Hydrothermal vent symbionts more useful than previously thought.

The first hydrothermal vents and associated communities were discovered in 1977. Since then two energy sources have been shown to be utilised by symbiotic bacteria: sulphur and methane. Today we have discovered a vast array of chemosynthetic ecosystems however despite this we have yet to find another energy source for metazoan chemosynthetic symbioses. This is extraordinary considering the extent of possible energy sources: hydrogen, ammonium, ferrous iron and manganese.

The study site is Logatchev vent field is found on the Mid-Atlanic Ridge. It has the highest hydrogen concentrations ever measured. Very high hydrogen concentrations can be found at hydrothermal vents due to the interaction of seawater with mantle-derived ultramafic rocks (composed of dark coloured minerals, high content of magnesium and iron).The authors predict that when hydrogen is oxidised it could provide 7x more energy/kg of vent fluid than methane oxidation and up to 18x more energy/kg of vent fluid than sulpha de oxidation. If this is such an abundant and effective source of energy why is it not obviously being utilised?

The focus of this study is on Bathymodiolus puteoserpentis. This mussel is the most abundant macrofauna in the Logatchev vent field. They were thought to only operate a duel symbiosis with methane-oxidising and chemoautotrophic sulphur-oxidising bacteria hosted in their gills. This paper shows that the use of hydrogen is also an important source of energy.

Using PCR they amplified the gene (hupL) that codes for the large subunit of the hydrogenase enzyme found in the symbionts. Phylogenetic analyses showed that its closes related sequence was from an alphaproteobacterium Oligotropha carboxidovorans which can utilise CO or H2 as an electron donor. This link shows that there is potential for hydrogen oxidation in the symbionts found on the mussels. This discovery of genetic potential to oxidise hydrogen led onto more experiments. They incubated the mussel gill tissues with hydrogen at partial pressures of ~100p.p.m. and measured the consumption over time. They found that hydrogen was taken up rapidly in gill tissues containing the symbionts. They found that hydrogen uptake is stimulated by an increase in hydrogen concentrations. They then tested the rates of carbon fixation with hydrogen compared to fixation by sulphide oxidation and concluded that hydrogen provides energy for the production of mussel biomass. Then applied the same tests to the mussels from the low hydrogen poor environments to see if they could also consume hydrogen. Although the uptake was much less they were still capable of using hydrogen as an energy source.

Few studies have addressed the hydrogen turnover in the oceans in comparison to the vast number that focused on terrestrial ecosystems. Interest in this field is rapidly increasing with the possibility of hydrogen fuel source becoming more viable. With a total population of 250,000 to 500,000 Bathymodiolus mussels prove to be a significant hydrogen sink. Currently it is not known if similar processes are happening at other mid-ocean ridges however we do know that the key genes necessary are present in a number of different communities across the globe.

Review of: J.M.Petersen et al. (2011). Hydrogen is an energy source for hydrothermal vent symbioses. Nature. 476 (3), 177.