Scientists are returning from the Atacama Trench in the East Pacific Ocean - Blogpost 9 from RV SONNE, April 2, 2018

From March 2^nd till April 2^nd, an international team of researchers, led by Professor Ronnie N. Glud, University of Southern Denmark and Dr. Frank Wenzhöfer, Max Planck Institute for Marine Microbiology in Bremen, were on a 32-day cruise on the German research vessel Sonne to the Atacama Trench in the eastern Pacific Ocean. During their cruise they collected samples from the water column and sediments from the shoulder and the bottom of the 8066 meters deep-sea trench. They covered a transect of 450 kilometers length by sampling 10 sites. Scientists from 15 different nations with a broad expertise in different scientific fields joined forces to elucidate the biology, biogeography and transport processes of this uncharted ecosystem. Now they report some of their findings.

Biological hot spots

Ronnie Glud and Frank Wenzhöfer both agree that the expedition was a great success. Their goal is to understand the trench system in its whole. The scientists collected large amounts of samples from the seafloor and the water column. They charted the seafloor with bathymetry and recorded  high-resolution videos and photos. From these data, they were able to identify fish species in several depths. Ronnie Glud says: “Looking at our results from all sites at the Atacama Trench, we find a very high biological activity in the trench. At the bottom it was much higher than on the shoulders. These trenches are indeed biological hot spots.”

Chief scientist Frank Wenzhöfer says: “On this expedition, we followed a holistic approach. To our knowledge this is the first such expedition to a deep-sea trench of this kind. Our scientific crew members are specialists on their fields: taxonomy, microbiology, molecular biology, biogeochemistry and oceanography. We wanted to find out how this deep-sea system works. Any life in this depth of more than 8000 meters has to be adapted to the high pressure and to find its niche. Our questions were: what are food sources for the communities and how is microbial cycling the elements?”

The food web of the trench: From the very small to the big sizes

The start of the food web is the action of photosynthetic organisms in the sunlit upper layer. These microscopic organisms are food for other species. As the coastal waters of Chile and Peru are influenced by supplies of cold nutrient rich water from the deep sea, the growth of the photosynthetic microorganisms should be boosted. As a consequence, the Atacama Trench should benefit from this material flow from above. The trench is assumed to contain large deposits of organic material, funneled down from trench slopes and overlying waters. In order to get the full picture, all processes involved have to be studied.
 
The different size scales observed by the scientists cover a range of more than 10 billion: From the microscopic scale with small virus particles (1/10 of a micrometer) to bacteria (micrometer), meiofauna (millimeter), macrofauna (centimeter), megafauna (meter) to transport processes across thousands of meters. On each of the 10 different sites in and close to the Atacama trench, they collected samples and measured the oxygen profiles in the water column and in the sediments. Oxygen is a very powerful agent used by microbes to gain energy from organic matter. The more active the microbes in the sediments, the earlier the oxygen is used up. This is demonstrated by a steep decline of the oxygen profiles.

Glud explains: “When comparing different sites at 8000 meters, the oxygen penetration depth and other parameters showed an unexpected high variation. That indicates a variation in biological activity and local variations in the delivery of organic matter. We speculate that two main processes are responsible for this pattern: It can either be a local variation in the biomass production in the upper part of the water column, or material transported from the shoulders of the trench by seismic activity.”