An article published in the journal “Science Advances” reports an abundance of ammonia-oxidizing archaea in microbial communities in sediments beneath the seafloor of the North Atlantic Ocean. A team of researchers led by William Orsi of the Ludwig-Maximilians-Universitaet (LMU) in Munich, Germany, discovered that archaea not only survived for millions of years in sediments that can reach over two kilometers below the ocean floor but adapted to those conditions better than bacteria and could play an important role in the geochemical cycles of carbon and nitrogen in that ecosystem.
Archaea, once called archaebacteria, are single-celled organisms lacking a nucleus in many ways still little-known. The situation is constantly improving thanks to advances in genetic techniques that allow to study their DNA and in techniques that improve the possibility of taking biological samples for analysis in environments where it might not be easy to work. It can be difficult to access the ocean floor to take samples, performing that kind of work beneath those floors is even more difficult. To carry out this research, a special 30 meter long drill was used to take sediment cores (Photo courtesy Christopher Griner. All rights reserved) beneath the seafloor at a depth of about 5,500 meters in an area of the Sargasso Sea, part of the North Atlantic Ocean.
Sedimentary cores represent 15 million years of history of those ocean floors and the researchers extracted DNA fragments as far as they found them. In this way they were able to identify at least the main groups to which the microorganisms belonged whose remains were present in those cores. The result was surprising because the researchers found that archaea and in particular of the phylum Thaumarchaea, of the ammonia-oxidizing type, were far more abundant than bacteria.
The conditions beneath the seafloor are not easy but in that environment there are microorganisms with metabolisms of various types, generally different from the ones that live in the water and metabolize organic carbon consuming oxygen. Near the seafloor there’s very little organic carbon available but this means that oxygen isn’t consumed and is available, even if in limited amounts, for microorganisms living in sediments.
The research showed that Thaumarchaea have a very efficient metabolic system that allows them to survive in sediments beneath the ocean floor using protein fragments from cells of dead organisms that sink. They also use the ammonia produced by protein degradation as the basis for another cycle of chemical reactions that provides them with energy. All this suggests that Thaumarchaea serve as the basal level of a food chain.
This research saw the participation of various scientists from the Deep Life Community, part of the Deep Carbon Observatory (DCO), a program that aims to study the biological carbon cycle in the depths of the Earth. There’s still a lot to discover about archaea and in general about the organisms that live underground, both on mainland and beneath the seafloor, and it’s not just a matter of scientific curiosity because they can be important in larger ecosystems. Understanding the processes underway in these ecosystems is essential to heal them after the damage caused by human activities.
