Hydrogen fuel cells found in deep-sea mussels

Mussels flourishing in the dark ocean depths at hydrothermal vents harness energy from hydrogen, thanks to symbiotic bacteria acting as 'fuel cells' inside their gills. This discovery unveils a third energy source, alongside sulphur compounds and methane, that power these sunlight-starved ecosystems in the deep-sea.

A research team led by Jillian Peterson and Nicole Dubilier from the Max Plank Institute for Marine Microbiology in Germany, explored a hydrothermal vent field called Logatchev at the Mid-Atlantic Ridge 3 km beneath the waves.

Superheated water gushing from these vents has the highest hydrogen levels ever measured in hydrothermal systems. Peterson and the team wondered if any organisms were tapping into this potentially rich source of energy - hydrogen is a great electron donor and the energy released by hydrogen oxidation is much higher than for methane or sulphur.

To find out, they sent down remotely-operated submersibles that brought up living samples of mussels, called Bathymodiolus, that cluster around these vents. Back on deck, researchers discovered that sulpur-oxidising bacteria living inside the mussels' gills can also be powered by hydrogen. They have a gene, hupL, that codes for a key enzyme involved in harnessing energy from hydrogen, which essentially breaks down hydrogen molecules into two hydrogen ions plus two electrons.

The team also showed that Bathymodiolus mussels mop up large amounts of hydrogen both in the lab and using in-situ equipment back down at the hydrothermal vent. They found that hydrogen levels were much higher in water spewing straight out of the vent compared to water after it had passed through a dense bed containing up to 500,000 mussels.

Until now, only free-living bacteria have been shown to use hydrogen as an energy source.

Other animals - including the giant tubeworm, Riftia, and the shrimp Rimicaris exoculata - live around hydrothermal vents where hydrogen is less abundant than at Logatchev but it turns out they also have hydrogen-harnessing capabilities. They too host symbiotic bacteria with the hupL gene, suggesting that an ability to harness hydrogen is widespread across vent communities.

The paper appears in this weeks edition of the journal Nature.