Phytoplankton-elevator to the deep sea

May 28, 2018 New phenomenon discovered under the Arctic sea ice

Tiny gypsum needles can make algae so heavy that they sink in a hurry, transporting large amounts of carbon into the deep sea along the way. Researchers at the Alfred Wegener Institute have now observed this phenomenon in the Arctic for the first time. This massive removal of algae could cause large amounts of nutrients to be lost to surface water in the future.

When sea algae die, they usually float in slow motion. During an Arctic expedition with the research icebreaker "Polarstern" in spring 2015, scientists from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) discovered a phenomenon that significantly accelerates this transport: tiny gypsum needles During the freezing of salt in the pore spaces of the Arctic sea ice, the algae pull down like heavy ballast within a few hours. As in an elevator, the carbon is transported away. "Until now, this mechanism was completely unknown," says marine bio-geologist Dr. med. Jutta Wollenburg, who discovered the deposits of algae weighted with gypsum needles on the seabed. Now, together with an international team of researchers, she has published an article about it in the journal Scientific Reports. "The rapid export of algae could have several consequences for the material flows and productivity of the Arctic, the extent of which we cannot yet estimate."

During photosynthesis, algae absorb carbon dioxide like terrestrial plants in order to build up high-energy sugar compounds. Algae remove carbon dioxide from the atmosphere. When the algae die, they sink. But only a part actually reaches the seabed. Because the vast majority of algae masses are reused immediately in the near-surface area of the sea: bacteria decompose the algae and thus release the nutrients contained in them and also the carbon dioxide. Therefore, only a fraction of the algae mass reaches the deep sea. The gypsum needles, however, seem to pull the clumps of algae down so quickly that there is no time left for the degradation. This allows more algae mass to get into the depths.

However, if the gypsum needles pull the algae down before the bacteria can process them, nutrients such as nitrate may be lost from the upper water layers. This can change the food web in the sea. Because the nutrients are in short supply for the growth of algae, which feed on small crabs, which in turn are food for fish. "On the other hand, more food gets into the deep sea through the gypsum transport, which is usually rather low in food," says Jutta Wollenburg. "We've seen before that changing food intake affects the Arctic communities in depth."

The newly observed phenomenon raises many questions. Jutta Wollenburg had become aware of this when she launched a so-called multicorer (MUC) on board the research vessel Polarstern. With this device, sediment samples are taken from the seabed. In addition, the apparatus is equipped with a video camera. "During the trip down to the bottom, we saw constantly thick algae flakes falling down quickly, and there were plenty of these lumps around the bottom of the sea." Which made Jutta Wollenburg startled: Such a density of clumps of algae under the closed ice cover in all water depths to the seabed had never been described by scientists before. With the Multicorer she brought algae lumps on board. Under the microscope, she saw that there were countless centimetre-long crystal needles between the algae. Back in Bremerhaven, colleagues examined the material and found that it was clearly plaster. Gypsum consists of calcium and sulphate minerals, which accumulate during freezing processes in the pore spaces of sea ice.

"In the meantime, we know that the needles form in the sea ice at low temperatures," explains the AWI sea ice physicist. Christian Katlein. "When the ice starts to melt in the spring, the gypsum needles are released in large quantities."

This is remarkable for Jutta Wollenburg, because with climate change, meanwhile, only one year of sea ice is melting ever earlier. In the future, gypsum needles will be released more frequently in times of plankton bloom. In addition, the sea ice is getting brittle and thus more translucent. As a result, under the ice, there are increasingly extensive algae blooms. "In the future, both phenomena could come together more frequently - the algal blooms and the release of gypsum needles," says the AWI ocean-ice ecologist. Ilka Peeken. "This could lead to large amounts of algae mass." This could have consequences for life in the Arctic waters: "It could be that in the long run in these regions, the nutrient concentration in the upper water layers decreases, which in the long run also affects the number of Fish and ultimately to fishing, "says Jutta Wollenburg.

Link to the study: https://www.nature.com/articles/s41598-018-26016-0