Hadal trenches continue to surprise researchers

The hadal trenches are the deepest places on Earth, and on the face of it, you would think life would struggle to exist at such dark and cold depths, which in many places reach more than 8 km below sea level.

However, there are special organisms that can withstand the extreme pressure, and now SDU researchers report that a certain type of bacteria even thrives particularly well.

The research team is led by Professor Bo Thamdrup, who researches the biogeochemical processes that take place in the ocean and in the hadal trenches.

Life reemerges in the depths

- These bacteria are attracted to the extreme environment of the hadal trenches because they can live off something down there, and we’re talking about outright hotspots brimming with activity, Bo Thamdrup explains.

We all know life is abundant in the oceans – at least in the top few hundred metres where there is enough sunlight. Below this depth, marine life decreases with increasing distance to the life-giving light, and intuitively one envisages the hadal trenches as wastelands devoid of life.

- But they are not. In recent years, research has shown that a lot of nutrients reach the hadal trenches. This attracts a number of creatures, especially microorganisms, which are specialists in living down there, he says.

The global nitrogen cycle

And now he and colleagues can report that this also applies to special bacteria that are attracted to nitrogen; the so-called anammox bacteria.

‘Anammox’ is an abbreviation for ANaerobic AMMonimun OXidation, which is an important microbial process in the planet’s nitrogen cycle.

The bacteria that provide for this process (the anammox bacteria) were first discovered in 1999, and it soon became clear to researchers that these bacteria play an extremely important role in the planet’s biogeochemical life.

The algae are of great importance

- Now we can see that not only are they highly active in the hadal trenches; they perform the same function down there which we otherwise usually see much higher up in overlying water, says Bo Thamdrup.

Broadly speaking, the function is that the anammox bacteria ‘eat’ nitrogen compounds in the form of ammonium and nitrite and convert these into nitrogen gas, which is the main component in the atmosphere. Ammonium and nitrite are bioavailable nitrogen compounds that can be used as nutrients (‘fertilizer’) by the algae, among others, in the oceans.

Algae, on the other hand, cannot utilise nitrogen gas. In other words, the anammox bacteria, through their lifestyle, remove nutrients from the ocean and thereby limit the growth of algae. As the algae are responsible for the oceans’ uptake of CO2, the anammox bacteria ultimately have an impact on the CO2 cycle. It is therefore important to understand the activity and distribution of the bacteria.

What else is going on in the hadal trenches?

As anammox bacteria are mainly known to flourish at shallower water depths, it was a big surprise to find them in abundance in the hadal trenches. Another surprise was that the anammox bacteria down there turned out to be very closely related to those found at lower water depths. Therefore, there is not one specific deep-sea variant of anammox bacteria. This suggests the bacteria have not found it difficult to adapt to the high pressure in the hadal trenches.

- We usually think of the hadal trenches as an extreme environment. It is, for instance, impossible for fish to live at such great depths. However, the results suggest that it is a different case with the anammox bacteria, which seem to be able to function across a huge range of pressure conditions, says Bo Thamdrup.

The researchers are now working on understanding the life and element cycles in the hadal trenches – especially how the pressure affects the different types of bacteria that drive the metabolism down there.