By Israh Ghobbar
Although conditions of the Mariana Trench are known to be dark and uninhabitable, research proves that is not the case. Images and extracted samples of water and sediment from the Challenger Deep have uncovered the existence of microbes and small organisms in the Mariana Trench.
The Mariana Trench is located in the western Pacific Ocean about 200 km east of the Northern Mariana Islands and Guam, measuring a length of 2550 km and a width of 69 km. This trench is a deep and curving fissure in the Earth’s surface, where the denser Pacific Plate subducts (forced under) the less dense Mariana Plate into the Earth’s mantle. Friction between the two plates causes the melting of the Pacific Plate, which triggers earthquakes and the rising of magma through cracks, where it flows onto the surface, forming a strato-volcano. At this destructive plate boundary, the relief is the 11034 meter Mariana Trench.
Formation of the Mariana Trench through the process of subduction. (credit: Pacific Ring of Fire 2004 Expedition, Noaa Office of Ocean Exploration; Dr. Bob Embley, Noaa Pmel, Chief Scientist/ National Geographic)
Half of the Mariana Trench seabed lies in the abyssal plain, which is about 4000-6000 meters high between a continental rise and a mid-ocean ridge, where there is limited bacterial activity. The Challenger Deep is the deepest point of the Mariana Trench, named after the British Research Vessel, Challenger II, which mapped it in 1951. The Sirena Deep is a neighbouring part of the trench, approximately 230 km from the Challenger Deep with a depth of 10676 meters. This location is known for being seismically active, which means that there are more active volcanoes and recurring earthquakes in the region. Due to ocean currents, this region is likely to be more nutrient-rich than other portions of the trench.
Expeditions in 2010 and 2012 led by National Geographic Explorers, James Cameron and Kevin Hand (who is also a NASA astrobiologist), and Professor Ronnie Glud of the University of South Denmark investigated whether bacterial communities can function effectively under high pressures. The other main objective was to understand the carbon cycle and how much CO2 the ocean fixes to determine the amount of oxygen on Earth.
The Mariana Trench is in perpetual darkness with extremely cold temperatures, similar to underwater caves. The seascape shows sparse signs of life and is dominated by drifting beige sediments. Rocks formed in the mantle thrust upwards by the moving crust, which produce an outcrop protruding through the sediments. As water is denser than air, going deep underwater leads to an increase in pressure being exerted on the object, which is the seabed of the Mariana Trench. The pressure can be more than a 1000 times greater than the pressure experienced by the Earth’s surface.
The 2012 Deepsea Challenge expedition used a robotic lander that is 4 meters tall and weighs 600 kg, which was constructed with designed gauges to withstand extreme pressures. After a 3 hour long descent, the lander reached the seabed, where it took images with a video camera using lights to illuminate the environment. Water and sediment samples were collected using an autonomous coring device and oxygen contents were measured using highly sensitive sensors as an indication of bacterial activity. This is to count the number of bacteria and observe the volume and age of the organic matter. Sampling was carried out 4 times successfully and the instruments were developed to perform pre-programmed measuring routines directly on the seabed. This is because microorganisms adapted to high pressure and low temperatures will die if brought to the surface.
Green and fuzzy mats affixed to the rock outcrops, dead organisms, and debris drifting to the ocean floor feed off chemicals produced when the seabed reacts with water. These mats have a complex structure of many layers of bacteria. Microbes are able to survive, due to their adapted cell membranes and enzymes and small size, meaning they are unaffected by extreme pressures. These microbes use chemosynthesis to survive, meaning carbon-containing molecules are converted into organic matter through the oxidation of inorganic compounds as a source of energy, instead of sunlight. This is due to the depth of the Mariana trench, where sunlight is not able to reach the organisms for photosynthesis. Serpentinization occurs when iron- and magnesium-rich rocks meet sea water. The reaction generates heat and compounds including methane and hydrogen, which can fuel microbial metabolisms. This is proven by the visible alterations to the rock and chemistry of the seawater. Therefore, the oxygen content is an indicator of bacterial activity, as bacteria use oxygen to convert the organic matter, meaning the less oxygen present, the more active the bacteria. These microbes could be the base of a food chain for amphipods (small crustaceans) e.g. Hirondellea gigas. However, more bacteria was present deeper in the Challenger Deep than the Sirena Deep because the trench captures sediments, collecting nutrients that land in shallower locations and are dislodged by earthquakes.
Bacterial structures found on a rock outcrop in the Sirena Deep during the 2012 DEEPSEA CHALLENGE expedition to the Mariana Trench. (credit: Kevin Peter Hand/ National Geographic)
This discovery is significant because it shows the potential habitability within oceans on other planets, including Jupiter’s moon, Europa and Saturn’s moon, Enceladus. This research is important for the assessment of bacteria’s contribution to the global carbon cycle and how this regulates our climate. Scientists are informed on how to develop instruments to explore these depths, promoting further research into the Kermadec-Tonga Trench near Fiji.
Drake, N. (2020, May 13). Possible microbes in the Mariana Trench hint at life on Jupiter's moon. Retrieved from: https://www.nationalgeographic.com/science/2020/05/possible-microbes-mariana-trench-hint-at-life-jupiters-moon-europa/
Sjøgren, K. (2013, March 21). Bacteria thrive at the bottom of the Mariana Trench. Retrieved from: https://sciencenordic.com/biology-denmark-earth/bacteria-thrive-at-the-bottom-of-the-mariana-trench/1384009
Stromberg, J. (2013, March 17). Nearly 8 Miles Down, Bacteria Thrive in the Oceans' Deepest Trench. Retrieved from: https://www.smithsonianmag.com/science-nature/nearly-8-miles-down-bacteria-thrive-in-the-oceans-deepest-trench-3298109/
Glud, R. & Wenzhöfer, F. & Middelboe, M. & Oguri, K. & Turnewitsch, R. & Canfield, D. & Kitazato, H. (2013, March 17). High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth. Retrieved from: https://www.nature.com/articles/ngeo1773
Max-Planck-Institut für Marine Mikrobiologie (2013). Retrieved from: https://www.mpi-bremen.de/en/Microbes-in-the-Mariana-Trench.html