In the cold darkness four kilometres beneath the waves, particles of marine snow drift slowly downward. These fragile aggregates of dead plankton, faecal matter and other organic debris have long been viewed as the ocean's main conveyor of carbon to the seafloor. Yet a careful experiment at the University of Southern Denmark has revealed a different story.
Using rotating pressure tanks that mimic conditions at abyssal and hadal depths, researchers created marine snow from diatoms and subjected it to the hydrostatic forces found between 2,000 and 6,000 metres down. The results were striking. Up to 50 percent of the carbon and between 58 and 63 percent of the nitrogen leaked out as dissolved organic compounds.
Pressure as a giant juicer
The pressure acts almost like a giant juicer, said Peter Stief, associate professor at the University of Southern Denmark and first author of the study. It squeezes dissolved organic compounds out of the particles, and microbes can use them immediately.
The leaked material proved highly reactive. Within two days of exposure, bacterial abundance increased thirty-fold and respiration rates rose sharply. The microbes did not need to wait for the particles to reach the sediment. Instead, the dissolved carbon and nitrogen became available directly in the water column.
The mechanism implies that less carbon reaches sediments for long-term burial than previously assumed, with more dissolved carbon remaining in deep waters for hundreds to thousands of years.
This finding refines our picture of the deep ocean. Earlier models assumed most particulate organic matter was either consumed by attached bacteria or buried for millions of years. The new work shows a significant fraction is instead converted into dissolved organic carbon with a residence time measured in centuries to millennia before it returns toward the surface.
A tradition of careful observation
The experiment stands in a long Western tradition of curiosity-driven science that prizes precise measurement over sweeping pronouncements. Peter Stief and his colleagues, including Jutta Niggemann, Margot Bligh, Hagen Buck-Wiese, Urban Wünsch, Michael Steinke, Jan-Hendrik Hehemann and Ronnie N. Glud, designed their pressure tanks to keep particles suspended while replicating the crushing forces of the deep. Their paper, titled Hydrostatic pressure induces strong leakage of dissolved organic matter from marine snow particles, appeared in Science Advances in February 2026.
