Estimating Symbiont Abundances and Gill Surface Areas in Specimens of the Hydrothermal Vent Mussel Bathymodiolus puteoserpentis Maintained in Pressure Vessels

Sébastien Duperron,Adrien Quiles,Nelly Léger,Bruce Shillito,Kamil M Szafranski

doi:10.3389/fmars.2016.00016

Abstract

The hydrothermal vent mussel Bathymodiolus puteoserpentis hosts gill-associated sulfur- and methane-oxidizing bacteria which sustain host nutrition and allow it to reach high densities at various sites along the northern Mid-Atlantic Ridge. Previous studies have demonstrated that in similar dual symbioses, relative abundances of each bacterial type could change following variations in symbiont substrate availabilities. In this study, pressurized recovery and incubations in pressure vessels were used to test whether B. puteoserpentis symbionts displayed similar behavior in the presence of symbiont substrates. The relative abundances of both types of symbionts were analyzed using fluorescence in situ hybridization (FISH) and group-specific gene copy numbers were assessed using qPCR. Specimens sampled using isobaric and non-isobaric recovery contained similar relative proportions (in surface coverage) of sulfur- and methane-oxidizing bacteria indicating that recovery type did not have impact on measured relative areas. Similarly, pressurized incubations with different substrates did lead to significant differences in the relative surface coverage of the two types of bacteria, although slight variations were measured with qPCR, suggesting changes in relative abundances of gene copy numbers but not in relative areas covered. Total gill surface areas and total bacterial numbers in specimens were estimated for the first time. Symbiont bearing-mussels display exchange surfaces about 20-fold higher than those found in similar-sized coastal mussels, and mean bacterial numbers of 2.5*1012 per specimen were estimated. This emphasizes that symbiotic mussels are a major reservoir of bacteria in vent ecosystems.

Highlights

The large mussels (Mytilidae: Bathymodiolinae) thriving at deep-sea hydrothermal vents and cold seeps owe their success to the symbiotic chemosynthetic bacteria they harbor in their gill tissues
MOX 16S rRNA gene copy numbers estimated by qPCR were comparable between the two recovery methods, SOX gene copy number was higher in non-isobaric recovery, again very close to the significance limit (2.03-fold)
The fact that gene copy numbers and areas are unaffected by non-isobaric recovery rRNA-encoding gene, and each copy would correspond to a bacterium

Summary

Introduction

The large mussels (Mytilidae: Bathymodiolinae) thriving at deep-sea hydrothermal vents and cold seeps owe their success to the symbiotic chemosynthetic bacteria they harbor in their gill tissues. Dual symbioses involving the simultaneous presence of sulfur- and methane-oxidizing bacteria (SOX and MOX) within the same host cells are a pervasive feature of bathymodiolins from vents and seeps in the Atlantic Ocean (Distel et al, 1995; Fiala-Médioni et al, 2002) The flexibility of these environmentally-acquired associations theoretically allows the symbiotic system to adapt to the reported spatial and temporal heterogeneities of symbiont substrate availability, and contributes to the ecological success of bathymodiolins (Won et al, 2003a; Halary et al, 2008). Despite that only two specimens were used for each treatment, exposure to bicarbonate ions and to a combination of sulfur and bicarbonate caused a spectacular increase in the relative abundance of sulfur-oxidizers measured as their volume within bacteriocytes (Szafranski et al, 2015)

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