Marine snow is an organic material shower that seeps into deep sea waters from the top layers of the water. 2017 Gulf of Mexico Seabed, Credit: NOAA Office of Ocean Exploration and Research
The global carbon dioxide balance is influenced by the oceans. Because billions upon billions of tiny algae inhabit the oceans, they absorb carbon dioxide through photosynthesis and incorporate it into their biomass. These algae die and are carried down with the excretionsof microscopic creatures that eat the "marine snow" to deeper areas. For thousands of years, about one percent of the carbon dioxide they produce is buried beneath the seafloor.
The quiet whisper of snow
Experts call it a biological pump because the constant rainfall of marine snowflakes transports carbon to the ocean's depths. It is driven by two processes, sinking organic flakes and bacteria's degradation. Sinking flakes increases the flux of carbon to depths while bacteria reduces this flux by removing the carbon from the particles. Current ocean models assume that sinking speed and degradation rate are independent. Uria Alcolombri, Institute of Environmental Engineering at ETH Zurich, says that sinking can actually increase degradation.
Alcolombri is the first author of a Nature Geoscience study that Roman Stocker's research team published. The researchers devised a clever technique to track sinking particles in seawater. Instead of using a satellite method, they placed individual microfluidic chambers containing millimeter-sized particles of alginate and then used artificial seawater to pump through them. "In our experiments, we found that the sea didn't move around the marine-snow particles. Alcolombri says that the relative speed of the marine snow is not different.
Video microscopy of an Alginate particle (large, light-colored circle) in artificial seawater. The water contains tiny dark green dots of marine bacteria, which can occasionally colonize the particle. Credit: Uria Alcolombri / ETH Zurich
Get rid of all by-products
Researchers colonized the alginate particles using genetically modified, green-glowing bacteria. The particles were broken down much faster when water was pumped through the chamber. However, the process takes about ten times longer if it is still standing water. The reason is that the water flows away any degradation products and allows bacteria's enzymes access to the particles directly without the need to decompose molecules that are already separated.
Alcolombri and Franois Pueaudecerf, based on these observations, have created a new model for the biological carbon pump. This model considers the impact of sinking on the degrading of marine snowflakes. Two things are apparent from the model calculations: First, the sinking effect increases the theoretical transport efficiency of a carbon pump by increasing particle degradation. Secondly, the model calculations show that most of the dead algae is found in the ocean's uppermost layers. This is consistent with real-world measurements of carbon flux in the sea.
Small things can have a huge impact
Alcolombri says that the research of the team was not intended to improve the performance of the biological carbon pumps. "We were interested in understanding natural processes fundamentally; we wanted the biology of the biological pump," he said. "This is crucial if we are to accurately predict how the oceans will react to climate change."
The microscopic transport dynamics was responsible for determining the rate at which marine snow degrades and, indirectly, the amount of global carbon dioxide in the atmosphere. This shows how even the smallest things can have a big impact on the overall picture.
Learn more Fishes produce approximately 1.65 billion tonnes of carbon each year through their feces, and other matter.
More information: Alcolombri, U., Peaudecerf, F.J., Fernandez, V.I. et al. Sinking increases the marine bacteria's ability to degrade organic particles. Nat. Geosci. (2021). Journal information: Nature Geoscience Alcolombri, U., Peaudecerf, F.J., Fernandez, V.I. et al. 2021. doi.org/10.1038/s41561-021-00817-x
