Give Hydrogen to Hungry Hydrothermal Vent Symbioses!

Have you ever heard of hydrothermal symbionts? Hydrothermal vent region looks very infertile since no light can come through, however, this special condition rather resulted in various uniquely-evolved symbionts. So far, only two energy sources that power these organisms, a kind of ‘food’ to these organisms, have been reported; methane and reduced sulphur compound. But recently, a research suggests another energy source, hydrogen. How could the researchers reach such a result, and what is the importance of this research?

Peterson M. J. and Zielinski U. F analyzed gill tissues of Bathymodiolus puteroserpentis(B. puteroserpentis) mussels from Logatchev which is a ultramafic-hosted vent and Bathymodiolus(B. spp.) mussels from southern MAR(Mid Atlantic Ridge) which is a basalt-hosted vent.

The key enzyme for hydrogen-using metabolism is called hydrogenase, which catalyzes the reaction , and therefore links hydrogen oxidation and energy production. This enzyme is encoded in a gene called hupL gene. Therefore, the researchers extracted hupL gene from gill tissues of the two mussels, incubated them in a headspace with hydrogen, and then measured the amount of hydrogen consumed. Result showed that concentration in headspace decreased, showing significant difference with that of symbiont-free tissues or boiled-tissues. (Figure 1-a) Also, by measuring hydrogen consumption depending on partial pressure, we can know that uptake of hydrogen generally increases with increasing concentration (Figure 1-b, c). This tells us that hydrogen uptake is affected by concentrations, and coincides with the result that hydrogen uptake in -rich ultramafic-hosted region is 20-30 times greater than -poor basalt-hosted region.

Meanwhile, because uptake is stimulated by B. puteroserpentis symbionts that take sulphur as an energy source, the researchers also included 14-bicarbonate hydrogen to the headspace to figure out whether hydrogen is source of autotrophic fixation. In both regions, the rates of uptake stimulated by hydrogen was comparable to those stimulated by sulphide oxidation, and was appreciably different to those of specimens without an electron donor like hydrogen or sulphides (Figure 1-d). Therefore, now we can say that hydrogen provides energy for the production in the both ultramafic and basaltic regions.

So far, there is only a paucity of data about global and terrestrial hydrogen cycle in oceans. According to this paper, B. spp. mussels that lives in Logatchev altogether can uptake up to 200, and this value is meaningful since hydrogen uptake rates by hydrothermal fluid or coastal water reported by previous studies are only nano-mole scale. Thus, this paper shows us that B. spp. mussels are important hydrogen sink in Logatchev. Moreover, it is known that both episymbionts of a hydrothermal vent shrimp Rimicaris exoculata from ultramafic-hosted vent and endosymbionts of giant tubewarm Riftia pachyptila from basalt-hosted vent have hupL genes. This suggests that hydrogen use is widespread in chemosynthetic symbioses.

Figure 1 | Hydrogen uptake and 14-C activity

a. consumption is much greater in gill tissues with symbionts than with no symbionts or boiled tissues. b, c. Hydrogen consumption generally increases as concentration increases. consumption in Logatchev is 20-30 times greater than that of south MAR. d. 14-C uptake rates by hydrogen are comparable to those of sulphides.