At the Black Smokers in 3000 meter depth there live exceptional symbiotic communities. MARUM
search for new energy sources to power mankind’s increasing needs is
currently a topic of immense interest. Hydrogen-powered fuel cells are
considered one of the most promising clean energy alternatives. While
intensive research efforts have gone into developing ways to harness
hydrogen energy to fuel our everyday lives, a natural example of a
living hydrogen-powered ‘fuel cell’ has gone unnoticed.
a recent expedition to hydrothermal vents in the deep sea, researchers
from the Max Planck Institute of Marine Microbiology and the Cluster of
Excellence MARUM discovered mussels that have their own on-board ‘fuel
cells’, in the form of symbiotic bacteria that use hydrogen as an energy
source. Their results, which appear in the current issue of Nature,
suggest that the ability to use hydrogen as a source of energy is
widespread in hydrothermal vent symbioses.
hydrothermal vents are formed at mid-ocean spreading centers where
tectonic plates drift apart and new oceanic crust is created by magma
rising from deep within the Earth. When seawater interacts with hot rock
and rising magma, it becomes superheated, dissolving minerals out of
the Earth’s crust. At hydrothermal vents, this superheated energy-laden
seawater gushes back out into the ocean at temperatures of up to 400
degrees Celsius, forming black smoker chimneys where it comes into
contact with cold deep-sea water. These hot fluids deliver inorganic
compounds such as hydrogen sulfide, ammonium, methane, iron and hydrogen
to the oceans.
organisms living at hydrothermal vents oxidize these inorganic
compounds to gain the energy needed to create organic matter from carbon
dioxide. Unlike on land, where sunlight provides the energy for
photosynthesis, in the dark depths of the sea, inorganic chemicals
provide energy for life in a process called chemosynthesis.
hydrothermal vents were first discovered more than 30 years ago,
researchers were astounded to find that they were inhabited by lush
communities of animals such as worms, mollusks and crustaceans, most of
which were completely unknown to science. The first to investigate these
animals quickly realized that the key to their survival was their
symbiotic association with chemosynthetic microbes, which are the
on-board power plants for hydrothermal vent animals.
now, only two sources of energy were known to power chemosynthesis by
symbiotic bacteria at hydrothermal vents: Hydrogen sulfide, used by
sulfur-oxidizing symbionts, and methane, used by methane-oxidizing
have now discovered a third energy source” says Nicole Dubilier from
the Max Planck Institute of Marine Microbiology in Bremen, who led the
team responsible for this discovery.
discovery began at the Logatchev hydrothermal vent field, at 3000 m
depth on the Mid-Atlantic Ridge, an undersea mountain range halfway
between the Caribbean and the Cape Verde Islands. The highest hydrogen
concentrations ever measured at hydrothermal vents were recorded during a
series of research expeditions to Logatchev.
to Jillian Petersen, a researcher with Nicole Dubilier, “our
calculations show that at this hydrothermal vent, hydrogen oxidation
could deliver seven times more energy than methane oxidation, and up to
18 times more energy than sulfide oxidation”.
The mussel beds at hydrothermal vents form a teeming expanse that contains an estimated half a million mussels. MARUM
the gills of the deep-sea mussel Bathymodiolus puteoserpentis, one of
the most abundant animals at Logatchev, the researchers discovered a
sulfur-oxidizing symbiont that can also use hydrogen as an energy
source. To track down these hydrogen-powered on-board ‘fuel cells’ in
the deep-sea mussels, the researchers deployed two deep-sea
submersibles, MARUM-QUEST from MARUM at the University of Bremen, and
KIEL 6000 from IFM-GEOMAR in Kiel.
the help of these remotely-driven submersibles, they sampled mussels
from sites kilometers below the sea surface. Their ship-board
experiments with live samples showed that the mussels consumed hydrogen.
Once the samples were back in the laboratory on land, they were able to
identify the mussel symbiont hydrogenase, the key enzyme for hydrogen
oxidation, using molecular techniques.
mussel beds at Logatchev form a teeming expanse that covers hundreds of
square meter and contains an estimated half a million mussels.
experiments show that this mussel population could consume up to 5000
liters of hydrogen per hour” according to Frank Zielinski, a former
doctoral student in Nicole Dubilier’s Group in Bremen, who now works as a
post-doctoral researcher at the Helmholtz Centre for Environmental
Research in Leipzig. The deep-sea mussel symbionts therefore play a
substantial role as the primary producers responsible for transforming
geofuels to biomass in these habitats.
hydrothermal vents along the mid-ocean ridges that emit large amounts
of hydrogen can therefore be likened to a hydrogen highway with fuelling
stations for symbiotic primary production” says Jillian Petersen.
the symbionts of other hydrothermal vent animals such as the giant
tubeworm Riftia pachyptila and the shrimp Rimicaris exoculata have the
key gene for hydrogen oxidation, but remarkably, this had not been
previously recognized. “The ability to use hydrogen as an energy source
seems to be widespread in these symbioses, even at hydrothermal vent
sites with low amounts of hydrogen” says Nicole Dubilier.