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-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.

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 + SO₄^2- → 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).

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.

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.

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.

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.