The hadal trenches are some of the least explored places on Earth. Most of what’s going on down there remains a mystery, but we’re on the brink of unlocking it, as a series of ambitious expeditions are planned.
In Greek mythology, Hades is the realm of the dead in the underworld. A
dark, lifeless place where man after death is forced to lead a miserable
existence in an eternally joyless shadow life. The first researchers who
ventured down into the world of the deep sea also reckoned it had to be a
dark and lifeless place.
Therefore, all of the globe’s oceanic depths between 6,000-11,000 metres
came to be described as ‘the hadal zone’. At the end of the day, what could
one expect to find down there, besides a dark, desolate wasteland?
As it turned out, quite a lot. The hadal zone is anything but as lifeless
as the Greek realm of the dead. On the contrary, the place is teeming with
life – even at the very bottom of the hadal trenches, which are up to
11,000 metres deep.
Discovering new species
– It is more the rule than the exception that we discover new species on
our major expeditions, says Ronnie N. Glud, Professor of Biogeochemistry
and Head of HADAL, the new Danish centre for basic research.
On his expeditions to hadal trenches with such exotic names as the Mariana
Trench, the Atacama Trench and the Kermadec Trench, he has repeatedly
encountered a myriad of life at the bottom of the trenches. For example,
despite the eternal darkness and crushing pressure, both sea cucumbers and
supergiant amphipoda live here, but these animals aren’t the ones that
preoccupy Ronnie N. Glud the most:
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– It is, of course, fascinating to see these bizarre supergiant deep-sea
amphipoda. However, the most abundant organisms, and those which truly
matter down here, are bacteria and viruses. There are huge amounts of them,
and their activity plays a crucial role in the conditions required for life
on Earth.
– For example, the microbial decomposition of organic matter in the
deep-sea sediments is of great significance for the oxygen and carbon
dioxide levels on Earth.
But how is it that such a desolate and dark environment, seemingly isolated
from everything, can sustain so much life?
How is this abundance of life even possible?
One of the secrets as to why the microbes are thriving is the relatively
large amount of organic matter available to them. A hadal trench acts a bit
like a funnel that accumulates most of the matter at the greatest depths,
such as sinking dead algae, plants and animals.
This process is strongly stimulated by seismic activity such as
earthquakes, where the mud slopes and all of their life forms subside and
end up at the bottom of the trenches.
– Another important factor is that the increasing pressure inhibits the
activity of the bacteria that originally colonised the organic matter at
the surface. Therefore, the matter reaches the depths relatively
undigested, and the specialists of the trenches, adapted to the crushing
pressure, can help themselves to the special of the day, explains Ronnie N.
Glud.
It’s teeming with viruses
Bacteria are thus found in huge numbers in the deep-sea sediments, but
greatest abundance of life forms is actually viruses.
– Viruses are by far the most numerous life form in the whole world, and we
now know this also applies in the deep sea, says Mathias Middelboe,
Professor of Marine Microbiology at the University of Copenhagen. He is
also affiliated with the HADAL Center and has joined Ronnie N. Glud on
several deep-sea expeditions.
Hadal trenches
Most hadal trenches are located in the Pacific Ocean and are the result of subduction; the convergence of tectonic plates. These trenches can be thousands of kilometres long and 20-60 kilometres wide.
The deepest place on Earth is the Challenger Deep in the Mariana Trench, which is almost 11,000 metres deep.
More people have visited the Moon than the deepest trenches.
The deep-sea viruses and bacteria are fighting an endless battle to sponge
on each other: On the one hand, viruses kill huge amounts of bacteria; on
the other hand, they also ‘help’ the bacteria thrive. This is partly
because they contribute to the organic matter being recycled when the
virus-infected bacterial cells die. And partly because they add important
genetic matter to the bacteria.
The bacteria thus gain from being invaded by viruses – but only to a
certain extent, and they fight back and develop defence strategies if their
community is threatened.
Viruses drive evolution
– Surprisingly, viruses have been shown to be extremely effective in
supporting the success of bacteria. They can spread genes from one host to
another and thus drive the evolution taking place in the trenches, says
Mathias Middelboe.
When a virus ingests a host – in the deep sea, typically a bacterium – it
carries genes. This may be a gene that the virus has taken from a previous
host, which is then passed on to the new host.
– Maybe the genes are valuable to the new host, or maybe they aren’t – in
which case they aren’t used for anything. But they remain present and can
be activated if it turns out the host can benefit from it. It’s selection
and evolution, he explains.
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This has been going on for billions of years, and therefore we should
expect to find an enormous number of genes in the deep-sea bacteria that
have been spread and shared by viruses, he believes.
As an example, he mentions the discovery of a gene that a virus has passed
on in a bacterium which he and his colleagues found in a deep-sea sediment.
This gene is also found in cholera bacteria on the surface of the Earth.
– It is also found in sewage bacteria as well as in many other types of
bacteria, which goes to show how far and wide viruses are able to spread
genes. It may have different functions in different bacteria, so it doesn’t
automatically follow that all bacteria with this gene pass on cholera to
us. It is utilised differently from bacterium to bacterium, he elaborates
and adds:
– Approx. 80% of the genes we find in viruses encode functions that we do
not yet know, so there is a great deal of elucidation ahead in describing
and mapping this gigantic pool of unknown genetic information.
Nuclear disaster evident in hadal trench
–
As it turns out, the hadal trenches are neither as void of life as
previously thought; nor are they completely isolated and inaccessible. In
fact, there are quite efficient and fast transport routes between the
surface and the deep sea. These take place via subsidence, oceanic currents
and seismic activity.
Therefore, it is not only algae, dead animals and plants that sink to and
accumulate at the bottom of the hadal trenches. So do environmentally
hazardous substances.
In 2011, Japan was shaken by a mega earthquake and the ensuing tsunami
flooded the cooling systems of the Fukushima nuclear power plant. Three
reactors melted down, triggering the biggest nuclear disaster since
Chernobyl.
Four months later, radioactive substances that could only originate from
Fukushima were detected in sediment samples from the Japan Trench.
– It shows us that tsunamis can efficiently transport large amounts of
matter from land and the coastal zone out to the open sea, where it
accumulates in the hadal trenches, among other places. Video footage also
showed that many aftershocks kept sending dead animals and other organic
matter down to the bottom of the trench, says Ronnie N. Glud.
Mercury and PCBs in crustaceans
Environmentally hazardous substances such as PCBs and heavy metals such as
mercury are also finding their way.
– Samples from hadal crustaceans contain much higher concentrations of
environmentally hazardous substances than one would expect, he says.
What this means is still too early to answer:
– We do not know. Maybe there are organisms down there that are good at
metabolising the substances. Perhaps the substances are incorporated into
organisms and thus form part of the hadal food web – but they will likely
never get out of the trench again.
He, Mathias Middelboe and their research colleagues in HADAL will search
for the answers in the sediments of the abyss.
Life must be examined in the ocean depths
For obvious reasons, it’s an extremely difficult and costly affair to put a
human or two in a deep-sea submersible and lower them to a depth of 10,000
metres. It’s certainly not harmless, either. A cheaper and less dangerous
alternative is to send robots with equipment and cameras into the abyss.
They can then perform experiments and measurements down there and retrieve
sediment samples for the scientists on the mother ship.
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– It is difficult to retrieve living organisms to the surface. For example,
due to the extreme hydrostatic pressure, the metabolism of bacteria will
change, so the challenge for us has been to find ways of conducting
scientific studies in their natural environment – and this means directly
on the abyssal seabeds, Ronnie N. Glud explains.
The team behind the research centre has also developed pressure chambers.
When used in the lab, these can mimic the extreme abyssal hydrostatic
pressure and thus investigate how various life processes are affected by
pressure.
What’s in it for us?
– Although we are constantly learning more about the conditions in the
hadal zones, we still know very little and basically we have no idea of
what we are going to find down there, says Mathias Middelboe, alluding
especially to viruses and the genes they carry.
– We will certainly find microorganisms that have adapted to the special
environment. These can make us wiser about how viruses and bacteria
interact under these conditions. Perhaps some of the properties we find in
the microbes will prove to be fit for industrial purposes, such as enzymes
that can operate efficiently at low temperatures and high pressure, he
says.
In the past, scientific explorations have led to beneficial spin-offs.
Without human curiosity and the urge to explore, we would have neither
electricity, penicillin nor washing powder today. No Velcro, no internet,
no Teflon pans.
– It would be fantastic to find something, such as genetic matter that can
be used in the pharmaceutical industry, for example, but it isn’t that
prospect which drives us initially, says Mathias Middelboe.
What do we basically know about life?
Ronnie N. Glud, too, is rather driven by curiosity and the fascination of
exploring life on the verge of the impossible. It’s more about conducting
basic research that can contribute to man’s understanding of himself and
the world.
– If we don’t know how our planet works, then what do we basically know
about life or ourselves, Ronnie N. Glud asks rhetorically and continues:
– 71 per cent of the Earth is covered by water, of which the vast majority
is more than 2,000 metres deep. We live on 29 per cent of the Earth’s
surface and may think we have a very good impression of how our planet
looks and works. But there is so much we do not know, and fundamentally, it
is important to understand how the chemistry and life of the oceans
interact to understand the planet we see today.
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The next opportunity to explore life on the verge of the impossible will be
in the autumn of 2021, when Glud and Middelboe, together with an
international research team, set course for the 9,780-metre deep Izu-Bonin
Trench in the western Pacific, a day’s sailing from Tokyo.
Ronnie N. Glud is a deep-sea researcher and Professor of Biochemistry at the University of Southern Denmark and the Tokyo University of Marine Science and Technology. He heads the Danish Center for Hadal Research.
Mathias Middelboe is a deep-sea researcher and Professor of Marine Microbiology at the University of Copenhagen and the University of Southern Denmark.