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Home , Cold seep, Marine geology, Sea, Seabed, Seawater, Trophosome

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Image courtesy of MARUM, Bremen University

Cold seeps: based on

Thousands of white crabs grazing on an extensive bed at a off the of

Glossary

Chemosymbiosis: a symbi- David Fischer takes us on a trip to the bot- otic association between a tom of the to learn about cold seeps – multi-cellular (the ), which provides a their ecosystems, potential fuels, and pos- protected environment, and a bacterium that oxidises sible involvement in global warming. specific chemicals to obtain and synthe- What are cold seeps? These hydrocarbons form up to sev- sise organic that is Cold seeps are often oases for eral kilometres below the surface of required by the host microbial and macrofaunal on the the when organic matter is Methanotrophic: a methan- sea floor – similar to hydrothermal degraded by either high temperatures otrophic organism vents, where hot emerges or micro-. When the hydro- metabolises as its under high pressure, several kilome- are produced in very large only source of energy and tres below the sea (see Little, 2010). In quantities, or where tectonic stress carbon contrast to hydrothermal vents, how- Pore water: the water that ever, cold seeps can occur at water fills the space between depths of between a few metres and individual grains of sedi- several kilometres, often along conti- ment nental margins. Thiotrophic: a thiotrophic They are places where hydrocarbons organism oxidises sulphur – mostly methane but also ethane, compounds propane, or even oil – seep from the sediment. Unlike at hydrothermal Trophosome: a specialised vents, the emanating fluids (gases and internal organ in tube Microbes covering cold seeps in an liquids) are no hotter than the sur- worms, hosting symbiotic anoxic , which does not support

BACKGROUND rounding , and they are not macrofauna necessarily under high pressure. Image courtesy of MARUM, Bremen University

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Science topics

squeezes the , the fluids rise One such energy source is methane, surface of the sediment (as in cold to the sediment surface through released at cold seeps. The anaerobic seeps), the sulphide produced by the cracks and fissures. The fluids may oxidation of methane (AOM) is a meta- micro-organisms can fuel an entire seep out at a continuous, low rate or bolic process with sulphate as the final . Microbial or faunal the rate may vary. electron acceptor, mediated by a sym- colonies at cold seeps can be between biosis of methane-oxidising (methan- 100 cm2 and several hundred square Life at cold seeps: anybody home? otrophic) archaeans and sulphate- metres in diameter. Where the sul- Some fascinating creatures can be reducing bacteria. phide-rich pore water escapes from 2- → - - found at deep-water cold seeps, CH4 + SO4 HCO3 + HS + H2O the sea floor, the seep site will be including giant tube worms, , AOM takes place in anoxic marine colonised concentrically around these clams and crabs. How are these sediments, wherever methane from spots: closest to them will be those ecosystems supported? deep down and sulphate from the organisms which tolerate the highest Almost the entire is home to seawater meet. The end products of concentrations of otherwise toxic sul- micro-organisms. However, to sup- AOM, and sulphide , phide (see upper image on page 62). port significant levels of microfauna are released into the surrounding sed- At the basis of these ecosystems in deep water, where the sunlight iment and pore water (see glossary are methanotrophic and thiotrophic does not reach, requires -rich for all terms in bold). bacteria. Some of them live in water and an alternative energy At locations where high methane chemosymbiosis with mussels source – such as hydrocarbons. concentrations are found close to the (methanotropic bacteria), clams and

Biology The text also provides valuable background reading to Chemistry introduce the origin of life at the bottom of the sea, for example with a comparison to hydrothermal vents. science Marine environments In addition, the article provides references to content- rich websites (MARUM, NOAA) where the reader can Ecosystems find further information and resources (including Energy teaching materials) on the topic. Therefore, I suggest completing the teaching unit with an activity from change one of the cited web resources. Ages 15+ The article can also be used as a comprehension exer- cise. Possible questions include: If you are curious about the extremes of life on Earth, In AOM this is the article for you. David Fischer reports on a) sulphate is oxidised to sulphide exotic biological communities living at the bottom of the sea and on the exploration of these environments. b) sulphide is reduced to sulphate The language is easy enough to understand, with a c) methane is reduced to bicarbonate glossary for technical terms. The article can be used in d) methane is oxidised to bicarbonate. different subjects (biology, , chemistry) Gas hydrates to address a whole range of topics, such as: ecosys- a) consist of water surrounded by gas tems, energy metabolism, food chains, natural molecules resources, sedimentary rocks, non-renewable energy b) consist of gas molecules surrounded by water sources, marine environments and biodiversity, air molecules pollution and the greenhouse effect, oceanographic c) are formed under high-temperature and research, hydrocarbons, clathrates, or reac- low-pressure conditions tions. Some of these topics (e.g. ecosystems, energy d) are formed under high-temperature and sources) are particularly suitable for an interdiscipli- high-pressure conditions. nary approach.

REVIEW Giulia Realdon, Italy

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our increasing energy demands. Cold Cold seeps are often Gas bubbles seeps generally indicate large colonised in a concentric amounts of hydrocarbons below the fashion Tube worms seabed, and they are comparably easy Chemosynthetic to identify because of their typical bacteria colonisation by specialised organisms. Most interesting in this respect are gas hydrates. In these ice-like crys- Mussels and clams talline compounds, water molecules form a cage structure called a clathrate around individual gas molecules (see image on page 63) – in hydrates, this is mostly methane. Gas Gas hydrate hydrates form where pore water is

Image courtesy of BGR, after Sahling et al. (2002) Low / high sulphide concentration saturated with methane gas, within a narrow window of low-temperature Plume: sucks in oxygen, carbon and high-pressure conditions found dioxide, and sulphide so only in deep permafrost soils – and in that microbes living inside the trophosome can process it as food marine sediments at depths below about 400 m (see lower image on page 63). At atmospheric pressure, gas Heart Ventral blood vessel hydrates are unstable and rapidly Dorsal blood vessel decompose into water and free gas. Gas hydrates store large amounts of Trophosome: micro-organisms that have a symbiotic relationship with chemically bound energy: because of Trophosome the tube worm live here. They gener- the specific molecular structure, one Capillary ate food for the tube worm through a process called litre of gas hydrate holds 0.8 l of Worm water and 164 l of methane gas. The Tube Tube: hard cylinder made of a total energy resources in gas hydrates chitin proteoglycan / protein on Earth are estimated to be greater complex that protects the soft than those of all other known insides of the tube worm fuels combined – ever. Several countries, including the Capillary Trunk: the tube worm has no USA, , South Korea, and anus; all of the waste from the microbial reactions are , are exploring ways to harvest stored here gas hydrates – safely, which is not a trivial matter. More importantly, how- ever, it is essential to stop the gas Chemosynthethic bacteria: These bacteria use sulphides hydrates from melting. Global warm- and to pro- ing is increasing the ’ tempera- duce oxygen and tures, and this could cause a large-

Image courtesy of Enduring Resources for Earth Science Education (ERESE) Image courtesy of Enduring Resources to be used by the tube worm scale melting of gas hydrates in the sediment. Tube worms host chemosymbiotic bacteria in their trophosome. This is a cross-section If they were to melt, the methane of , which is found at hydrothermal vents released into the would react with atmospheric oxygen to

tube worms (thiotrophic bacteria). The formation and fate of gas form CO2, very efficiently enhancing The mussels and clams harbour the hydrates the greenhouse effect. Left undam- bacteria in their , whereas the Cold seeps are not only interesting aged, gas hydrates act as stabilising tube worms shelter the bacteria in because of the ecosystems they host: agents for the continental slopes. If their trophosome; the bacteria, in they could be important contributors they were to melt, the slopes could return, supply their host with organic to and valuable new destabilise, resulting in huge subma- carbon (see image above). sources of hydrocarbons, to satisfy rine and .

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Gas Images courtesy of MARUM, Bremen University molecules

Image courtesy of IfM-Geomar, Kiel, Germany Image courtesy of IfM-Geomar, White gas hydrates consolidate Water molecules Burning gas hydrates sea floor sediments Clathrate structure of gas hydrates

How do we study cold seeps? they had filled with sample material. to collect samples or taking photos of Studying cold seeps is obviously a This of course made it very difficult a larger area to provide an overall pic- big challenge for scientists: how can to have any visual control of where ture of a cold seep location. gases, water, sediment and organisms the tools hit the ground. There is still a lot of research to be sampled up to several kilometres Matters have improved dramatical- be done into the development and below the sea surface? First, you have ly with the development of sophisti- stability of cold seeps over time, and to reach your sampling location – this cated underwater technology, such as little is known about the organisms may take several days by , even remotely operated vehicles (ROVs) colonising them. Yet the most for cold seeps on the , and autonomous underwater vehicles important question about cold seeps and maintaining such a research ves- (AUVs), equipped with an array of is how much methane is transferred sel costs several tens of thousands of cameras, lamps and sampling devices. into the and subsequently into Euros a day. And when you get there, MARUMw1 has its own workshop to our atmosphere, where it will con- how do you reach the sea floor? Until develop such tools and robots for tribute to global warming. For me, it the 1990s, the only way was to lower ocean research. Researchers use them is very exciting to take part in this special tools on the end of a long steel to investigate cold seeps by either research. cable, and recover them as soon as having them dive to individual spots

Temperature in oC

Under specific combinations of high pressure (water depth) and low temperature, methane can crystallise with water to form gas hydrates, as indicated by the phase boundary line. At higher temperatures or shal- lower depths, methane will instead dissolve in water. In the ocean, there are additional complications: in the , temperature decreases with increas- ing water depth, whereas in the sediment, temperature increases with increasing depth. The points at which these temperature profiles cross the theoretical phase boundary determine the depths at which gas hydrates may be found (the gas hydrate stability zone). Depth in m Furthermore, it is mostly in the sediment that methane concentrations are high enough to form gas hydrates (marked in white) – they have seldom been observed in the water column. Note that the scale in this diagram is an example and can vary depending on conditions

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clams found at cold seeps. See http://www.marum.de/marumTV. html The US National Oceanic and Atmospheric Administration (NOAA) offers a number of lesson plans and downloadable teaching activities on cold seeps for all ages, plus background information. See the NOAA website Image courtesy of MARUM, Bremen University MARUM-QUEST 4000m, an ROV Close-up photograph of the symbiotic (http://oceanexplorer.noaa.gov) suitable for depths of up to vestimentiferan tube worm or use the direct links: 4000 m, equipped with a large from a cold http://tinyurl.com/36zuhfb number of cameras, spotlights, seep at 550 m depth in the of Image courtesy of Boetius (2005); image source: Wikimedia Commons Wikimedia Image courtesy of Boetius (2005); image source: (‘Windows to the Deep’) corers and other tools to collect Mexico. The tubes of the worms are samples from the sea floor as stained with a blue chitin stain to http://tinyurl.com/32f6me6 small as single organisms only a determine their growth rates. (‘Expedition to the Deep Slope 2006’) few millimetres long Approximately 14 months of growth http://tinyurl.com/35g3qrk is shown by the staining here (‘Expedition to the Deep Slope 2007’) For some general information about gas hydrates, see the MARUM Acknowledgements Web reference website (www.marum.de) or use The author would like to thank Dr w1 – To learn more about MARUM – the direct link: w1 Pape (MARUM ) for valuable com- the Center for Marine http://tinyurl.com/3xzzpjj ments, particularly about gas Environmental , an inde- For more information on hydrocar- hydrates. Moreover, Science in School pendent DFG-funded research facil- bons, including gas hydrates, see: and the author thank the publisher ity at the University of Bremen, van Dijk M (2009) Hydrocarbons: a Inter-Research for permission to reuse Germany, see: www.marum.de fossil but not (yet) extinct. Science in the image from Sahling et al. (2002). School 12: 62-69. Resources www.scienceinschool.org/2009/ References MARUM offers a large selection of issue12/energy Boetius A (2005) Microfauna-macro- German-language resources and If you enjoyed reading this article, interaction in the seafloor: activities for teachers and school you might like to browse our other lessons from the tubeworm. PLoS students, including videos and arti- articles on science topics in Science Biology 3(3): e102. doi: 10.1371/ cles on research topics, a large selec- in School: www.scienceinschool.org/ journal.pbio.0030102 tion of workshops for both primary- sciencetopics and secondary-school students in Little C (2010) Hot stuff in the deep the MARUM teaching lab, work- sea. Science in School 16: 14-18. shops for primary-school teachers, Born in Jülich, Germany, David www.scienceinschool.org/2010/ experimental science theatre work- Fischer has always been fascinated by issue16/hotstuff shops for primary-school children, science and the sea. After graduating Sahling H et al. (2002) Macrofaunal and much more. See: in , marine geolo- structure and sulfide www.marum.de/en/entdecken.html gy and biology from the University of flux at gas hydrate deposits from MARUM has produced a wonderful Bremen, he is now working towards a the Cascadia convergent margin, (English-language) video on cold PhD in marine at MARUM, NE Pacific. Marine Progress seeps and methane hydrates. See investigating the biogeochemistry of Series 231: 121-138. doi: www.marum.de/marumTV.html cold seeps. He has participated in a 10.3354/meps231121 number of research expeditions to the In addition, MARUM has pro- The article can be downloaded free , the , the Arabian duced, in co-operation with the of charge from the Inter-Research Sea, the central east Atlantic, and the Deutsche Forschungsgemeinschaft, website: www.int-res.com near the Antarctic a 12-episode series of films about its Peninsula. research (DFG Science TV). Episode six (German only) tells the story of

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