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Deep-sea pressure preserves food for microbes

Experts at the University of Southern Denmark have investigated the fate of marine snow under the crushing pressures of the deep sea. The findings are the result of innovative experiments simulating the intense conditions of deep-sea trenches.

Marine snow

Marine snow consists of tiny, millimeter-sized particles that descend from the upper layers of the ocean. These particles are a combination of dead cells, fecal matter, and other organic debris. 

As the particles journey downwards, they resemble snowflakes. This organic matter is a crucial food source for a multitude of microbes that dwell on the ocean floor.

“Photosynthetic biomass production by phytoplankton is a major source of organic particles in the euphotic zone of the ocean. These particles sink to depth as dense aggregations of microalgal and prokaryotic cells or repackaged into zooplankton fecal pellets, collectively referred to as marine snow,” wrote the study authors. 

Increasing pressure 

“Not much is known about how marine snow responds to the increasing pressure when it sinks. But it is known that marine snow is food for an enormous amount of microbes and small animals on the seabed,” explained study lead author Peter Stief.

“In fact, there are more microbes in the part of the ocean that lies at or below 1000 meters depth than anywhere else on Earth. This habitat is extremely large, and there can be a long distance between the microbes down there, but nevertheless a huge number of Earth’s organisms thrive under high pressure, and we don’t know how.”

Series of experiments 

The research team conducted a series of experiments that subjected marine snow to pressures up to 1000 bar – equivalent to ocean depths of 10 kilometers.

In a controlled laboratory setting, the team created flakes of marine snow from diatoms and bacteria and placed them in pressure tanks filled with seawater. 

The tanks, designed to withstand extreme pressures, were kept in continuous horizontal rotation to mimic the perpetual descent of flakes through the ocean. Over four days, the pressure was incrementally increased to simulate the flakes’ journey to greater depths.

What the researchers learned 

Upon examining the flakes at various pressures, the researchers discovered a significant reduction in bacterial respiration with increased pressure, indicating less consumption of organic carbon. 

At 600 bar, bacterial respiration ceased entirely, yet at 1000 bar, about half of the marine snow flake remained, serving as potential nourishment for the uniquely adapted seabed microbes.

Study implications 

The implications of this research extend beyond biological interest. Stief remarked on the geological significance of marine snow. 

“Perhaps only 1% of the marine snow gets stored on the seabed. But over time, it accumulates to huge amounts. The oil and gas we are currently extracting is largely created in this way,” said Stief.

Furthermore, the team’s research sheds light on the mechanisms by which organic matter reaches the deep sea, not only through sediment slides, which can rapidly transport nutrient-rich sediment to deep trenches, but also through the preservation of marine snow under high pressure conditions.

The study offers a profound understanding of the deep-sea ecosystem’s functioning and the role of pressure in the preservation of organic matter. It also provides a glimpse into the vast and often overlooked cycle of carbon in our planet’s oceans, with far-reaching implications for our understanding of carbon sequestration and deep-sea life.

The study is published in the journal Communications Earth & Environment

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