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Hydrothermal vents teem with life on a lifeless seabed. How are they colonised?

Mike Follows, Sutton Coldfield, West Midlands, UK

Hydrothermal vents occur at hotspots on the ocean floor, often at mid-ocean ridges where two oceanic plates are moving apart.

Vents in the deep ocean are beyond the reach of sunlight yet support a diverse ecosystem and appear like oases in a desert. Creatures living in this environment obtain their energy via chemosynthesis from the chemicals dissolved in the vents instead of the photosynthesis that is essential to life on Earth’s surface. The discovery that ecosystems can be hosted by these vents increases the likelihood of extra-terrestrial life, even within our solar system – in the water beneath the ice of Jupiter’s moon Europa, for example.

Giant tube worms (Riftia pachyptila) and other animals around hydrothermal vents reproduce by releasing eggs and sperm into the water. These fuse and develop into microscopic larvae – the equivalent of seeds – that are dispersed by ocean currents. Hot water from the vents forms buoyant plumes that cool as they mix with the surrounding water and become neutrally buoyant about 200 metres above the ocean floor. Local currents then carry this water over long distances.

The larvae carry enough food reserves to survive a journey of several weeks. If they arrive at another suitable vent before they starve, they could start a new colony. The same species can be absent from closer vents but present at more distant ones, which may be because of differences in the depth, pressure, temperature or chemistry at different vents.

This colonising mechanism is reminiscent of the mass spawning events on tropical coral reefs. It is also like panspermia, the idea that life came to Earth aboard an asteroid, which was championed by scientists like Svante Arrhenius and Fred Hoyle. They believed that the seeds of life pervade the universe and, when they land on a habitable planet, life flourishes.

Other scientists doubt that life has an extraterrestrial origin. There is an increasing body of opinion that alkaline hydrothermal vents like those found at the Lost City site in the mid-Atlantic Ocean might be where life on Earth began.

In 1993, before alkaline vents were discovered, geochemist Michael Russell predicted that life could harness the energy created when electrical charges are separated. Acids have more hydrogen ions – or protons – than alkalis, so separating an acid from an alkali leads to a separation of charges, just like in a battery.

This could occur when alkaline water inside a vent is separated by porous rock from the more acidic seawater of the ancient oceans outside. This may have been the precursor of the proton pump that acts across biological membranes in cells and allows respiration to take place.

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