Microbes are alive and well in the deepest ocean

Posted by Thomas Anderson

An international research team has announced the first scientific results from one of the most inaccessible places on Earth.

Yes, this is the bottom of the Mariana Trench located nearly 11 kilometers below sea level in the western Pacific, which makes it the deepest site on Earth.

According to the team, a highly active bacteria community is thriving in the sediment of the trench - even though the environment is under extreme pressure at almost 1,100 times higher than at sea level.

To be sure, the trench sediments house almost 10 times more bacteria than in the sediments of the surrounding abyssal plain at much shallower water depth of 5-6 km water.

"Deep sea trenches act as hot spots for microbial activity because they receive an unusually high flux of organic matter, made up of dead animals, algae and other microbes, sourced from the surrounding much shallower sea-bottom," explained Professor Ronnie Glud from Nordic Center for Earth Evolution at the University of Southern Denmark.

"It is likely that some of this material becomes dislodged from the shallower depths during earthquakes, which are common in the area. So, even though deep sea trenches like the Mariana Trench only amount to about two percent of the World Ocean area, they have a relatively larger impact on marine carbon balance - and thus on the global carbon cycle."

As Glud notes, one of the team's primary research methods was to measure the distribution of oxygen in the trench sediments - and how it relates to the activity of microbes in the sediments.

While it is technically and logistically challenging to perform such measurements at great depths, such analysis is necessary in order to calculate 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 at these extreme conditions will die, due to the changes in temperature and pressure," said Guld.

"Therefore, we have developed instruments that can autonomously perform preprogrammed measuring routines directly on the seabed at the extreme pressure of the Marianas Trench."

Indeed, the research team has, together with various companies, designed the underwater robot which stands almost 4 m tall and weighs 600 kg. The robot is fitted with ultrathin sensors that are gently 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," Glud confirmed.

"Rather, we find a world dominated by microbes that are adapted to function effectively at conditions highly inhospitable to most higher organisms."

The original expedition of the Mariana Trench took place in 2010. Since then, the research team has sent their underwater robot to the bottom of the Japan Trench which is approximately 9 km deep, 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.

"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 which 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 how the general conditions are at extreme depths, but also the specific conditions for each particular trench – that may experience very different deposition regimes. This will contribute to our general understanding of Earth and its development," Glud concluded.