Details are outlined in an international study involving the ICM-CSIC, which concludes that the organic substances keeping iron dissolved in the Southern Ocean mainly come from algae and bacteria that form plankton.
New research led by the University of Geneva and the Ecole Polytechnique Fédérale de Lausanne, with participation from the Institut de Ciències del Mar (ICM-CSIC) and the universities of Cape Town and Canberra, has revealed the mechanism by which plankton keeps iron available for its survival, preventing it from sinking with mineral particles. The details are presented in a recently published study in the journal Nature Communications, which concludes that the organic substances that keep iron dissolved in the Southern Ocean mainly originate from the algae and bacteria that form plankton.
"Iron is essential for life, including marine life, but it is highly insoluble in seawater unless bound to organic substances. The source of these substances was one of the main enigmas to solve," explains Rafel Simó, a researcher at ICM-CSIC and one of the authors of this study conducted during the Antarctic Circumnavigation Expedition, an international project led by the Swiss Polar Institute in 2017. This expedition travelled around the Southern Ocean for three months, making it the most ambitious project ever undertaken in those waters.
Anaemic oceans
Iron is an essential element for the cellular machinery of photosynthesis and, therefore, for the development of life. This is because photosynthesis first appeared on Earth around 3.5 billion years ago, at a time when iron was abundant in the sea. However, photosynthesis itself eventually oxygenated the atmosphere, oxidising iron and causing it to precipitate to the seabed. Today, iron is scarce in the surface ocean, especially in the Southern Ocean far from coastal areas, where life suffers from chronic anaemia.
What helps to keep the small amount of residual iron available to plankton are organic compounds that chemically bind to it and keep it in the water, preventing it from settling to the seabed. Most of these compounds are known as humic substances.
"Until now, we knew that most of the humic substances in the oceans came from land, being discharged into the sea by rivers and the atmosphere," explains Christel Hassler, a researcher at the École Polytechnique Fédérale de Lausanne and the lead author of the study."We have found that, in the Southern Ocean, far from rivers and exposed continental land, a significant proportion of humic substances come from plankton itself," Hassler continues.
Iron captors
To carry out the study, researchers travelled across the Southern Ocean, starting and ending in Cape Town (South Africa), stopping at the ports of Hobart (Tasmania) and Punta Arenas (Chile). For much of the journey, they sailed far from the coast of the frozen continent, except in the regions of the Ross, Amundsen, and Bellingshausen Seas in West Antarctica, where they approached the ice shelves. Chemical and electrochemical measurements of iron and its organic ligands, taken at various depths in the ocean, have allowed researchers to determine that most of the iron is bound to substances produced by plankton.
"This is very interesting," says Simó, "because we still do not fully understand what enables or limits biological activity in the Southern Ocean, to the extent that the models we use underestimate atmospheric CO₂ consumption for photosynthesis. This discovery will help us improve the models we use to make predictions."
Overall, the research provides key information about how life functions in the Southern Ocean, one of the most crucial regions for global climate regulation and, at the same time, one of the most vulnerable to climate change. "This is yet another example of how life develops strategies to survive when a fundamental resource is scarce," concludes the researcher.