Deep-sea Microbes Get Unexpected Energy Boost

For many years, the deep ocean has been seen as a nutrient-poor environment where microbes living in the water survive on very limited resources. But new research from the University of Southern Denmark challenges that idea.

A study led by SDU-biologists at the Department of Biology shows that nutrients might not be so sparse after all in the deep and that microbes have access to a hitherto unknown source of dissolved organic food.

The study shows that sinking organic particles—known as marine snow—begin to leak dissolved carbon and nitrogen when they reach depths of 2–6 kilometres, presenting microbes in the surrounding seawater with nutrients. The leakage is caused by the intense hydrostatic pressure in the deep ocean.

Like a giant juicer

“The pressure acts almost like a giant juicer,” says first author of the study, biologist and Associate Professor Peter Stief, “It squeezes dissolved organic compounds out of the particles, and microbes can use them immediately.”

The research team has published their findings in the article “Hydrostatic pressure induces strong leakage of dissolved organic matter from ‘marine  snow’ particles”, which has been published in the journal “Science Advances”. 

According to the study, the sinking particles may leak up to 50% of their initial carbon and 58-63% of their initial nitrogen.

How today’s oil and gas was created

The discovery also has implications for the global carbon cycle. When particles lose such large fractions of their carbon before reaching the seabed, less carbon is buried in deep-sea sediments than previously thought.

Instead, the dissolved carbon remains in the deep ocean water column, where it may stay for hundreds to thousands of years before returning toward the surface ocean and the atmosphere. In the sediment, however, carbon will be buried for millions of years, and over time, it will accumulate to huge amounts. The oil and gas we are currently extracting was largely created in this way.

“This process affects how much carbon the ocean can store and for how long,” says Peter Stief, “It’s relevant for understanding climate processes and for improving future models.”

Special built pressure tank

In nature, sinking particles are tiny clumps of different kinds of organic material: Dead algae, microbes and other organic particles floating in the water. When they lump together and sink down the water column they can look like a snowflake, so often they are called marine snow.

For this study, the researchers created their own marine snow in the laboratory from diatoms. Diatoms are small microalgae that naturally clump together and form marine snow in the ocean. To test how much carbon and nitrogen they would leak under pressure, the researchers placed them in specially designed pressure tanks in their lab. Since the tanks were constantly rotating, the sinking particles were kept in suspension and did not settle at the bottom of the tank.

The researchers observed that up to half of a particle’s carbon content can leak out during its descent. The leaked material consists mainly of proteins and carbohydrates, which are easily utilised by microbes living freely in the deep-water column.

Next step is an expedition to the Arctic

Their tests showed that microbes respond rapidly. Within two days, bacterial abundance increased 30-fold, and respiration rates rose sharply. This indicates that the leaked organic matter serves as a fast and valuable energy source at great depth.

The team found the same leakage behaviour across multiple diatom species, suggesting that the mechanism is widespread in the ocean.

The next step is to search for evidence of the observed process directly in the ocean – more specifically to search for matching molecular fingerprints in surface and deep waters. The researchers hope to be able to do this in an upcoming expedition to the Arctic with the German research vessel Polarstern.

Science Advanced: Hydrostatic pressure induces strong leakage of dissolved organic matter from “marine snow” particles. Peter Stief, Jutta Niggemann, Margot Bligh, Hagen Buck-Wiese, Urban Wünsch, Michael Steinke, Jan-Hendrik Hehemann og Ronnie N. Glud.

This work was supported by the Danish National Research Foundation, the European Union’s Horizon 2020 Research and Innovation and the Independent Research Fund Denmark.