Hadal trenches are dynamic hotspots in the deep sea

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Sometimes I have to remind myself, why I work with sediments in the deepest parts of the ocean. It is extremely exciting, scientifically rewarding, but at times also very challenging.  I have been doing this kind of work for many years. Traditionally, scientists have recovered sediment and studied microbiology and biogeochemistry in the laboratory. But when I was a young scientist there was a growing awareness on the challenges of this approach, and that chemical and physical changes during recovery could compromise the investigations. This initiated the development on in situ instrumentations, instruments that generally are called landers. This happened just when I was doing my PhD study – in which I designed my first lander. In the beginning I studied the shallower areas, but when I got a chance to go deeper, a new world opened up! The deepest trenches are called hadal trenches after the Realm of Death in the Greek mythology, as they were thought to have little or no life! The trenches stretch from 6-11 km depths – with the Challenger Deep in the Mariana Trench being the deepest site on Earth.

The sediment lander being deployed in the Atacama Trench from the German research vessel "RV Sonne" in March 2018

Very little is known about the life or biogeochemistry in the hadal trenches or their importance for the overall function of the global ocean.

Together with many good colleagues I have now visited several hadal trenches, and we now realize that the trenches are far from being dead! Actually, they act as hot spots for early digenesis and microbial activity in the deep sea. This was a very exiting discovery and lead to a series of exciting questions, including:

What is the importance, and what are the sources, pathways, and rates of material deposition in hadal trenches?

 What is the nature of the organic material that sustain hadal life?

Who are the important microbial players in the trenches?

Do they differ among trenches and are they distinctly different than microbial communities in the rest of the ocean?

How does the detrital food chain function at these extreme settings?

How does hadal conditions and the extreme pressure shape basic microbial interactions and functions?

One major challenge when doing research at great depths, is the high hydrostatic pressure. Instruments often need to be autonomous and they need to be adapted to operate at the extreme conditions. The deployments are always nerve wracking!  Will the instrument come up to the surface, and will we be able to get it back onboard? Has everything worked and what kind of data and samples will be recovered by the instrument?

Another challenge doing this kind of work is the involved logistics of chartering large research vessels and sending researches and containers full of equipment to remote areas of the planet, and ultimately organizing shipments of samples for further on-land analysis.  

On top of these challenges - we are at the mercy of “the weather gods” to be successful!

But the novel discoveries are well worth the hassle and give important insights on life at one of the most extreme and remote part of the ocean.

https://doi/s43247-020-0087-2 

Ronnie N. Glud

professor, University of Southern Denmark

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