Oceanospirillales, friend or foe?

The study of microbial symbionts has been a hot topic recently, and it is well known that these bacterial house guests often play a huge role in the health of their hosts. However not all symbionts have a clear role. A study by Beinart et al., 2014 recently discovered a previously overlooked bacterial symbiont of the hydrothermal vent snail Alviniconcha.

Alviniconcha species are known to play host to a variety of chemoautotrophic symbionts, either of the group γ- or ε-proteobacteria, which are found mainly in the gill tissues of these snails, and are thought to provide the majority of the host’s nutrition. The hosts are found to contain either γ-proteobacteria or ε-proteobacteria. This study found a novel phylotype of the order Oceanospirillales (AOP), residing in vacuoles of Alviniconcha which hosted γ-proteobacteria.

Using FISH and TEM, it was found that this bacteria lives within vacuoles of the gill tissues in these vent snails. AOP was only found within cells which already contained other γ-proteobacteria. Q-PCR was also employed to compare population densities of each type of bacteria within the host samples, this showed that in comparison to the ‘main symbionts’ (these being either the regular γ- or ε-proteobacteria), AOP had a very minor population. Despite AOP density being ‘minor’, it was stated that it was found with frequency and specificity in terms of how many and which Alviniconcha’s it was found in.

2 vent sites were studies, 10s of kilometres apart. 96% of γ-proteobacteria hosts sampled contained AOP whereas only 5% ε-proteobacteria hosts were found to have it. It is so far unknown what this particular symbionts function is within Alviniconcha, and whether it is parasitic or mutualistic. Both of the sites studied contained both types of host (although not in equal numbers), which suggests that AOP shows a host specificity, and the result is not likely influenced by other geographical factors.

An interesting observation made by the team was that Alviniconcha which contain γ-proteobacteria were also found to have sulfur granules within their gill tissues, whereas the ε –proteobacteria hosts did not. Although it was not discussed in detail, I think this could be an interesting lead into further research to determine the role of AOP as a symbiont, and how this bacteria functions in general. The authors stated that AOP is similar to other Oceanospirillales members which have been shown to degrade DMSP and other sulfur containing compounds.

While its close relation to another Oceanospirillales which is parasitic to hydrothermal vent mussels points towards AOP being a parasite, I believe that the presence of sulfur granules in AOP host tissues could suggest something about that AOP may play a role in sulfur cycling, especially with the proximity to hydrothermal vents, and that it is much more likely that AOP is a mutualistic symbiont of these snails, rather than a parasite. Understanding the functions and behaviors of even the seemingly insignificant symbionts is vital for a better understanding of many ecosystems.

R. A. Beinart, S. V. N. N. D. P. R. G., 2014. Intracellular Oceanospirillales inhabit the gills of the hydrothermal vent snail Alviniconcha with chemosynthetic, γ-Proteobacterial symbionts. Environmental Microbiology Reports, pp. 656 - 664.