Dual symbiosis of the vent shrimp Rimicaris exoculata with filamentous gamma‐ and epsilonproteobacteria at four Mid‐Atlantic Ridge hydrothermal vent fields

Abstract

SummaryThe shrimp Rimicaris exoculata from hydrothermal vents on the Mid-Atlantic Ridge (MAR) harbours bacterial epibionts on specialized appendages and the inner surfaces of its gill chamber. Using comparative 16S rRNA sequence analysis and fluorescence in situ hybridization (FISH), we examined the R. exoculata epibiosis from four vents sites along the known distribution range of the shrimp on the MAR. Our results show that R. exoculata lives in symbiosis with two types of filamentous epibionts. One belongs to the Epsilonproteobacteria, and was previously identified as the dominant symbiont of R. exoculata. The second is a novel gammaproteobacterial symbiont that belongs to a clade consisting exclusively of sequences from epibiotic bacteria of hydrothermal vent animals, with the filamentous sulfur oxidizer Leucothrix mucor as the closest free-living relative. Both the epsilon-and the gammaproteobacterial symbionts dominated the R. exoculata epibiosis at all four MAR vent sites despite striking differences between vent fluid chemistry and distances between sites of up to 8500 km, indicating that the symbiosis is highly stable and specific. Phylogenetic analyses of two mitochondrial host genes showed little to no differences between hosts from the four vent sites. In contrast, there was significant spatial structuring of both the gamma-and the epsilonproteobacterial symbiont populations based on their 16S rRNA gene sequences that was correlated with geographic distance along the MAR. We hypothesize that biogeography and host-symbiont selectivity play a role in structuring the epibiosis of R. exoculata.

Inorganic carbon fixation by chemosynthetic ectosymbionts and nutritional transfers to the hydrothermal vent host-shrimp Rimicaris exoculata

“…That the presence of an electron donor increases the carbon fixation rate indicates that Na 2 S 2 O 3 , and Fe 2 þ at least contribute to fuelling R. exoculata epibionts, and our autoradiography data suggest that chemosynthesis occurs at least in the large filamentous bacteria (epsilonproteobacteria of Marine Group 1), which appeared more intensely labelled than the thin filamentous and rod-shaped ones (epsilonproteobacteria of Marine Group 2 and gammaproteobacteria). The view that R. exoculata epibionts have sulphide-oxidising activity is consistent, moreover, with the fact epsilon-and gammaproteobacteria are assumed to have such activity, on the basis of the group affiliation of the former (Petersen et al, 2009;Goffredi, 2010;Guri et al, 2012) and because the latter possess functional SoxB and aprA genes (Hü gler et al, 2010(Hü gler et al, , 2011. We cannot say, however, what the epibiotic bacteria of R. exoculata use as predominant energy source.…”

“…Schulz and Jørgensen (2001) interpret the metabolic flexibility of filamentous sulphur-oxidising bacteria, especially those of the Thiothrix group, and their ability to grow heterotrophically or autotrophically on different sulphur species, as a need to overcome disadvantages of their sessile life and to adapt their activity to changes in substrate availability (oxygen, sulphide). In R. exoculata, the obvious variability of bacterial assemblages according to the gill chamber area, the individual and the vent site (Zbinden et al, 2008;Petersen et al, 2009;Hü gler et al, 2011) suggests a consortium composition plasticity as a response to varying vent fluid composition Schmidt et al, 2008) and to fluctuating geochemical or micro-environmental conditions in the zone of mixing with deep water.…”

“…By combining 16S rRNA and functional gene analysis with in-situ hybridisation and transmission electron microscopy observations on R. exoculata epibionts and their free-living or symbiotic relatives from vent habitats, investigators (Campbell et al, 2006;Zbinden et al, 2008;Petersen et al, 2009;Goffredi, 2010;Hü gler et al, 2010Hü gler et al, , 2011Guri et al, 2012) have evidenced epsilonproteobacteria of Marine Groups 1 (the new family Thiovulgaceae) and 2, as thick (3-mm wide) and thin (1-mm wide) filaments, respectively. Both should oxidise sulphur compounds through the Sox pathway and fix carbon through the reverse tricarboxylic acid cycle (SoxB and aclA gene sequences).…”

“…The shrimp R. exoculata, listed as a model organism of an extreme deep-sea environment (CAREX, 2010), is exceptional among crustaceans for its association with bacteria. Recent studies suggest that R. exoculata gill chamber epibionts constitute a diversified community with various morphotypes, phylotypes and metabolisms (Zbinden et al, 2004(Zbinden et al, , 2008Petersen et al, 2009;Hü gler et al, 2011;Guri et al, 2012), but no study has directly demonstrated which substrate(s) the bacteria oxidise to fuel their 'chemosynthetic' metabolism(s). This shrimp also harbours a digestive bacterial community (Zbinden and Cambon-Bonavita, 2003;Durand et al, 2010) whose role remains hypothetical (detoxification, nutrition and/or pathogen control).…”

Epibiotic bacterial community of Sphaeroma serratum (Crustacea, Isopoda): relationship with molt status

“…The branchial chamber of R. exoculata and its mouthparts harbour a rich epibiotic microbial community (Van Dover et al 1988, Segonzac et al 1993, Zbinden et al 2004. The bacterial community of R. exoculata comprises different bacterial morphotypes, affiliated with 2 major phylogenetic groups (γ-and ε-Proteobacteria) and corresponding to 3 metabolic types, including iron sulphide and methane oxidation (Segonzac et al 1993, Polz & Cavanaugh 1995, Zbinden et al 2004, Petersen et al 2010. The role of this ectosymbiotic community still remains unclear.…”