Hydrogen provides energy for bacteria in 'extreme' habitats

The discovery of deep hydrothermal springs(1) in 1977 revolutionized our understanding of the ocean depths, and also of the energy sources that fuel primary production on Earth. In the perpetual darkness of the deepest waters, living organisms proliferate around these vents. These ecosystems are dominated by animals living in symbiosis with bacteria(2) that are capable of using chemical energy to produce organic matter, rather than the light energy used by most plants.

An international team led by German researchers, primarily from the Max Planck Institute, in collaboration with researchers from CNRS and CEA-Genoscope in France and Harvard University in the United States, has explored the Logatchev hydrothermal site, a series of vents located 3,200 meters below the surface at 14˚ north latitude on the Mid-Atlantic Ridge, the vast ocean floor accretion zone that runs through the middle of the Atlantic Ocean. Previously, the bacteria found near hydrothermal sp rings were known to use two sources of chemical energy: reduced sulfur compounds and methane. Now the team's researchers have determined that the symbiotic bacteria of the hydrothermal mussels in the Mid-Atlantic Ridge are also capable of using hydrogen as an energy source to fix carbon and produce organic matter.

At the Logatchev site, large quantities of hydrogen are emitted in the hydrothermal fluids. Stéphane Hourdez, a CNRS researcher at the Laboratoire Adaptation et Diversité en Milieu Marin (Laboratory for Adaptation and Diversity in the Marine Environment, Roscoff Marine Station, CNRS/UMPC), has shown that the mussel beds consume much of this hydrogen, reducing its flow into the deep ocean by 45 to 50%. The total hydrogen consumption in the Logatchev zone (with an estimated mussel population of 250,000 to 500,000) has been calculated at 4,460 liters per hour, or 39 million liters of hydrogen per year.

In addition, a team from the Laboratoire Genomique Metabolique (Metabolic Genomics Laboratory, CNRS/CEA-Genosc! ope/Univ ersité d'Evry Val Essonne) helped identify the key gene for the chemical transformation of hydrogen. This same gene is also present in the bacteria associated with many other hydrothermal organisms (including worms and shrimps), which would imply that the capacity to use hydrogen as an energy source is widespread in hydrothermal symbioses, especially in hydrogen-rich sites like Logatchev.

These findings open promising new perspectives, since the use of hydrogen by these bacteria is simpler and more efficient, by a factor of 7 to 18, than energy production from the other sources (methane and hydrogen sulfide). In the longer term, the discovery of this new metabolic pathway and the identification of the key gene for the oxidation of gaseous hydrogen could be of interest in biotechnology, and hold definite promise for the future of renewable energies.

(1) Hydrothermal springs or vents form on the oceans' underwater ridges at depths varying from 500 to 4,000 m. These ridges are characterized by powerful volcanic activity, which creates fissures. Seawater penetrates these fissures and heats up as it approaches the underlying magma chambers. This causes it to become acidic and dissolve the basaltic components of the rock. This acidic fluid then rises to the ocean floor charged with metals (iron, zinc, manganese, lead, copper) and reduced elements (including hydrogen sulfide, hydrogen and carbon dioxide). Contact with the seawater at 2°C triggers the precipitation of the minerals, which accumulate around the vents, forming "chimneys" that can reach heights of around 20 m.

(2) Symbiosis is a sustainable and mutually beneficial association between two living organisms. Each of the organisms is referred to as a "symbiote" or "symbiont" in the symbiotic relation.