Black Smokers: Buoyant Plumes and Life Without Sunlight

Seawater cooked underground and spat back out

A hydrothermal vent begins with ordinary seawater seeping down through cracks in newly formed oceanic crust near a mid-ocean ridge. As it descends it gets closer to magma chambers a few kilometres below the seafloor, and its temperature climbs past 350°C. At those temperatures the fluid becomes chemically aggressive — it leaches metals like iron, copper, zinc and manganese straight out of the surrounding rock and picks up dissolved hydrogen sulfide from magmatic gases. Being now hot and buoyant, it rises rapidly back up through the crust and erupts from a chimney-like vent structure back into the near-freezing (around 2°C) water of the deep ocean floor.

Why the plume is black, and why it rises

The instant that scalding, metal-laden fluid meets cold seawater, the dissolved sulfides precipitate almost instantaneously into microscopic particles of iron sulfide and other metal sulfides — the "smoke" of a black smoker is really a dense cloud of freshly formed mineral dust, not combustion. Because the vent fluid is both hotter and less dense than the surrounding seawater, the whole plume is buoyant and rises through the water column exactly the way hot air rises above a candle, entraining and mixing with the surrounding cold water as it climbs, cooling and diluting until it reaches a level where its density matches the ambient water and it spreads out sideways instead of rising further.

buoyancy force per unit volume  ≈  (ρ_seawater − ρ_plume) · g

ρ_plume < ρ_seawater   while the plume is still hotter than its surroundings
→ net upward force → plume rises and entrains cold water as it goes
→ plume cools, ρ_plume rises → buoyancy shrinks → plume spreads at its
  "neutral buoyancy" height instead of rising forever

Chemosynthesis: an ecosystem with no sunlight at all

Almost every ecosystem on Earth's surface ultimately runs on photosynthesis — plants and algae turning sunlight into sugar, and everything else eating them or eating something that ate them. Vent communities, thousands of metres below any sunlight, run on a completely different energy source: chemosynthesis. Free-living and symbiotic bacteria oxidise the hydrogen sulfide streaming out of the vent, using that chemical reaction — rather than light — to build sugars from carbon dioxide and water. Those bacteria form the base of a food web that supports giant tube worms, vent crabs, eyeless shrimp and mussels found nowhere else on the planet, all clustered tightly around the narrow band of warm, chemical-rich water near each vent.

Tubeworms: a body built entirely around bacteria

Giant tube worms (Riftia pachyptila) are the most striking result of this partnership. As adults they have no mouth, no gut and no way to eat at all — instead, a specialised internal organ called the trophosome is packed with billions of chemosynthetic bacteria. The worm's blood-red plume, which is what protrudes from its white tube, absorbs oxygen and hydrogen sulfide directly from the vent water and ferries both to the trophosome, where the resident bacteria oxidise the sulfide and manufacture organic compounds that feed the worm from the inside. It is one of the most complete examples of symbiosis known: the worm exists purely as life support for its bacteria, and the bacteria feed it in return.

Frequently asked questions

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