Subsea microbes outperform bacteria

Archaea micbrobes. Credit: David Schart/Corbis

Deep sea sediments are notoriously nutrient poor, however, microbes belonging to the Archaea group of unicellular microorganisms have been efficiently scavenging dead cells, and play an important role in the geochemical carbon and nitrogen cycles of the ecosystem, researchers have found.

Research led by Professor William Orsi of the Department of Earth and Environmental Sciences at Ludwig-Maximilians-Universitaet (LMU) in Munich, Germany, in cooperation with American colleagues belonging to the Deep Life Community (part of the Deep Carbon Observatory (DCO) have released a study on Archaea, featured in the online journal Science Advances.

The study called Archaea dominate oxic subseafloor communities over multimillion-year time scales, shows that the species of Archaea have not only managed to survive for many millions of years in sediments far below the floor of the open ocean, but that they cope with the difficult living conditions in the subseafloor biosphere far better than bacteria do.

Further analysis showed that the Thaumarchaea possess a highly efficient metabolic system, as they utilise protein fragments from dead cells to enable them to survive in sub-seafloor sediments. They also generate energy by using the ammonia produced by the degradation of proteins.

The results of the study show that Thaumarchaea convert inorganic carbon into biomass and thus serve as the basal level of the food chain in oxygen-bearing deepsea sediments. This activity has been going on for at least 15 million years, “In this ecosystem, they have been outperforming bacteria for many millions of years,” said Orsi, “and this may also hold in other ecological niches below the seafloor.”

Sediment that is deposited near continental margins in the oceans often has low levels of oxygen, this is due to organic carbon constantly being exported to the seafloor. The carbon is then quickly metabolised by microorganisms, which consume all the available oxygen. However, in the open ocean, the main source of nutrients in the seafloor is made up of dead organisms that have sunk to the seafloor. Conversely, in this situation there is less carbon available for metabolism while the oxygen in the sediments on the seafloor is preserved.

“Modelling studies suggest that between 10 and 40% of these sediments, from the seafloor down into the underlying crust, are supplied with oxygen,” says Orsi. “This is a huge region whose biosphere is as yet largely unexplored.”

To understand this habitat better, the researchers used a 30 m-long drill core to acquire samples of sub-seafloor sediments from an approximate depth of 5,500 m at a site in the Sargasso Sea, situated in the North Atlantic.

DNA fragments were extracted from sedimentary deposits and analysed, these identified that ammonia-oxidising Thaumarchaea members of the Archaea species were present more than bacteria.