August 13, 2013
Unique Worm Community Releasing Methane From Ocean Floor
April Flowers for redOrbit.com - Your Universe Online
A super-charged methane seep has been discovered in the ocean off the coast of New Zealand. The seep has created its own unique food web, resulting in elevated amounts of methane escaping from the ocean floor into the water column, according to scientists at Oregon State University.
Methane is a greenhouse gas 23 times more potent than carbon dioxide at warming our atmosphere. Most of the methane in the seep is likely consumed by biological activity in the water, the study suggests. This means the methane will not make it into the atmosphere to exacerbate global warming. This discovery, however, highlights the limited understanding scientists have of the global methane cycle – and specifically the biological interactions that create the stability of the ocean system.
The study results were published online in the journal Limnology and Oceanography. “We didn’t discover any major ‘burps’ of methane escaping into the atmosphere,” said Andrew R. Thurber, a post-doctoral researcher at Oregon State University. “However, some of the methane seeps are releasing hundreds of times the amounts of methane we typically see in other locations, so the structure and interactions of this unique habitat certainly got our attention.
“What made this discovery most exciting was that it is one of the first and best examples of a direct link between a food web and the dynamics that control greenhouse gas emissions from the ocean,” Thurber added.
The new series of methane seeps were first discovered in 2006 and 2007 in approximately 2,000 to 4,000 feet of water off North Island of New Zealand. The researchers found the amount of methane emitted from the seeps was surprisingly high. A unique habitat dominated by polychaetes, a class of segmented marine worms from the family Ampharetidea, was created by the high methane emissions.
"They were so abundant that the sediment was black from their dense tubes,” Thurber pointed out.
The researchers say these tubes, or tunnels in the sediment, are critical. The worms essentially created tens of thousands of new conduits for the methane by burrowing into the sediment. These conduits release the methane trapped below the surface, most of which is then consumed by bacteria and converted to carbon dioxide. The worms feast on the enriched bacteria, thus bolstering their population and leading to even more tunnels and higher levels of methane release.
One further element is necessary for the creation of this unique habitat: oxygen rich waters near the seafloor. The bacteria use this rich water to consume the methane more efficiently. The higher levels of oxygen also allow the worms to breathe better and in turn consume the bacteria at a faster rate.
“In essence, the worms are eating so much microbial biomass that they are shifting the dynamics of the sediment microbial community to an oxygen- and methane-fueled habitat – and the worms’ movements and grazing are likely causing the microbial populations to eat methane faster,” said Thurber, who works in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “That process, however, also leads to more worms that build more conduits in the sediments, and this can result in the release of additional methane.”
Many other areas of the world, including the Pacific Northwest, have methane seeps and worm communities. The deep water in many of these locations, however, has low levels of oxygen. The scientists believe this is a factor that constrains the growth of the worm populations. The New Zealand sites, in contrast, are bathed in cold, oxygen-rich water from the Southern Ocean that fuels these unique habitats.
“The large amounts of methane consumed by bacteria have kept it from reaching the surface,” Thurber said. “Those bacteria essentially are putting the pin back in the methane grenade. But we don’t know if the worms ultimately may overgraze the bacteria and overtax the system. It’s something we haven’t really seen before.”