Cold seeps: marine ecosystems based on hydrocarbons Understand article

David Fischer takes us on a trip to the bottom of the sea to learn about cold seeps – their ecosystems, potential fuels, and possible involvement in global warming.

What are cold seeps?

Cold seeps are often oases for microbial and macrofaunal life on the sea floor – similar to hydrothermal vents, where hot water emerges under high pressure, several kilometres below the sea (see Little, 2010). In contrast to hydrothermal vents, however, cold seeps can occur at water depths of between a few metres and several kilometres, often along continental margins.

They are places where hydrocarbons – mostly methane but also ethane, propane, or even oil – seep from the sediment. Unlike at hydrothermal vents, the emanating fluids (gases and liquids) are no hotter than the surrounding seawater, and they are not necessarily under high pressure.

These hydrocarbons form up to several kilometres below the surface of the sediment when organic matter is degraded by either high temperatures or micro-organisms. When the hydrocarbons are produced in very large quantities, or where tectonic stress squeezes the sediments, the fluids rise to the sediment surface through cracks and fissures. The fluids may seep out at a continuous, low rate or the rate may vary.

Life at cold seeps: anybody home?

Some fascinating creatures can be found at deep-water cold seeps, including giant tube worms, mussels, clams and crabs. How are these ecosystems supported?

Almost the entire seabed is home to micro-organisms. However, to support significant levels of microfauna in deep water, where the sunlight does not reach, requires oxygen-rich water and an alternative energy source – such as hydrocarbons.

One such energy source is methane, released at cold seeps. The anaerobic oxidation of methane (AOM) is a metabolic process with sulphate as the final electron acceptor, mediated by a symbiosis of methane-oxidising (methanotrophic) archaeans and sulphate-reducing bacteria.

CH₄ + SO₄^2- → HCO_3- + HS^– + H₂O

AOM takes place in anoxic marine sediments, wherever methane from deep down and sulphate from the seawater meet. The end products of AOM, bicarbonate and sulphide ions, are released into the surrounding sediment and pore water (see glossary for all terms in bold).

At locations where high methane concentrations are found close to the surface of the sediment (as in cold seeps), the sulphide produced by the micro-organisms can fuel an entire ecosystem. Microbial or faunal colonies at cold seeps can be between 100 cm² and several hundred square metres in diameter. Where the sulphide-rich pore water escapes from the sea floor, the seep site will be colonised concentrically around these spots: closest to them will be those organisms which tolerate the highest concentrations of otherwise toxic sulphide (see image above).

At the basis of these ecosystems are methanotrophic and thiotrophic bacteria. Some of them live in chemosymbiosis with mussels (methanotropic bacteria), clams and tube worms (thiotrophic bacteria). The mussels and clams harbour the bacteria in their gills, whereas the tube worms shelter the bacteria in their trophosome; the bacteria, in return, supply their host with organic carbon (above).

The formation and fate of gas hydrates

Cold seeps are not only interesting because of the ecosystems they host: they could be important contributors to climate change and valuable new sources of hydrocarbons, to satisfy our increasing energy demands. Cold seeps generally indicate large amounts of hydrocarbons below the seabed, and they are comparably easy to identify because of their typical colonisation by specialised organisms.

Most interesting in this respect are gas hydrates. In these ice-like crystalline compounds, water molecules form a cage structure called a clathrate around individual gas molecules (see image on left) – in natural gas hydrates, this is mostly methane. Gas hydrates form where pore water is saturated with methane gas, within a narrow window of low-temperature and high-pressure conditions found only in deep permafrost soils – and in marine sediments at depths below about 400 m (see image below). At atmospheric pressure, gas hydrates are unstable and rapidly decompose into water and free gas.

Gas hydrates store large amounts of chemically bound energy: because of the specific molecular structure, one litre of gas hydrate holds 0.8 l of water and 164 l of methane gas. The total energy resources in gas hydrates on Earth are estimated to be greater than those of all other known fossil fuels combined – ever.

Several countries, including the USA, Japan, South Korea, India and China, are exploring ways to harvest gas hydrates – safely, which is not a trivial matter. More importantly, however, it is essential to stop the gas hydrates from melting. Global warming is increasing the oceans’ temperatures, and this could cause a large-scale melting of gas hydrates in the sediment.

If they were to melt, the methane released into the atmosphere would react with atmospheric oxygen to form CO₂, very efficiently enhancing the greenhouse effect. Left undamaged, gas hydrates act as stabilising agents for the continental slopes. If they were to melt, the slopes could destabilise, resulting in huge submarine landslides and tsunamis.

How do we study cold seeps?

Studying cold seeps is obviously a big challenge for scientists: how can gases, water, sediment and organisms be sampled up to several kilometres below the sea surface? First, you have to reach your sampling location – this may take several days by ship, even for cold seeps on the continental shelf, and maintaining such a research vessel costs several tens of thousands of Euros a day. And when you get there, how do you reach the sea floor? Until the 1990s, the only way was to lower special tools on the end of a long steel cable, and recover them as soon as they had filled with sample material. This of course made it very difficult to have any visual control of where the tools hit the ground.

Matters have improved dramatically with the development of sophisticated underwater technology, such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), equipped with an array of cameras, lamps and sampling devices. MARUM^w1 has its own workshop to develop such tools and robots for ocean research. Researchers use them to investigate cold seeps by either having them dive to individual spots to collect samples or taking photos of a larger area to provide an overall picture of a cold seep location.

There is still a lot of research to be done into the development and stability of cold seeps over time, and little is known about the organisms colonising them. Yet the most important question about cold seeps is how much methane is transferred into the ocean and subsequently into our atmosphere, where it will contribute to global warming. For me, it is very exciting to take part in this research.

Acknowledgements

The author would like to thank Dr Pape (MARUM^w1) for valuable comments, particularly about gas hydrates. Moreover, Science in School and the author thank the publisher Inter-Research for permission to reuse the image from Sahling et al. (2002).

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