Cold-seep communities
Cold seeps are places on the seafloor where cold hydrocarbon-rich water escapes. They occur most often at tectonic plate boundaries. Carbonate deposits and communities of organisms are often found at these sites. The base of this community is chemosynthetic bacteria that use the methane and hydrogen sulfide present in the seep water as an energy source.
Cold seeps explained
Dr Ashley Rowden, a principal scientist with the NIWA Marine Benthic Ecology group based in Wellington, explains what cold seeps are, where they are located in New Zealand and why rich chemosynthetic communities of specialised organisms develop around them.
Cold seeps and cold-seep carbonates
Dr Steve Hood explains that a cold seep is the expulsion of hydrocarbon-rich fluids at the seafloor. Over time. a specialist type of limestone can form. These are quite unlike shelf limestones found on land. Ancient cold-seep limestone deposits have been found on land in the East Coast region of the North Island.
Points of interest
Look for the following micrographs:
Micrograph a: Thin section image as seen under cathodoluminescent light of cold-seep limestone. The worm tube cross-section is infilled with radiating needles of aragonite crystals that have a blue colour.
Micrograph b: Thin section image as seen under cathodoluminescent light of cold-seep limestone. Needles of aragonite crystals growing off a bivalve shell fragment.
Cold-seep chemosynthetic bacteria
The microbial communities thriving at cold-seep sites are the basis of life around the seep system. They are found within the seafloor sediment, as bacterial mats on the seafloor, within larger invertebrate organisms in the community and in the water column above the seep, and they act as the base of the food chain for an extensive and unique collection of organisms.
Cold-seep bacterial mat
Scanning electron micrograph of methanotrophic bacteria found at a cold-seep site. The cylindrical bacteria ‘feed’ on the methane present in the water column issuing from the seep.
It is the methane and hydrogen sulfide present in the cold-seep water that serves as the energy source for these microbial communities.
Some of the bacteria (methanotrophic archaea) carry out a process called the anaerobic oxidation of methane (AOM). This permits the energy present in the methane to be used by the bacteria to carry out living processes such as growth, reproduction and the production of organic compounds such as proteins, lipids and sugars. Carbon dioxide dissolved in seawater provides a source of carbon for the bacteria to make these compounds.
CH4 + SO42- → HCO3- + HS- + H2O
methane sulfate bicarbonate hydrogen sulfide water
Unlike the community of organisms that humans live in, which is totally dependent upon photosynthesis, cold-seep communities are chemosynthetic, dependent only upon a supply of methane and hydrogen sulfide from deep within the seafloor.
Symbiotic relationships
The abundance of life around the methane-rich cold seeps would not be as prolific without symbiotic relationships. A symbiosis is an association between two forms of life from which both organisms benefit (for example, one can provide protection and the other can provide food).
Cold seeps and living things
NIWA's Dr Ashley Rowden describes how a community of organisms develops around a cold-seep site. Forming the base of such a community are specialised bacteria that use methane gas present in the seep water as their energy source. Other organisms such as clams, mussels and tubeworms use these chemosynthetic bacteria by developing symbiotic relationships with them.
Invertebrate organisms, like certain species of mussels, clams and tubeworms, can absorb and adsorb the chemosynthetic bacteria into or onto their tissues. Here, the bacteria supply the energy needs of these organisms using methane and hydrogen sulfide present in the surrounding water as the energy source. In return, these organisms provide protection in a sheltered environment.
A typical cold-seep community
A diagram of a typical cold-seep community of organisms. The community is totally dependent on the hydrocarbon-rich water flowing from the cold seep.
Consumers in the community
The large numbers of bacteria serve as a food source for bacteria-feeding animals (bacteriovores) such as snails, worms and crabs.
Organisms such as shrimps and crabs feed on the detritus produced by the mussels, clams and tubeworms, and predatory organisms such as octopus, fish and crustaceans are then attracted to this vibrant community to complete the food chain.
Many of the species present are only found in association with cold seeps. To date, over 400 named species have been identified.
Locating New Zealand cold-seep communities
Not a great amount was known about cold seep sites present on the Hikurangi Margin off the east coast of the North Island prior to 1990. Some of the sites had been located first by fishermen who had recovered unusual clams or had seen on their echo-sounders distinctive patterns they thought might be gas plumes.
Exploring New Zealand’s cold seeps
In 2006, NIWA’s deep-sea research vessel Tangaroa was used to explore the Hikurangi Margin for cold-seep sites. Communities of organisms at the sites were monitored by video and specimen samples taken for identification and analysis. Dr Ashley Rowden, from NIWA, summarises the research expedition.
During the 1990s, surveys were carried out by NIWA, and a number of potential seep sites were identified. In 2006 and 2007, three major research cruises to the Hikurangi Margin were conducted to study methane seepage and gas hydrates in areas previously reported as positive locations – 32 new seep sites were detected, bringing the total of known sites to 36. Most of these sites had a community of organisms associated with them, with bacterial mats, clams and tubeworms being observed and sampled for identification purposes.
Analysing cold-seep data and samples
NIWA's Dr Ashley Rowden describes how the data from the echo-sounders and the video camera images are overlaid. The net result gives a habitat map of the site. In 2006, eight seep sites were studied in detail, and a follow-up voyage in 2007 discovered more. At least 20–30 seep sites exist, each with a community of organisms.
Recent analysis at the species level has confirmed that the organisms collected are new to science or endemic to New Zealand seeps. It may well be that the Hikurangi Margin may represent a new biogeographic region for cold-seep organisms.
Nature of science
The world we live in is understandable. As the breadth of our knowledge expands, so to does the depth of our understanding. Recent discoveries made in and around the Hikurangi Margin cold-seep sites serve to illustrate this point.
Related content
Find out more about Cold-seep carbonates and Exploring for cold-seep sites.
In the activity, Multibeam seafloor survey students create a model seafloor and create a map of it through taking depth readings.
Useful link
Read more about cold seeps and the ChEss (Chemosynthetic Ecosystem Science) field project of the Census of Marine Life programme.
