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23.08.2016 Diversity of habitats at natural oil seeps

Aug 23, 2016
A research team investigates the allegedly inhospitable sea-floor area in the southern Gulf of Mexico - and finds it surprisingly diverse and multifaceted.
 
Habitats surrounding natural oil seeps on the sea floor are multifaceted and diverse. During an expedition organized by MARUM, an international team of researchers including participants from the Max Planck Institute for Marine Microbiology (MPI Bremen) discovered gas-bubble streams, massive gas hydrates, oil-soaked sediments, and deposits of heavy oil, all closely spaced at a depth of around three kilometers. Each of the different constituents - gas, light oil, and heavy oil congealed to asphalt - is home to its own characteristic group of organisms. The scientists have now published their initial results, along with photos from the remotely operated vehicle MARUM-QUEST, in the journal Biogeosciences.
 
“Recent years have seen a minor revolution in the field of marine research,” explains Heiko Sahling from MARUM, the Center for Marine Environmental Sciences, and the Geosciences Department of the University of Bremen. Many German research ships have been outfitted with state-of-the-art multibeam echosounders. These are of great help in the systematic search for natural seeps of oil and gas on the sea floor. “In the past, this was more like the proverbial search for a needle in a haystack”, says Sahling. “Now we have discovered habitats on the sea floor that were unknown before.”
 
MARUM - Center for Marine Environmental Sciences
Asphaltstrukturen am Mictlan Knoll in 3100 Meter Tiefe, aufgenommen mit dem Tiefseerobotoer MARUM-QUEST.
Scientists use the echosounders to detect gas bubbles in the water column. Hydrocarbons leaking from the seafloor amplify the acoustic signal in the water, and to some extent also within the sea floor. The team of scientists from Bremen, Kiel, Vienna (Austria), Mexico City (Mexico) and Talahassee (USA) now applied this modern technology during an expedition to the Bay of Campeche in the southern Gulf of Mexico. There they discovered hundreds of gas seeps and investigated a number of them in detail with the submersible vehicle MARUM-QUEST. Their goal was to reveal the ways of hydrocarbon movement at natural seeps. In what form are they released? How do they affect the local organisms? How rapidly does the oil break down? What happens to the released gas?
 

“The gas is partly converted to gas hydrates (an ice-like form of water and gas), which form small mounds on the sea floor. These are densely populated by meter-long tube worms,” explains Sahling. “Sometimes the mounds are prised, allowing a view into several-meter-thick gas hydrates, a very rare observation. The gas hydrates are overlain by a reaction zone where microbial communities convert methane, carbonate is precipitated, and dense colonies of tube worms develop. These keep the mounds together and live on reduced sulfur compounds. It is truly a remarkable habitat,” Sahling simmarizes.

In addition to the gas, also liquid oil escapes from the sea floor. It ascends slowly through small white chimneys, the drops of oil forming elongated threads or seeping through the sediments. “For organisms that are not adapted, the oil is harmful,” Sahling explains. “But the bountiful life at these sites shows that there are certain organisms that can thrive even on these hydrocarbons.”

The basis for this life is formed by microorganisms which degrade the various components of the oil. They are the focus of research at the MPI Bremen. Many of these microorganisms live anaerobically, i.e. without oxygen, in the oily sediments. Gunter Wegener from MPI Bremen currently investigates which microorganisms are actually using the asphalt and its constituents „What’s really exciting is that very different microorganisms – namely bacteria and so-called archaea – join forces to break up the hydrocarbons. We call this a syntrophy.“ Other bacteria degrading hydrocarbons cannot live without oxygen. As a result, their partnerships look quite different: They form symbioses with invertebrates. „We find these bacterial tenants in mussels and different sponges inhabiting the the oily crusts and blick of gas hydrates“, says Christian Borowski from MPI Bremen. „It’s noteworthy that these symbionts are closely related to bacteria that played a major role in degrading hydrocarbons in the Gulf of Mexico after the Deep-Water-Horizon-Oilspill.“ Researchers at the MPI Bremen now investigate, which metabolic pathways the symbiotic bacteria use, and which role the play fort he host.

While these bacterial processes are not apparent to the eye, the oil seeps do allow for some spectacular views. Some components of the so-called “heavy oil” dissipate. What remains forms flow structures of asphalt on the sea floor. “During the expedition we documented many of these unique structures,” says Sahling. “The asphalt covers hundreds of meters of the sea floor and thus also forms a habitat that is colonized by tube worms and bacterial mats."

 
 
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