Unlocking the Secrets of the Deep Ocean’s Carbon Engine

Why should you care about what happens six miles beneath the ocean’s surface?

It is easy to view the "Hadal zone," those jagged, lightless trenches that plunge from 6,000 to 11,000 meters, as a remote, alien world. However, these trenches are the ocean’s ultimate recycling centers. They act as "hotspots" for carbon processing, where organic matter, often called "marine snow," settles and is consumed by specialized microorganisms.

Understanding how these microbes function is not just a matter of scientific curiosity; it is essential for accurately modeling the Earth’s carbon cycle. The ocean absorbs nearly a quarter of the CO₂ produced by human activity, and much of that carbon eventually sinks. If we do not understand the rate at which carbon is processed or sequestered in the deep, we cannot fully understand the ocean’s role in regulating our global climate. To get to the bottom of this mystery, researchers must go to the bottom of the world—or at least bring the conditions of the bottom into the lab.

As Urban Wünsch, Assistant Professor at Chalmers University of Technology’s thematic area Ocean and guest researcher at the Danish National Research Foundation’s HADAL Center, explains, the ocean is not a uniform environment. The "average" ocean floor, the abyssal plain, sits at roughly 4,000 meters. But about 1% of the seabed goes much deeper, all the way to 11,000 meters.

"It’s incredibly difficult to sample this environment," Wünsch says, reflecting on the logistical nightmare of deep-sea research. "When you do sample, you have to deal with the fact that you're bringing up microorganisms from the deep to the surface, which already has an impact."

Because deep-sea expeditions are prohibitively expensive and technically cumbersome, the HADAL Center team complements field campaigns with a "simplified system" in the lab. The goal was to conduct high-throughput experiments with different cultures and strains of microorganisms under varying conditions of depth and temperature.

In the lab in Odense, Denmark, the team utilizes specialized pressure tanks to replicate the "marine snow" sinking process. By slowly pressurizing samples, they can simulate a particle of organic matter as it sinks through the water column. This allows researchers to observe the "mechanistic" side of the ocean, measuring what happens as the hydrostatic pressure mounts.

In a new study, the HADAL team designed a miniature pressure tank with windows that allowed them to see what happens under pressure from the safe confines of the laboratory just outside the tank. The focus of this research, as detailed in Limnology and Oceanography: Methods (Papadimitraki et al., 2022)ⁱⁱⁱ, is Extracellular Enzymatic Activity (EEA). Microbes are the primary drivers of organic matter degradation in the ocean. However, they cannot "eat" large polymers like proteins or carbohydrates directly. Instead, they secrete enzymes, biological "scissors," into the surrounding water to snip these large molecules into smaller, transportable pieces.

"We wanted to have a real-time monitoring system in the lab that would allow us to do high-throughput experiments... under different conditions," Wünsch explains. “A high-pressure cuvette system with optical windows allows us to achieve that”.

Quantifying these microscopic reactions inside a thick-walled high-pressure cell (HPC) is an optical challenge of the highest order. The standard method for measuring enzyme activity involves using "fluorogenic substrates," molecules that change their fluorescence once they have been "snipped" by a specific enzyme. By measuring the increase in fluorescence over time, scientists can calculate the rate of enzyme activity.

However, the deep-sea environment and the specific assays used, often involving compounds like 4-methylumbelliferone (MUB) and 7-amino-4-methylcoumarin (AMC), introduce a major source of error: the Inner Filter Effect (IFE).

The IFE occurs when the concentration of the sample is high enough that the sample itself absorbs either the incoming "excitation" light or the outgoing "emitted" light. This results in a measured fluorescence signal that is significantly lower than the actual activity occurring in the sample. In a high-pressure cell, where the light must already pass through thick synthetic sapphire windows and a small sample volume, this distortion can render the data useless.

This is where HORIBA’s resources proved instrumental. To accomplish this research, Wünsch’s team turned to the HORIBA Aqualog (now the Aqualog-NEXT) spectrofluorometer.

"We know the Aqualog really, really well," Wünsch says. "It’s a familiar instrument, a workhorse of our research."

The Aqualog is unique because it utilizes A-TEEM technology (Absorbance-Transmittance and Fluorescence Excitation-Emission Matrices). Unlike a standard fluorometer that only measures light emitted from the sample, the Aqualog captures both fluorescence and absorbance simultaneously.

"This is the main advantage," Wünsch points out. "In order to push things a little, the Aqualog would be quite instrumental because it measures absorbance and fluorescence at the same time... these assays work with very, very fluorescent compounds that also absorb a lot. The instrument allows us to collect both at the same time, which, when we combine that with high-pressure measurements, was the key point."

By integrating a custom-built high-pressure cell (HPC) into the Aqualog’s sample chamber, the researchers could monitor enzyme kinetics under pressures equivalent to those found 10 kilometers deep. Because the Aqualog provided real-time absorbance data alongside the fluorescence matrices, the team could mathematically correct for the Inner Filter Effect instantly. Without this simultaneous measurement, the researchers would have been "blind" to the optical distortions caused by the pressure and the sample concentration, potentially leading to inaccurate conclusions about carbon turnover in the trenches.

The results of these studies, as seen in the Science Advances paper, reveal that Hadal trenches are far from biological deserts. In fact, they are hotspots of organic matter processing. Even though these trenches make up a small fraction of the total ocean surface area, they process a disproportionate amount of carbon.

Wünsch emphasizes that understanding the mechanisms of this processing, how enzymes are either inhibited or stimulated by pressure, is the only way to build reliable global models. "We need to understand how they work at depth, at those conditions," he says. "People have found that when you actually do incubate at the pressure that these microorganisms are under, sometimes you have half the turnover rate... that’s why it’s really important to have systems that can accurately quantify processes."

By proving that many enzymes are actually "piezophilic" (pressure-loving) or have adapted to remain stable under extreme stress, the research shows that the deep ocean is far more active than previously thought.

For the non-scientist, the takeaway is clear: the deep ocean is not a static tomb where carbon goes to be forgotten. It is a dynamic, high-pressure engine that plays a vital role in the health of our planet. Thanks to the investigative rigor of scientists like those at the HADAL center and the application of advanced spectroscopy, we are finally beginning to understand the true pulse of the abyss.

As Wünsch concludes, "This was based on cultures that were isolated... replicating the conditions that we would find in the deep sea. The more mechanistic studies, I would say, those can happen in the lab."

And with the right tools, the lab can now reach the deepest points on Earth.^iv

ⁱ (1) Stief, P.; Elvert, M.; Glud, R. N. Respiration by “Marine Snow” at High Hydrostatic Pressure: Insights from Continuous Oxygen Measurements in a Rotating Pressure Tank. Limnol Oceanogr 2021, 66 (7), 2797–2809. https://doi.org/10.1002/lno.11791.

ⁱⁱ Stief et al. (2026)

ⁱⁱⁱ “A high-pressure system for the measurement of extracellular enzymaticactivity in the deep sea" (Papadimitraki et al., 2022): This paper details the engineering and validation of the high-pressure system used in conjunction with the HORIBA Aqualog. It provides the protocol for A-TEEM-based Inner Filter Effect correction in deep-sea research.

^iv "High-pressure adaptation of microbial communities in the deepest ocean trenches" (He et al., 2022): This study in Science Advances explores the biological implications of pressure on microbial communities, proving that Hadal microbes have unique adaptations that allow them to thrive where other life forms would fail.