At a depth of 6 km: Scientists discovered a food source they did not know existed
A new study by researchers from the University of Southern Denmark reveals how microorganisms find food at depths of several kilometers. It turns out that organic particles sinking from the upper ocean layers release nutrients under immense pressure, providing a vital food source for deep-sea bacteria.
A new study by researchers from the University of Southern Denmark reveals how tiny creatures manage to find food even at a depth of several kilometers below the ocean surface. The researchers found that particles sinking from the upper layers of the sea release nutrients on their way – and these become an available food source for bacteria living in the depths. The finding may change an assumption that has been accepted for years, according to which the deeper one goes into the ocean, the amount of available food decreases significantly.
At any given moment, tiny particles containing, among other things, remains of algae and microscopic creatures sink in the oceans. These particles connect to each other and slowly descend towards the ocean floor. Scientists call this phenomenon "marine snow," because under the water the sinking particles resemble snowflakes. One can think of them as a kind of small packages of organic matter falling from the top of the ocean towards the depths. However, the new study reveals that these "packages" leak along the way.
As one goes deeper into the ocean, the water pressure increases. At a depth of about 2 to 6 kilometers, the pressure is already enormous, and the researchers discovered that it affects those same particles that sink in the water. The pressure actually causes them to release some of the substances contained within them into the surrounding water. Peter Stief, one of the leaders of the study, described it simply: "The pressure acts almost like a giant juicer. It squeezes dissolved organic compounds out of the particles, and the microbes can use them immediately." According to the study, during the sinking, the particles can lose up to half of their carbon and between 58% and 63% of their nitrogen.
The substances released from the particles include, among other things, proteins and carbohydrates. For the tiny bacteria living in the ocean depths, this is a food source they can utilize quickly. In an experiment conducted by the researchers, the result was particularly striking: within just two days, the amount of bacteria increased 30-fold. Their activity also increased sharply.
In other words, instead of imagining the ocean depths as an environment where there is almost no food, the study points to a relatively simple mechanism: organic matter arrives from above, the pressure in the depths "squeezes" nutrients out of it – and the surrounding bacteria utilize them.
To test the process, the researchers created in the laboratory particles similar to "marine snow," using microscopic algae. They placed them in special containers where it is possible to create pressure similar to that which exists at a depth of several kilometers in the ocean. Thus, they managed to measure how much material is released from the particles as the pressure increases. The researchers repeated the tests with several types of algae and received similar results. Therefore, they estimate that the phenomenon may be common and not limited to a specific type of "marine snow."
The study does not only deal with the question of what bacteria eat in the deep sea. It may also be important for understanding the Earth's climate. The oceans are capable of absorbing and storing huge amounts of carbon. Part of this carbon is inside those same particles that sink to the depths. Until today, the assumption was that a significant part of the carbon could reach the ocean floor with the particles and be buried there. When carbon is buried in sediments on the floor, it can remain there for very long periods.
However, if a large part of it is released along the way, as the study shows, less carbon may reach the floor and be buried there. Instead, it remains in the deep waters, where it can stay for hundreds and even thousands of years before it gradually returns to the upper layers of the ocean – and subsequently to the atmosphere as well. Stief explained: "This process affects the amount of carbon the ocean can store and for how long. It is relevant for understanding climate processes and improving future models."
However, the study has an important limitation: the experiments were done under conditions created in the laboratory. Therefore, the next step for the researchers will be to go out into the field and check if the same process occurs in the ocean itself. They plan to go on a research expedition to the Arctic region on board the German research vessel Polarstern. During the journey, they will look for signs in the water that could indicate that the substances are indeed released from the particles at depth as observed in the laboratory.
If the process is verified in nature as well, the significance could be broad: it is possible that scientists will need to update the way they understand both the food chain in the ocean depths and the amount of carbon that reaches the floor and remains there for the long term. In simple words, the discovery points to a process that has not received enough attention until now: tiny particles fall into the sea depths, the enormous pressure "squeezes" food out of them, and the bacteria living in the darkness enjoy the meal along the way.
