The strange world of the ocean deep

3 Apr 2023

Life exists even miles under the sea, where microorganisms create peculiar ecosystems. Geobiologist William Orsi explores this fascinating wilderness in the remote depths, a place where time has another meaning. A portrait from the magazine EINSICHTEN.

Vast muddy plains cover millions of square kilometers of the deep seafloor. Extreme conditions prevail far below the surface of oceans, at depths of up to 6,000 meters. Low temperatures, absolute darkness, and immense pressures present huge challenges for the existence of life. While the uppermost layer of this soft mud remains in continuous contact with water, go down further and a great, largely unknown wilderness begins. Researchers such as William Orsi call this mysterious place the deep biosphere. And if you thought it was lifeless, you would be wrong. By analyzing drill cores of deep sea sediments, researchers have discovered single-cell organisms: bacteria, archaea, fungi. Viruses have also been found. If anything can carve out a meager existence down there in the thick mud layers and even in the rock below, then it is microorganisms like these, which are among the most flexible and diversified creatures on Earth.

Search for Life

Drilling into the sediments of the deep-sea trench off Puerto Rico, at around 8,000 meters the deepest marine region in the Atlantic Ocean.

© William Orsi / LMU

What nature asks of us

Read the answers in the new issue of our research magazine EINSICHTEN at | © LMU

In the ocean underworld, it pays to be frugal. “The cells of the bacterial species in the deep-sea sediments are relatively small,” explains William Orsi, Professor of Geosciences, Palaeontology, and Geobiology at LMU. “Moreover, the metabolism of these organisms is ten thousand times slower than that of their counterparts on the surface.” Both these things save energy – important qualities given that overlying waters in the open ocean are very unproductive and any energy-rich organic compounds that could serve as food are at an absolute premium below this blue desert. And so the organisms in the sea floor lead a very slow life, close to the energy limit to life.

It is an existence in extreme slow motion. Whereas bacteria, for example, normally divide at a daily or hourly rate, the fastest species in the deep ocean split every few months. Some of them need years, or possibly even centuries, and may end up having a Biblical lifespan – at least in a certain sense. “The components of the cells completely renew themselves piece by piece over time – such that they’re effectively no longer the same individual they once were.” Can we call this aging? This habitat, says Orsi, stretches the concept of what the true limits to life are.

Precious commodity from drill cores

The geobiologist wants to investigate the laws and patterns at play here. “When we bring mud samples to the surface, we can draw conclusions about the metabolic rates of the organisms we find in them,” explains Orsi. “You have to be careful, though, because bacteria often grow very quickly once you take them out of the deep sea mud, and into this new environment.” Under the extreme conditions at the bottom of the ocean, things are different. Therefore, researchers also use biochemical markers in the sediment layers as indicators – traces of the life and death of its inhabitants. “When analyzing the drill cores, you generally get a curve, with the number of living cells falling off sharply the deeper the sediment layer,” says Orsi.

The farther you penetrate into the layers, and the age of the sediment increases, the rarer life becomes. “At some point, the death rate in a population exceeds the growth rate – and for some microbial life they may be existing on the very border of death, so to speak, for a very long time.”

The remoteness of the habitat poses a serious challenge for researchers. Without excellent cooperation between international teams, it would be impossible to drill these technically difficult and expensive deep-sea bores. “In terms of complexity, the individual drilling operations are comparable with those carried out by the oil industry – albeit on a grander scale – when searching for new deposits,” says Orsi. Samples from the precious drill cores are therefore distributed to researchers around the world, who grapple with many unanswered questions about the biotic communities that survive there in the darkness.

It is still largely unknown, for example, how the cooperation between different species works, as deep-sea mud is very fine-pored. This isolates the organisms beneath the surface not only from the world above, but also from each other. “Indeed, the current consensus is that individual organisms in deep sea mud tend to move very little,” says Orsi.

Mud from the deep:

In his laboratory, William Orsi studies the organisms in the sediments.

© Oliver Jung / LMU

No place for biodiversity

Nevertheless, bacteria, archaea, fungi, and viruses probably form a complex and connected ecosystem. “There are indications of syntrophy between various bacteria and archaea,” notes Orsi. This means that one species can feed on what the other produces. Meanwhile, viruses lurk as so-called prophages in the genome of the single-cell organisms and generally pounce when their hosts are stressed. “The death of the host cells benefits other individuals, who make use of the cell components released during virus-induced cell lysis.” In view of the extreme shortage of food, therefore, viruses presumably play an important ecological role. Fungi also utilize such released organic matter, breaking it down into inorganic materials.

People expecting abundant species diversity in these depths, however, will be disappointed. Barren and often permeated with oxygen – occasionally all the way down to the Earth’s crust beneath – the sediments below the ocean floor, such as the vast plains under the North Atlantic, do not yield much. “The biodiversity in deep-sea sediments is much lower than in sea floors in more shallow waters such as the North Sea or in soil on dry land.”

Furthermore, the biotic communities living below the ocean floor are anything but rich in individuals. The contrast is sharp with areas near the coast, where the sea is richer in nutrients and therefore more productive because of the influence of the land and the deposition of more organic sediments. This in turn leads to less oxygen in the ocean floor. Under these nutrient-rich conditions, we see a comparatively thriving biotic community at the seafloor surface with an anaerobic microbial community developing a few centimeters below.

On the track of survivalists from the deep sea

"When we bring samples to the surface, we can draw conclusions about the metabolism of the organisms found in them," says William Orsi.

© Oliver Jung / LMU

Life at the limit

But proximity to land is not the only factor that shapes communities. “We expect to see differences in composition and diversity in different regions and latitudes. This becomes apparent when you compare drill cores from, let’s say, the ocean before Alaska with cores from the Caribbean.” There is also evidence to suggest that water depth influences the microbial community of the ocean floor. “I’m afraid our knowledge is rather scanty here. To remedy this deficiency, we’re planning to drill several holes at a deep-ocean slope at depths of between 3,000 and 8,000 meters,” explains Orsi.

How deep into the seabed must we drill down before life becomes impossible? The search for this limit, reports Orsi, is not yet complete. To date, living cells have been found more than two kilometers beneath the ocean floor. “But only in very low concentrations – and we have now definitively reached the detection limit of our current methods. All the same, I don’t believe that we’ve yet reached the true limit of life. Rather, only the detection limit to life.” At some depth, of course, the heat from the Earth’s interior increases to the point that microorganisms would begin to boil. Then the jig is most certainly up, even for the most tenacious survivalists.

But what brought them to these darkest depths in the first place? Most of the species way down in the sediment have are also found at the seafloor surface, and are even found in the overlying water. Presumably some organisms endured being pushed down meter by meter by the pressure of the sediment floating down upon them over the course of millions of years. “We think that better adapted individuals survived to great depths, whereas most others that get trapped in this harsh environment died.”

This resulted in very stable ecosystems, which have survived for millions of years of time. William Orsi has discovered, for example, that the biotic communities in the oxygen-permeated red mud seafloor in the middle of the North Atlantic have been dominated for up to 15 million years by single celled microbes called Archaea. These Archaea have a unique strategy to survive under these extreme conditions, they obtain their energy by oxidizing ammonia.

As isolated from the rest of the world as these microorganisms are, will they remain untouched in the long run by all the environmental changes that are presently altering the land and the oceans, the planet as a whole? “I believe to a large extent, they will,” says Orsi. “In sediments located in shallower regions of the ocean, we do see the effects of past climatic changes – such as those of the ice ages – on the microorganism communities.“ This is not the case in deep-sea sediments: “This ecosystem and its inhabitants appear to have not changed for millions of years, and as such don’t respond to shorter-term climatic fluctuations.”

So will this habitat, this world of extremes, actually remain the last untouched wilderness in the future? Possibly not, conjectures Orsi: “The deep ocean is rich in raw materials such as metal ores, and industries have already expressed interest in mining manganese nodules from the sea bottom on a large scale.” Who knows what would become of the ecosystems and their inhabitants at those sites?

Text: Nicole Lamers

© Oliver Jung / LMU

Prof. Dr. William Orsi is Professor at the Department of Earth and Environmental Sciences, Palaeontology, and Geobiology at LMU. Born in 1984, Orsi studied biology at Temple University in Philadelphia. He completed a doctorate in the Biology Department at Northeastern University, Boston. Before joining LMU in 2016, he was a researcher at the Geology and Geophysics Department and the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, and at the University of Maryland Center for Environmental Science in Cambridge, Maryland.

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