Mussels of the deep

Cold seeps are areas of the ocean floor which are rich in the seepage of hydrocarbon-rich fluids such as hydrogen sulphide and methane, this most often occurs in the form of brine pools. These areas support their own endemic species the most dominant of which are usually mussels of the genus Bathymodioline. It has been found that these organisms survive and thrive in these habitats in the deep oceans due to the endosymbiotic chemosynthetic bacteria that reside within the gills of the mussels.

 This study focuses upon the Idas genus which has been found upon sunken wood and whale fall bones which may have served as an evolutionary stepping stone between mussels colonising the rocky shores of the world and the deep sea cold seeps. The authors of the paper comment on the data collected in the past about the endosymbiotic bacteria within the bacteriocytes of the mussels, mentioning that it was collected using only Transmission Electron Microscopy (TEM) and observation and enzyme assays. The methods of this study expand upon that using 16SrRNA to identify the phylotypes present within the bacteriocytes of the mussels epithelial gill cells. This approach is a far more accurate approach to identifying the differing species of endosymbiotic bacteria than was previously used.

The need for caution when using RubisCO as a marker for Sulphide Oxidising bacteria as the enzyme itself is very widespread in other bacteria for other processes, perhaps this is best used in conjunction with other techniques used for identifying enzymes such as APS reductase which is widely recognised as a good marker for the presence of bacteria which are involved in the sulphur cycle. The caution used in this paper in this regard is well deserved as otherwise the data can become confounded or end up misrepresenting the phylotypes that are present.

Previous papers had found that there were only two endosymbiont phylotypes associated with the Bathymodioline genus; however, this paper found that there are around six 16SrRNA phylotypes associated with Bathymodioline which raises particular questions about the evolution of the relationship between the mussel and the endosymbionts, in particular how the bacteria came to be symbionts at all. An increased permeability theory was put forward by this paper to attempt to provide a hypothesis for it; this theory suggests that the epithelial cells increased their permeability as a defence system change. I think that there needs to be further research into this area before any conclusions like this can be drawn but inferences about evolutionary history are difficult to make as we have very little opportunity to study how organisms were in their past.    

Duperron S., Halary S., Lorion J., Sibuet M., Gaill F., 2008, Unexpected co-occurrence of six bacterial symbionts in the gills of the cold seep mussel Idas sp. (Bivalvia: Mytilidae) Environmental Microbiology, 10, 433-445