From Vienna to the Atlantic Ocean: Tracing the deep sea's missing carbon

We are currently sailing in the Atlantic Ocean, towards the Azores. Our journey began in Reykjavik on the 1st of August and after stops in the Norwegian, Greenland and Labrador Seas, we were expected to end in the Azores at the end of the month. But nature sometimes likes to remind us that it always has the final say: Hurricane Erin forced us to reroute earlier than planned, sending us towards the Azores ahead of schedule. But even here, in the middle of the Atlantic, as we evade the upcoming storm, we continue collecting samples.

Carbon in the dark

At the ocean’s sunlit surface, photosynthetic organisms, mostly phytoplankton, capture carbon dioxide and convert it into organic matter, forming the base of the food web. But sunlight fades quickly with depth, and below 200 meters the ocean becomes a world of permanent darkness. Surprisingly, scientists now know that far from being barren, the deep sea is teeming with life.

How is this possible? The prevailing explanation has been "marine snow": a slow rain of organic particles made of remains of living beings, decaying plankton, excretions, and microbes that sink from the surface. This process, which results in the removal of carbon dioxide from the atmosphere and subsequent storage in the deep, is called the biological carbon pump. It is critical for life in the oceans and for regulating the world’s climate, removing about one-third of the CO₂ humans emit into the atmosphere. 

But here lies the mystery. The amount of "marine snow" measured sinking into the abyss is not enough to sustain the abundance of life we now know exists there. Something is missing.

Gerhard Herndl and his team from the Functional and Evolutionary Ecology department at the University of Vienna recently identified a new clue: a type of non-living organic particle that does not sink at all but instead lingers in deep waters in surprisingly stable concentrations. "These particles had never been accounted for in models of the deep ocean carbon budget," explains Herndl. "We don’t yet know where they come from, how long they persist, or how microbes use them. But they could be the hidden source that feeds life in the dark."

Work at sea

To test this hypothesis, the team spends long days collecting and processing water samples from multiple depths, from the surface down to 4000 meters, using several instruments deployed from the ship.

The CTD (Conductivity, Temperature, Salinity) is one of the most important instruments on board: a large frame carrying sensors for salinity, temperature, and depth, along with bottles that can be triggered to close at specific depths. As it descends, it records a detailed profile of those parameters throughout the water column.

When the CTD surfaces, the deck comes alive. Each scientist collects the volumes required for their experiments from the bottles, measuring microbial respiration, analysing DNA and proteins, or studying microbial communities. Every task is different, but together they form pieces of a larger puzzle:  how carbon moves and transforms in the deep ocean.

By measuring microbial respiration and enzymatic activity, researchers can assess how active microbes are on and around marine snow particles, and which compounds they consume. DNA and protein analyses reveal which species are present, how they interact with marine snow, and which metabolic pathways they use. Together, these approaches provide a clearer picture of microbial–particle interactions and their influence on the ocean’s carbon cycle. 

The work doesn’t end there. Other instruments may be deployed, like in-situ pumps and deep-sea incubators. Afterwards, samples must be processed, which can mean hours of filtering water, pipetting and measuring. At the end of the cruise, many will be shipped to Vienna for further analysis.

One day aboard

A "typical" day starts early with the CTD cast, followed by long shifts in the lab or on deck that often stretch late into the night. As the science communicator onboard, I follow this rhythm too, helping where I can, but mostly documenting the work to share with others. Yet life here is more than science. With 25 people living onboard, trust and camaraderie are essential. When time allows, we watch sunsets together, stargaze, look for whales and seabirds, and play games. There’s always a special bond that forms when you spend this much time together on a 66-meter floating house, with nothing but the North Atlantic as our neighbour. And after a long day’s work, few things compare to watching the sun sink below the horizon, the sky alive with colours, with the ocean all around, together with my shipmates.

Out here, I miss family, friends, trees, long walks, running, and even Aperol spritz on my balcony. But for a month, I happily trade them for sea-friends, shared purpose, and the rare chance to wake up in the middle of the Atlantic with science unfolding around me. It feels like a privilege to share this unique world with those on land.

Science under uncertainty

Life at sea is an exercise in resilience and adaptability. Storms, equipment malfunctions, or unexpected findings can change the day’s plans in minutes. The looming threat of Hurricane Erin reminded us of that vividly. With forecasts predicting waves up to 16 meters, the captain and chief scientist decided to cut short operations in the Labrador Sea and head south to safer waters one week ahead of schedule.

Yet even setbacks can be turned into opportunity. Arriving earlier than expected in the Azores means the team can now sample a unique meeting point of subtropical and temperate waters, a natural crossroads that may offer fresh insights into the interactions between marine snow and microbes.

Why this matters

The ocean is Earth’s largest carbon reservoir and our strongest natural ally against climate change. But it is also vulnerable. Rising temperatures and acidification threaten to disrupt the balance of carbon cycling.

Understanding how carbon moves and transforms in the deep sea will help us better forecast our planet’s climate.

As our ship "Pelagia" steams toward the Azores, researchers spend long hours filtering seawater, logging data, and storing samples that may hold answers to one of the ocean’s great mysteries. Out here, surrounded by nothing but water and sky, it is impossible not to feel both the vastness of the unknown and the urgency of the research.