Deep sea: Bacteria feast for stone crabs

Insight into the highly specialized food web at the methane sources of the deep sea

Cancer on the Bacterial Mat near Mud Volcano Mound 12 © GEOMAR
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Cold seeps, cold springs, form a kind of oases of the deep sea. Now explorers off the coast of Costa Rica have discovered that certain cancers also use these oases: they are grazing the bacteria mats that grow at the deep-sea wells. This proves that not only sedentary organisms benefit from the productivity around the cold seeps, the researchers report in the journal "PLoS ONE".

The bottom of the deep sea is dreary and lifeless for long stretches - a desert, covered with water miles high. But there are also oases there: they form around so-called cold seeps, cold springs where water carries up dissolved elements from the seabed. There, specialized microbes can convert methane and sulphate from seawater to hydrogen sulphide. This benefits worms and mussels, which live in close symbiosis with other bacteria that use hydrogen sulfide for their growth. In their body cells, they also incorporate carbon derived from the chemical reaction of methane.

"Coexistence in organisms that have settled firmly on the cold seeps has already been well researched, " explains Peter Linke from the GEOMAR Helmholtz Center for Ocean Research Kiel, one of the two main authors of the study. "We have now been able to prove that crabs are also beneficiaries of the methane sources. In this way, we are approaching an answer to the question of which organisms benefit from the cold seeps: migratory mixed-breeders apparently also belong to it. "

Stone crab caught while grazing

During dives with the ALVIN dive boat and the underwater robot QUEST off the coast of Costa Rica, in the summer of 2005, the researchers observed stone crabs that were abrading bacterial mats at a methane source near the mud volcano "Mound 12". "As far as we know, deep-sea crabs have been discovered once when eating on bacterial mats, " explains Linke. "Our team was the first to succeed in producing photographic documentation over a longer period of time, allowing for scientific interpretations."

For this, a deep-sea observatory was equipped with a camera and placed over a bacterial mat. About 400 hours, the camera automatically triggered every 30 minutes. "Crabs were visible on 184 images that crawled across the bacterial layer and apparently rubbed off the bacterial lawn, " says Niemann, describing the observations. "After the 'grazing' it took a few hours each, until the animals returned. This allowed new bacteria to regrow. "Display

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With the dive boat ALVIN, the marine biologists brought one of the crabs aboard their research vessel ATLANTIS. For comparison with the bacteria, the diving robot QUEST pulled short sediment cores from the seafloor on board the METEOR. DNA and isotope analysis at the Max Planck Institute for Marine Microbiology showed that the cancers actually feed on the bacterial mat and absorb large quantities of chemically produced carbon.

"However, we also found traces of carbon in the body cells, which were formed under the influence of light through photosynthesis, " summarizes lead author Niemann the results. Therefore, we assume that cold seeps make an important, but not the only, contribution to the feeding of migrating animals at the bottom of the deep sea, and in this way carbon, which is obtained by methane chemosynthesis, into the marine food web is received. (PloS ONE; 2013; doi: 10.1371 / journal.pone.0074894)

(GEOMAR, 08.10.2013 - NPO)