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Weird Science: Hydrothermal Vents and Cold Seeps

NGSS Crosscutting Concepts:

Scientists once assumed that fish and other animals living at great depth were all somehow linked to food chains that begin with photosynthesis at the surface. However, a completely alternate source of energy forms the basis for the unique food webs found near hydrothermal vents.


Hydrothermal vents are isolated areas where the ocean floor has cracks that produce geothermally heated water. Organisms that live near the vents are able to live in very hot (65°C to 100°C) water that would kill most surface organisms. In addition, organisms at this depth are adapted to extreme pressure. The pressure at around 2,500 meters deep, where the vents are located, is so great that the superheated water from the underwater springs doesn’t boil! In fact, the pressure at this depth prevents water from boiling until it reaches about 370°C.


In 1977, scientists in deep-sea vehicles explored deep-ocean hot springs off the Galapagos Islands. They found a single-celled bacterium that contains no chlorophyll but can produce sugar. They also discovered that water from the vents is rich in hydrogen sulfide (H2S). The H2S molecule is energy rich, but it is also toxic to most living things. However, the vent bacteria are adapted to use the energy in H2S in the same way that plants harness sunlight energy. Rather than conducting photosynthesis, these bacteria conduct chemosynthesis. The chemical reaction for chemosynthesis is shown below:

6 CO2 +
6 O2
+ 24 H2S =
C6H12O6 +
24 S
+ 18 H2O
carbon dioxide
(energy source)


Hydrothermal vent animals feed on these bacteria, filtering them from the water. Other vent animals, like the giant tubeworms in the genus Riftia, have evolved a symbiotic relationship with the chemosynthetic bacteria (SF Fig. 2.3 C). Adult Riftia worms have a specialized organ that provides a home for the bacteria. In fact, Riftia lose their stomach early in life—as soon as they acquire their bacteria symbionts.

<p><strong>SF Fig. 2.3.</strong> (<strong>A</strong>) Deep sea hydrothermal vents are home to a variety of living organisms.</p><br />
<p><strong>SF Fig. 2.3.</strong>&nbsp;(<strong>B</strong>) A fuzzy, white, chemo-autotrophic bacteria covers coral and rocks near a hydrothermal vent.</p><br />
<p><strong>SF Fig. 2.3.</strong>&nbsp;(<strong>C</strong>) The tubeworm, <em>Riftia pachypitila</em>, contains chemo-autotrophic bacteria that use hydrogen sulfide as an energy source.</p><br />


Cold seeps are another environment of the deep sea loaded with energy-rich chemicals. Cold seeps occur at fissures, or cracks in the seafloor, that are caused by the movement of earth’s tectonic plates. The environment of hydrothermal vents and cold seeps is different in terms of temperature and longevity. Unlike the chemicals around hydrothermal vents, cold seeps are similar in temperature to the surrounding waters. Seeps also tend to be more stable than hydrothermal vents. Hydrothermal vents are relatively short-lived, but cold seeps are long-lasting. In fact, worms that live in the cold seep environments are among the longest living invertebrates in the world.


The base of the food web is also different in cold seeps. In cold seeps, hydrogen sulfide, methane, and other hydrocarbon-rich chemicals leach from petroleum deposits. These chemicals began as organic matter, which was transformed under high temperature over millions of years—they are a form of fossil fuel. Thus, the energy contained in cold seep chemicals can be traced back to the sun. In contrast, the energy in hydrogen sulfide (H2S) found at hydrothermal vents, comes from deep in the Earth’s core—not from the sun.

<p><strong>SF Fig. 2.4.</strong> (<strong>A</strong>) A cold seep tubeworm surrounded by orange bacterial mats of the sulfide-oxidizing bacteria at 550 m depth</p><br />
<p><strong>SF Fig. 2.4.&nbsp;</strong>(<strong>B</strong>) Mussels like <em>Bathymodiolus childressi</em> dominate cold seep communities in the Gulf of Mexico.</p><br />
<p><strong>SF Fig. 2.4.</strong>&nbsp;(<strong>C</strong>) Tube worms and mussels are often found together at cold seeps.</p><br />


Special Feature Type:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.