Deep ocean pressure may feed microbes by squeezing marine snow

Researchers at the University of Southern Denmark say extreme pressure in the deep ocean can force nutrients out of sinking organic particles, giving microbes an unexpected food supply. The finding could change estimates of how much carbon reaches the seafloor and how long the ocean stores it, according to the university.

The work, published in Science Advances, focuses on marine snow: small clumps of dead algae, microbes and other organic material that fall through seawater. The study found that as those particles sink to depths of about 2 to 6 kilometers, hydrostatic pressure can drive dissolved carbon and nitrogen out of them and into the surrounding water.

Pressure releases usable nutrients

Peter Stief, a biologist and associate professor at the Nordcee research center and the Danish Center for Hadal Research, said the pressure works like a press that pushes dissolved organic compounds from the particles. According to the University of Southern Denmark, microbes in deep water can use that material quickly.

The researchers estimated that sinking marine snow can shed as much as half of its original carbon before reaching the seafloor. The study also found losses of 58% to 63% of original nitrogen during descent through deep water.

According to the university, much of the released material consisted of proteins and carbohydrates. Those compounds are readily available to free-living microbes in the deep ocean, a region often described as poor in accessible nutrients.

Carbon storage estimates could shift

The results matter for the ocean’s biological carbon pump, the process by which carbon fixed near the surface moves downward through sinking organic matter. Scientists have expected a portion of that carbon to end up buried in deep-sea sediments, where it can remain stored over very long periods, according to the University of Southern Denmark.

If pressure causes substantial leakage during the descent, less particle-bound carbon may reach the seabed than models assume. The university said dissolved carbon left in deep waters can persist there for hundreds or thousands of years before gradually returning to surface waters and, eventually, the atmosphere.

Carbon that is buried in seafloor sediments can be stored for millions of years, the university said. Stief said the process affects both the amount of carbon the ocean can hold and the length of that storage, making it relevant for climate research and future modeling.

Lab tests simulated a falling particle

To test the mechanism, the research team made marine snow in the laboratory from diatoms, microscopic algae that naturally form sinking clumps. The particles were placed in rotating pressure tanks designed to keep them suspended while exposing them to conditions similar to the deep ocean, according to the university.

The experiments showed substantial leakage of carbon and nitrogen under high pressure. The team also observed a rapid microbial response: within two days, bacterial abundance increased 30-fold, and respiration rates rose strongly, according to the University of Southern Denmark.

The same pattern appeared across several diatom species, which the researchers said suggests the process may occur widely in the world’s oceans. The study was authored by Peter Stief, Jutta Niggemann, Margot Bligh, Hagen Buck-Wiese, Urban Wünsch, Michael Steinke, Jan-Hendrik Hehemann and Ronnie N. Glud.

The team plans to look for molecular evidence of the process outside the laboratory during an expedition to the Arctic Ocean aboard the German research vessel Polarstern. The research was supported by the Danish National Research Foundation, the European Union’s Horizon 2020 Research and Innovation program, and the Independent Research Fund Denmark.

This story draws on original reporting from ScienceDaily.