis of microbial episymbionts of deep-sea squat lobsters (Shinkaia crosnieri) by applying in situ RNA stabilization equipment. Compared to conventional metatranscriptomics (i.e., RNA stabilization after retrieval), our method provided substantially different RNA expression profiles. Moreover, we discovered that S. crosnieri and its episymbiotic microbes constitute complex and resilient ecosystems, where closely related but various episymbionts are stably maintained by horizontal exchange and partly by their sulfur storage ability for survival even when sulfur compounds are not usable, likely also in a detached and free-living state.

Dual energy metabolism of the Campylobacterota endosymbiont in the chemosynthetic snail Alviniconcha marisindica

Some deep-sea chemosynthetic invertebrates and their symbiotic bacteria can use molecular hydrogen (H₂) as their energy source. However, how much the chemosynthetic holobiont (endosymbiont-host association) physiologically depends on H₂ oxidation has not yet been determined. Here, we demonstrate that the Campylobacterota endosymbionts of the gastropod Alviniconcha marisindica in the Kairei and Edmond fields (kAlv and eAlv populations, respectively) of the Indian Ocean, utilize H₂ in response to their physical and environmental H₂ conditions, although the 16S rRNA gene sequence of both the endosymbionts shared 99.6% identity. A thermodynamic calculation using in situ H₂ and hydrogen sulfide (H₂S) concentrations indicated that chemosynthetic symbiosis could be supported by metabolic energy via H₂ oxidation, particularly for the kAlv holobiont. Metabolic activity measurements showed that both the living individuals and the gill tissues consumed H₂ and H₂S at similar levels. Moreover, a combination of fluorescence in situ hybridization, quantitative transcript analyses, and enzymatic activity measurements showed that the kAlv endosymbiont expressed the genes and enzymes for both H₂- and sulfur-oxidations. These results suggest that both H₂ and H₂S could serve as the primary energy sources for the kAlv holobiont. The eAlv holobiont had the ability to utilize H₂, but the gene expression and enzyme activity for hydrogenases were much lower than for sulfur-oxidation enzymes. These results suggest that the energy acquisitions of A. marisindica holobionts are dependent on H₂- and sulfur-oxidation in the H₂-enriched Kairei field and that the mechanism of dual metabolism is controlled by the in situ H₂ concentration.