What are cold seeps?
Lamellibrachia tube worms © MacDonald, Texas A&M
Cold seep ecosystems were found for the first time as recently as 1984 in the deep Gulf of Mexico! The animals that thrive on cold seeps are similar to those found on hydrothermal vents and also depend on the production of microorganisms that use reduced chemicals as source of energy.
Deep water cold seeps occur in both passive and active continental margins, at depths ranging between 400 and 8000 m. The continental margin is the slope that extends from the shelf edge (~200 m) to the abyssal plains at 4000 - 6000 m of water-depth. Passive continental margins are non-seismic margins, and cold seeps here are usually associated with oil and gas reservoirs. Active continental margins are associated with subductions zones. Subduction occurs where an oceanic plate collides with another oceanic plate or with a continental plate. One of the plates bends and sinks under the other, forming an oceanic trench.
© Scripps Insitute of Oceanography
Cold seeps are characterised by seepage of cold fluids with a high concentration of methane. This methane can have a biological origin from the decomposition of organic matter by microbial activity in anoxic (without oxygen) sediments, or a thermogenic origin from fast transformation of organic matter caused by high temperatures. Another important factor in some cold seeps is a high concentration of sulfide in the sediments, produced by the reduction of sulfates. Both, methane and sulfide play a major role in the sustainment of the high-productive cold seep communities.
"Cold seep" communities were first discoverd in 1984 just off Monterey Bay, California by geologist, Professor Charles Paull from MBARI, USA. He was one of the first scientists to study and name these communities. At the same time, cold seeps were quite unexpectedly discovered on the Florida Escarpment in the Gulf of Mexico by Texas A&M University and by LGL Ecological Research Inc., then under contract to the Minerals Management Service and the oil and gas industry. Since these discoveries, cold seeps have been discovered in many other parts of the world. The deepest known cold seep community is located in the Sea of Japan at depths of 5 - 6,500m.
Mussels & tube worms on Florida Escarpment © MacDonald, Texas A&M
The communities associated with cold seeps are dense and composed by animals related to the animals found in hydrothermal vents in that they utilize chemoautotrophy (in the form of methane and sulphides).
What animals live there?
The dominant fauna found at cold seeps are clams, mussels and tubeworms. These animals largely base their nutrition on chemosynthetic production via symbiotic bacteria.
Mussels, Clam and Tubeworms © MacDonald, Texas A&M
The tubeworms often form bush-like aggregations as shown in the photograph and these bushes are themselves a habitat for associated organisms including crabs and a variety of sponges, bryozoans, and polychaetes.
A new kind of animal was recently discovered on cold seeps in the Gulf of Mexico (at depths of ~500m). It has been named the "ice worm", Hesiocaeca methanicola. It inhabits extensive burrows that it excavates in seafloor deposits of gas hydrate. Gut contents of the ice worms include sediments and large bacterial cells, which could be obtained by grazing hydrate surfaces, but their food sources are still being determined.
The Ice Worm © MacDonald, Texas A&M
As for hydrothermal vents, cold seep communities are sutained by chemoautotrophic bacteria. Metabolic processes of any living organism require a source of energy and a source of carbon for the production of organic compounds which are at the base of life. Until the discovery of hydrothermal vents, photosynthesis was the best known metabolic process for the sustainment of life on earth. Photosynthesis uses light as the source of energy and CO 2 as the inorganic source of carbon. In chemoautotrophy, the source of carbon is again inorganic (CO 2), but the source of energy is chemical, obtained from sulfide or methane.
Chemoautotrophic Bacterial Mat © Levin, Scripps Institute of Oceanography
The chemoautotrophic bacteria of cold seeps are found both free living (as seen in the photograph above) and in symbiotic associations with invertebrates such as tube worms, mussels and clams. A symbiosis is an association between two forms of life from which both organisms benefit (for example one can provide protection and the other one can provide food). We will talk with more detail about specific symbiosis when we observe the animals from the submbersible window.
Dive with us to a seep
It's 07h45 in the morning. You are on board Research Vessel Edwin Link in the Gulf of Mexico. Today, you will be diving in a research submersible to explore cold seep communities. You had your dive briefing last night and have the notes of your tasks in your notebook.
© M. Baker, NOCS, UK