Mixotrophic Mussels

Bathymodiolus azoricus (Ba) is a dominant species of mussel which inhabits many hydrothermal vents of the mid-Atlantic Ridge (MAR). They contain two metabolically distinct endosymbionts (methanotrophic (Mb) and thiotrophic (Sb)) however have also been shown to capture particulate organic matter. The gill filaments contain three different zones consisting of a ciliated frontal zone, followed by a transitory zone, followed by the bacteriocyte zone. The objective of this study was to see the effects of starving the mussels of methane and dissolved sulphur on their gill filaments using microscopy and PCR based methods. The study also gives insight into how to maintain this deep-sea species in laboratory conditions for extended periods of time.

Mussels were collected from Menez Gwen as it is the shallowest location which removes the need to keep them under pressure. For the main experiment mussels were kept in tanks which were sulphide free for thirty days followed by fifteen days of exposure to normal sulphide conditions. However many other tanks were set up in different ways to provide controls. Light and Electron Microscopes were used at different time periods to see the effects on the gill filaments. They also did DNA extractions from gills using an old school method (not kits) followed by precipitation to clean the samples for PCR. In the PCR two sets of primers were used to target the endosymbiont 16s rRNA and ATP Sulphuryase genes to see whether the endosymbionts were present at different time periods. This was analysed using agarose gels. The method seemed to focus on Sb as no methane was introduced to any of the tanks.

The authors present the results extremely clearly with very nicely described images. There were very dramatic changes in mussel gills from tanks without dissolved sulphide. The bacteriocyte layer was highly reduced and all brown pigment was lost. The PCR and images clearly showed that all Sb and Mb were lost from the mussel gills very quickly. There was also an increase in the numbers of amoebocytes and lysosomes in gill filaments. The authors do not mention any mortality figures so I guess that there were no significant mortalities over the thirty days. Once they were re-acclimatised with dissolved sulphide the gill filaments seem to produce pit-like structures which had Sb associated with them and can be clearly seem in the images. The gill filaments seem to return to the form shown in the control group however do not seem to have fully “recovered” after fifteen days. It is very important to note that only mussels which were re-acclimatised by being placed in the control tanks showed this “recovery.” In all mussel gills Mb were shown to dissapear after twenty-four hours.

The study definitely seems to provide evidence for lateral transmission (possibly from other mussels or free living bacteria) of these endosymbiont bacteria. It also shows that the mussels can survive without these bacteria for at least thirty days. The paper provides evidence that the mussels gain bacteria through pit-like structures present on the gill filaments. Another key point which the paper makes which I have not really focused on is their method for keeping stable dissolved sulphide concentrations, which they proved did work. They also highlight other experimental procedures which they found to be highly important in keeping the mussels alive. Overall I thought the study was pretty impressive and the paper itself very nicely written. I thought it was a bit of a shame that they did not also try to see how long the mussels can survive without their endosymbionts to see how dependant the relationship is. I thought the mixotrophic nature of the mussels was quite cool and they also seem to show some level of plasticity.

A Review of Kadar, E. et al. (2005) Experimentally induced endosymbiont loss and re-acquirement in the hydrothermal vent bivalve Bathymodiolus azoricus, Journal of Experimental Marine Biology and Ecology 318, 99– 110.