**An Insightful Model to Study Innate Immunity and Stress Response in Deep‐Sea Vent Animals: Profiling the Mussel** *Bathymodiolus azoricus*

Raul Bettencourt, Inês Barros, Eva Martins, Inês Martins, Teresa Cerqueira, Ana Colaço, Valentina Costa, Domitília Rosa, Hugo Froufe, Conceição Egas, Sergio Stefanni, Paul Dando and Ricardo S. Santos

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/68034

#### **Abstract**

Deep‐sea environments are, in some cases, largely unexplored ecosystems, where life thrives driven by the geochemical features of each location. Among these environments, chemosynthesis‐based ecosystems, in the Mid Atlantic Ridge, have an exclusive combi‐ nation of high depth, high sulfur, and high methane concentrations. This is believed to modulate the biological composition of vent communities and influence the overall vent animal transcriptional activity of genes involved in adaptation processes to extreme envi‐ ronments. This opens, thus, the possibility of finding gene expression signatures specific to a given hydrothermal vent field. Regardless of the extreme physicochemical conditions that characterize deep‐sea hydrothermal vents, the animals dwelling around the vent sites exhibit high productivity and thus must cope with toxic nature of vent surrounding, seemingly deleterious to the animals, while developing surprisingly successful strategies to withstand adverse environmental conditions, including environmental microbes and mechanical stress whether ensuing from animal predation or venting activity. The deep‐ sea vent mussel *Bathymodiolus azoricus* has adapted well to deep‐sea extreme environ‐ ments and represents the dominating faunal community from hydrothermal vent sites in the Mid‐Atlantic Ridge, owing its successful adaptation and high biomasses to special‐ ized exploitation of methane and sulfide sources from venting activity. Its extraordinary capabilities of adapting and thriving in chemosynthesis‐based environments, largely devoid of photosynthetic primary production and characterized by rapid geochemical regime changes are due to symbiotic associations with chemosynthetic bacteria within its large gills. In an attempt to understand physiological reactions in animals normally set to endure extreme deep‐sea environments, our laboratory has undertaken, for the

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last few years, a series of investigations, aimed at characterizing molecular indicators of adaptation processes of which components of the host defense system has received most attention. This study reviews recent advances on the characterization of molecules and genes participating in immune reactions, using *in vivo* and *ex vivo* models, to elucidate cellular and humoral defense mechanisms in vent mussels and the strategies they have adopted to survive under extreme environments.

**Keywords:** innate immunity, deep‐sea hydrothermal vents, chemosynthetic ecosystems, long‐term acclimatization, host‐symbionts interactions, endosymbionts, differential gene expression, transcriptomics, mollusc bivalve, hydrothermal vent mussel, *Bathymodiolus azoricus*

### **1. Introduction**

Deep‐sea hydrothermal vents were discovered 40 years ago in the Galapagos Rift [1] reveal‐ ing for the first time, to the amazement of the scientific community, unusual life forms that have developed unique biochemical adaptations to high temperatures and toxic chemical nature of vent surrounding, otherwise harmful to life as we know it on the surface of the planet [2–5]. The animals dwelling around the vent sites exhibit high productivity and thus must cope with the seemingly deleterious physical and chemical conditions, while develop‐ ing surprisingly successful strategies to withstand adverse environmental conditions, includ‐ ing environmental microbes and mechanical stress whether due to animal predation or from deep‐sea volcanic eruptions [6–8].

At such depths and in the absence of light, life is thriving in chemosynthesis‐based ecosys‐ tems where most abundant marine invertebrates have developed mutualistic relationships with chemosynthetic bacteria. These symbiotic interactions are believed to play a crucial role in the survival of hydrothermal vent animals, driving their transcriptional activities, and their successful adaptation strategies to subsist under extreme environmental conditions. They essentially rely on the establishment of endosymbiosis relationships between vent animals and sulfur‐oxidizing (SOX) or methane‐oxidizing (MOX) bacteria [9–13].

Deep‐sea vent mussels of the *Bathymodiolus* genus are dominant members at hydrothermal vents and cold seep habitats. These mussels have the peculiarity of sheltering both endosymbi‐ otic sulfide‐oxidizing and methane‐oxidizing bacteria in their gills [9–13], supporting thus their endurance within this type of environment. *Bathymodiolus azoricus* is also the dominant species in deep‐sea hydrothermal vents in the Azores region and is well adapted to extreme conditions particularly to toxic concentrations of heavy metals, acidic pH, and absence of light [14–17].

In an attempt to understand physiological reactions of animals normally set to endure extreme conditions, in deep‐sea environments, our laboratory has undertaken, for the last 6 years, a series of investigations aimed at characterizing molecular indicators of adaptation processes of which components of the immune and stress‐related systems have received most of our attention [18, 19]. Central to our studies is the long‐term maintenance of vent mussels to atmospheric pressure proven to be a useful model to study unique molecular relationships under which the regulation of gene transcription may be affected by aquaria conditions and by the gradual disappearance of endosymbiont bacteria from gill epithelia [20]. Nonetheless, vent mussels subsist for months at atmospheric pressure in aquaria supplemented with plain sea water in or with artificial diet. This has allowed us to focus on developing experiments to investigate new physiological responses of animals sustaining experimental challenges involving immunological and stress‐related reactions and to provide new approaches to assess the effect of natural microorganisms and metal toxicity at vent environments [21, 22]. As a research model, the choice of the vent mussel *B. azoricus* is of great significance given its unique symbiosis with SOX and MOX bacteria. It has provided us with the means to under‐ standing the molecular mechanisms underlying immune reactions in animals normally set to endure extreme deep‐sea environments and the role of their symbiotic bacteria in controlling immune gene transcriptional activity.

In line with this, we have investigated main constituents of the vent mussel immune system and demonstrated how immune and stress genes could be modulated upon different experi‐ mental challenges in the absence of the characteristic high hydrostatic pressure found at deep‐ sea vent sites without methane and/or sulfide supplementation [21–23]. The proximity to the nearby hydrothermal vent fields, in the Azores region, has given us a geographical advan‐ tage for earning first insight into immediate physiological responses comprising both cellular and humoral responses of live mussels, freshly collected from the hydrothermal vents, which upon arrival, are acclimatized to our aquarium system, LabHorta [23–25]. The maintenance of live mussels in our laboratory is thus a key factor in gaining knowledge into the physiology of vent animals including the study of evolutionary conserved immune, inflammatory, and stress‐related factors commonly found in other marine bivalves [19, 26].
