GEOMAR scientists show that manganese, in addition to iron, can also limit the growth of plankton with photosynthesis
HELMHOLTZ CENTER FOR OCEAN RESEARCH KIEL (GEOMAR)
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PICTURE: The study is based on an expedition by the British research vessel RSS JAMES CLARK ROSS, which is shown here before it departs from the Falkland Islands. Show more CREDIT: THOMAS BROWNING / GEOMAR
The term plankton usually describes very small organisms that drift with the currents in the seas and oceans. Despite their small size, they play an important role for our planet due to their immense abundance. Photosynthetic plankton, called phytoplankton, for example, produce half of the oxygen in the atmosphere and bind large amounts of carbon dioxide (CO2). Since the Southern Ocean around Antarctica is very rich in nutrients, phytoplankton can thrive there. It is therefore a key region for controlling atmospheric CO2 concentrations.
Since other nutrients are abundant, scientists have previously assumed that the amount of available “micronutrient” iron determines how well phytoplankton thrive in the Southern Ocean or not. Researchers at the GEOMAR Helmholtz Center for Ocean Research in Kiel and the British National Oceanography Center have now published a study in the international journal Nature Communications which for the first time shows that in some areas of the Southern Ocean manganese and not iron is the limiting factor for phytoplankton growth.
“This is an important finding for our ability to assess future changes, but also to better understand phytoplankton in the past,” says Dr. Thomas J. Browning of GEOMAR, lead author of the study.
Earlier studies indicate that greater phytoplankton growth in the Southern Ocean over the past 2.58 million years contributed significantly to the outbreak of the Ice Ages. More phytoplankton could bind more CO2 that was removed from the atmosphere. As a result, global average temperatures continued to decline. “It is therefore important that we understand exactly which processes regulate phytoplankton growth in the Southern Ocean,” emphasizes Dr. Browning.
In fact, besides iron, manganese is another essential “micronutrient” that every photosynthetic organism needs, from algae to oak. In most of the ocean, however, the phytoplankton has enough manganese available not to restrict its growth.
In contrast, measurements in remote regions of the Southern Ocean have shown much lower concentrations of manganese. During an expedition on the British research vessel RRS JAMES CLARK ROSS through the Drake Passage between Tierra del Fuego and the Antarctic Peninsula in November 2018, Dr. Browning and his team water samples. While on board, they used these water samples and the phytoplankton it contained to conduct experiments to determine which nutrients affect growth and which do not.
“In this way we were able to prove a manganese restriction for phytoplankton growth in the center of the Drake Passage for the first time. As expected, iron was the limiting factor closer to the coast, ”reports Dr. Browning.
After the expedition, the team used additional model calculations to assess the impact of the experimental results. Among other things, they found that the manganese limitation was possibly even more widespread during the Ice Ages than it is today. “This would make this previously neglected factor a central part of understanding the Ice Ages,” says Dr. Browning.
However, since this is the first record in any particular region of the Southern Ocean, more research is needed to better understand the geographic extent and timing of the Southern Ocean’s manganese limit. “We also have to investigate which factors control the manganese concentration in seawater and how phytoplankton adapts to the manganese shortage. All of this is critical to building more accurate models of how the Earth system works, ”concludes Thomas Browning.
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From EurekAlert!
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