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Cold, oxygen-rich water from the Southern Ocean combined with a super-charged methane seep in the seafloor off New Zealand’s North Island have created a unique food web according to an international team of scientists.

Bacteria convert methane to carbon dioxide

Beds of tens of thousands of burrowing worms are creating conduits in the seafloor releasing large amounts of methane trapped just below the surface into the water column. However, lead researcher Dr Andrew Thurber from Oregon State University says a thriving bacterial community is “putting the pin back in the methane grenade” by consuming the released methane and converting it to carbon dioxide. In turn, the fat happy worms are gobbling up the bacteria.

In a press release from Oregon State University, Dr Thurber says they didn’t discover any major burps of methane (a greenhouse gas 23 times more potent than carbon dioxide at warming our atmosphere) escaping into the atmosphere. “However, some of the methane seeps are releasing hundreds of times the amounts of methane we typically see in other locations [as much as 100–300 times more than other methane seep 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.”

Unique polychaete habitat

The scientists discovered seven new methane seeps in 600–1200 m of water in 2006 and 2007. The amount of methane emitted from the seeps was surprisingly high, fuelling a unique habitat dominated by polychaetes or worms from the family Ampharetidae.

“They were so abundant that the sediment was black from their dense tubes,” says Dr Thurber.

The other critical element necessary for this unique habitat are the oxygen-rich waters near the seafloor that the bacteria harness to consume the methane efficiently. The oxygen also enables the worms to breathe better and in turn consume the bacteria at a faster rate.

Global methane cycle

“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-fuelled habitat – and the worms’ movements and grazing are likely causing the microbial populations to eat methane faster. 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.”

The research team writes that the discovery highlights scientists’ limited understanding of the global methane cycle – and specifically the biological interactions that create the stability of the ocean system.

Results of the study were published in the August 2013 issue of Limnology and Oceanography. NIWA’s Dr Ashley Rowden is one the authors of the research report.

Activity idea

Your students may like to watch this video clip, in which Dr Ashley Rowden describes how a community of organisms develops around a cold-seep site.
Cold seeps and living things

    Published 30 September 2013