Hydrogen is an energy source for hydrothermal vent symbioses

Deep sea hydrothermal vent ecosystems are dominated by organisms that are in a symbiotic relationship with chemosynthetic bacteria. Before this paper only two types of metabolism in the symbiotic bacteria had been established, these were the reduction of sulphur compounds and the reduction of methane. The aim of the investigation was to determine if symbiotic bacteria in hydrothermal vent organisms could utilise hydrogen as an energy source. The authors state that it is likely there are different types of metabolism in symbiotic bacteria because bacteria which can utilise hydrogen, ferrous iron, ammonium and manganese have been found in hydrothermal vents, however these metabolism pathways have not been found in bacteria which are in a symbiosis.

Hydrogen would be an ideal source of energy, as it is a favourable electron donor, using a thermodynamic model it was predicted that hydrogen oxidation could release as much as 7 times more energy per kilogram of hydrothermic vent fluid compared to methane oxidation while compared to sulphur oxidation as much as 14 times more energy could be released. Furthermore there are some hydrothermal vents in which the vent fluid has a high concentration of hydrogen, for example the Logatchev vent field is located in a ridge segment characterized by ultramafic outcrops, the fluids that vent from this site have the highest hydrogen concentration ever measured in a hydrothermal vent system, this would act as a rich energy source for chemosynthetic microbes.

The experiment began by sequencing genes from bacteria found in the gill tissue of mussels from the Logatchev vent field (high hydrogen concentration). The enzymes that are involved in hydrogen metabolism are hydrogenases, enzymes of the group 1 NiFe hydrogenases are membrane bound respiratory enzymes and provide a link from hydrogen oxidation with energy production by channelling hydrogen into the Quinone pool. The genes which encode for the membrane bound uptake hydrogenases is the hupL gene, the gene was amplified and sequenced from B. puteoserpentis symbiont containing mussel gill. The sequence then underwent phylogenetic analysis and was compared with a similar sequence from Oligotropha carboxidovorans which is known to utilise both CO or H₂ as electron donors. The enzyme from B. puteoserpentis was found to be placed in a cluster with other group 1 NiFe hydrogenases, showing that B. puteoserpentis has the genetic capability to utilise hydrogen as an energy source. As a comparison endosymbionts of mussels found in hydrogen poor vents were also analysed for the hupL gene, it was found that the gene could be amplified in hydrogen poor endosymbionts suggesting that the potential to use hydrogen as an energy source is not restricted to those microbes in high hydrogen concentrations.

As the genetic potential to utilise hydrogen as an energy source was expressed, gills from the mussels were then incubated with hydrogen of 100ppm. The mussel gills from the Logatchev site (B. puteoserpentis) were found to take up hydrogen at a rate of 650+200nmol h^-1 (g wet weight)^-1 , whereas the symbiont free gill tissue did not take up hydrogen above the rates of the negative control. Furthermore mussels from the basalt hosted vent fields (low hydrogen) were found to take up hydrogen, however this was 20-30 times lower than the mussels from the Logatchev vent.

Furthermore to determine if hydrogen was in fact acting as an energy source, because hydrogen uptake is not necessarily coupled with carbon fixation, the mussel gills were incubated in water containing ¹⁴C-bicarbonate. The control gills used were those incubated with sulphur and those incubated without an electron donor; it was found that ¹⁴C uptake was stimulated by sulphur (which is known to be en energy source for the symbiont) and also hydrogen. As the rates of ¹⁴C uptake were similar in both hydrogen oxidation and sulphur oxidation the authors concluded that hydrogen was in fact acting as an energy source.

The effect of hydrogen concentrations on hydrogen uptake was also tested; some hydrogen oxidising microbes only express hydrogenases in the presence of hydrogen, while some express them at low levels and then increase them in the presence of hydrogen. The mussel gills were incubated with hydrogen at partial pressures up to 3,000 ppm, the hydrogen uptake increased by 135, 21 and 8 nmol h^-1 (g wet weight)^-1 respectively for each increase of 100 ppm in hydrogen partial pressures, therefore hydrogen uptake is clearly stimulated by increasing hydrogen concentrations.

The final experiment which was conducted was to test to see if sulphur oxidising symbionts can also utilise hydrogen as an energy source. There were two different conditions, those were the fluid from the vent had not been exposed to B. puteoserpentis mussels, and situations where they had. The slope of the regression line calculated for hydrogen concentration versus temperature was significantly lower in the mussel bed compared to the source fluid, therefore indicating that the fluids inside the mussel bed are depleted of hydrogen in comparison with the sources fluid. This result indicates that the hydrogen is being utilised by the bacteria and therefore showing that significant amounts of hydrogen are consumed at the Logatchev trench. This is corroborated through the findings that sulphur oxidising symbiont of Bathymodiolus was found to possess the genes needed for uptake and oxidation of hydrogen, these were found on the same genome fragment that contained the genes for sulphur oxidation and carbon fixation showing the hydrogen oxidising genes were from the symbiont.

The paper was extremely well written and easy to understand the results and the reasoning behind the experiments. However when it came to addressing the significance of finding that many bacteria in a symbiotic relationship use hydrogen as an energy source, I feel this was poorly addressed, only really stating that it may act as a major hydrogen sink, but not really explaining why this would be important.

Reference: Petersen, J.M., Zielinski, F.U., Pape, T., Seifert, R., Moraru, C., Amann, R., Hourdez, S., Girguis, P.R., Wankel, S.D., Barbe, V., Pelletier, E., Fink, D., Borowski, C., Back, W., Dubilier, N. 2011. Hydrogen is an energy source for hydrothermal vent symbioses. Nature., 476, pp. 176-180.