Ultrastructural and molecular evidence for potentially symbiotic bacteria within the byssal plaques of the deep-sea hydrothermal vent mussel Bathymodiolus azoricus.

Bacterial symbiont metabolic activity is thought to be key at deep sea hydrothermal vents. This is because they are key to sustaining the ecosystem as the main primary producers and because of this the symbiosis between chemosynthetic bacteria and invertebrates is characteristic of vent habitats. Despite 30 years of vent research very little research has been carried out into metal-microorganism interactions at the vents. It is thought that the symbionts have a metal-detoxification role preventing the host from absorbing harmful metals.

Mussels where collected from two different sites along the Mid-Atlantic ridge after collection the samples where fixed in ways relevant to the tests to be carried out on them. These tests were; light and electron microscopy, metal content analysis and FISH.

High concentrations of the essential metals, Fe, Cu and Zn were found in the byssal threads. With non-essential and toxic metals being found at concentrations of at least one order of magnitude less than the essential metals. Part of the byssal thread is an adhesive plaque that consists of a protective coating and an internal spongy matrix. It was discovered upon examination of the Electron microscope imaging that a bacterium was residing within the spongy matrix in all samples from both sites. The bacteria were filamentous and frequent through the matrix. The bacteria are also thought to be gram negative due to the presence of a double membrane. Bacteria were present in several stages both alive and tangled in the fibres and dead with mineral deposits present at the bacterial surface.

To localise the different metals thin sections of the byssal threads were prepared for scanning electron microscopy and then observed using back scatter imaging. Any areas with high electron density were then scanned for elemental composition. Most metallic elements and others associated with hydrothermal vents were found in the spongy collagen matrix. FISH analysis of bacteria on and in the byssal thread found several different morphotypes. One probe used was specific for bacteria found within the gill bacterioctes of B. azoricus and this confirmed the intra-byssal presence of only one type of bacteria present in the gills, a larger methane oxidising bacteria. A thiotroph specific probe found small bacteria only present on or in the protective sheet of the byssus plaque. A different methanotroph-related probe detected large bacterium found both outside the protective layer and within the collagen matrix of the byssal plaque. Both morphotypes shared several common morphological features with that of bacteria present in the gills.

One of the main suggestions is that marine microbe driven Fe/Mn oxidation at hydrothermal vents is much more common place than previously thought. The way in which the bacteria enter the byssus despite the strong antibacterial activity is unknown although the bacteria tend to colonise the end of the thread that is in contact with the rock. The authors have hypothesised that free-living bacteria may enter the thread for nutrition or shelter but no mention of the mechanism of entry was mentioned. The high levels of metal within the treads has been put down to bacterial assimilation of metals.

This paper is a well written and researched piece which has found some interesting symbiotic bacteria which seem to have bypassed the antibacterial properties of the mussel and which assimilate hydrothermal elements such as toxic metals. The further research into the bio-mineralisation mechanisms of deep sea bacteria could be very interesting and useful.

Kádár, E. and Bettencourt, R., 2008. Ultrastructural and molecular evidence for potentially symbiotic bacteria within the byssal plaques of the deep-sea hydrothermal vent mussel Bathymodiolus azoricus. Biometals, 21(4), pp.395-404.