In the deepest ocean site on earth, nearly 11 kilometers below sea level, an international team of researchers has found thriving bacteria communities existing in the sediment of one of the planet’s most inaccessible places.
At the bottom of the Mariana Trench in the western Pacific, the scientists led by Professor Ronnie Glud from the University of Southern Denmark have discovered that the sediment there houses almost 10 times more bacteria than the sediments of the surrounding abyssal plain at a much shallower depth of 6 km. This is in spite of the extreme pressure (almost 1,100 times greater than sea level) being exerted on the sunless environment.
The results published today by Nature Geoscience are the first scientific results to be analysed from such extreme locations. The team, which explores the deepest parts of the world’s oceans, includes researchers from Denmark, Germany, Japan and SAMS in the UK.
Deep sea trenches act as hot spots for microbial activity because they receive an unusually high flux of organic matter, made up of remnants of dead animals, algae and other microbes. This organic matter comes from the overlying sunlit waters and the much shallower surrounding seafloor from where it is thought it is shaken loose during earthquakes, which are common in the area. Currents may also transport extra sediment down the trench slopes. “We measured the distribution of a naturally occurring substance called lead-210 to determine how much sediment is transported down the trench slopes in addition to the material that settles straight from the overlying waters, and it turns out the slope transport may in fact double the amount of sediment reaching the bottom of the trench“, says Dr Robert Turnewitsch from SAMS. This may mean that even though deep-sea trenches, like the Mariana Trench, only amount to about two percent of the seafloor of the world ocean, they could have a relatively larger impact on marine carbon cycling than previously thought, which would affect the amount of carbon dioxide in the atmosphere and impact climate regulation.
To measure the activity of the Mariana microbes, the team determined the distribution of oxygen in the trench sediment. It is technically and logistically difficult to attain such measurements from great depths, but it is necessary for getting accurate data on rates of bacterial activity. “If we retrieve samples from the seabed to investigate them in the laboratory, many of the microorganisms that have adapted to life in these extreme conditions will die, due to the changes in temperature and pressure. Therefore, we have developed instruments that autonomously perform preprogrammed measuring routines directly on the seabed at the extreme pressure of the Mariana Trench”, says Ronnie Glud. Among the instruments on the deep sea robotic lander are ultrathin sensors that are inserted into the seabed to measure the distribution of oxygen at a high spatial resolution.
“We have also made videos from the bottom of the Mariana Trench, and they confirm that there are very few large animals at these depths. Rather, we find a world dominated by microbes that are adapted to function effectively under conditions too inhospitable for most higher organisms”, says Professor Glud.
The unmanned submersible went to the bottom of the Mariana Trench in 2010. Since then, the research team has deployed their underwater robot to and collected samples from 9km deep at the bottom of the Japan Trench and later this year they are planning a dive in the world’s second deepest trench, the 10.8 kilometers deep Kermadec-Tonga Trench near Fiji in the Pacific.
“It is crucial to explore the extremes of the global ocean as this helps us understand how organisms adapt to a broad range of environmental conditions. Most of the Earth’s organic carbon is in the sediments of the deep sea and this is why it is so important to understand how they form,” says Dr Turnewitsch.
“The deep sea trenches are some of the last remaining “white spots” on the world map. We know very little about what is going on down there or what impact the deep sea trenches have on the global carbon cycle as well as climate regulation. Furthermore, we are very interested in describing and understanding the unique bacterial communities that thrive in these exceptional environments. Data from multiple deep sea trenches will allow us to find out about general conditions at extreme depths, but also the specific conditions for each particular trench – that may have very different deposition regimes. This will contribute to our general understanding of Earth and its development”, says Ronnie Glud.
The scientific paper, “High rate of microbial carbon turnover in sediments in the deepest oceanic trench on Earth” can be read in Nature Geoscience.
University of Southern Denmark
University of Copenhagen
Max Planck Institute for Marine Microbiology
Alfred Wegener Institute
Japan Agency for Marine-Earth Science and Technology and
Scottish Association for Marine Science