Iron is an important micronutrient without which phytoplankton cannot grow. Pursuing a unique approach, a team from the Alfred Wegener Institute has now, for the first time, demonstrated experimentally that Antarctic phytoplankton can utilise iron from glacial meltwater. By contrast, however, the microscopic algae were unable to utilise iron released from groundwater into seawater. This contradicts previous assumptions, which were not experimentally proven, that iron from groundwater is also bioavailable, as the team has now published in the journal Communications Earth and Environment.
Natural sources of iron such as glacier water, shelf sediments and groundwater contribute to fertilising large phytoplankton blooms occurring in the region around South Georgia (an island in the Atlantic sector of the Southern Ocean) and playing a significant role in CO2 uptake. Global warming is causing glaciers to melt faster and permafrost to thaw. Until now, the bioavailability of these iron sources has only been estimated in many geochemical studies. This has hampered the ability to model biological CO₂ uptake in the Southern Ocean, which is mainly controlled by iron availability.
“For the first time, we have quantified the iron bioavailability of these sources by measuring the iron uptake rates of Antarctic phytoplankton in experiments,” reports Jasmin Stimpfle, doctoral student in the Chemical Ecology Section at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and first author of the study. “In the process, we discovered that iron bioavailability varies greatly depending on whether the source was glacier water or groundwater.” Jasmin Stimpfle reports: “We had not expected that the algae could not use the iron from groundwater. We had assumed that it would be just as readily available as we were able to demonstrate for iron from glacier water. We need to conduct further research to find an explanation for this.”
Furthermore, the researchers were able to show for the first time that not only the iron source alone, but also the chemical composition of the seawater to which the iron source is added, is decisive for the bioavailability of the iron for Antarctic phytoplankton. If dissolved organic material accumulates in seawater, for example, it binds the iron regardless of the added source (glacial meltwater or groundwater), making it unavailable to phytoplankton. Only when the seawater was free of accumulations of dissolved organic material did differences between the various iron sources become apparent: the iron from glacial water was usable, while the trace metal was not bioavailable when groundwater was added as the iron source.
The experiments were conducted during the Polarstern expedition “Island Impact” around South Georgia in November and December 2022. Pursuing a unique approach, the researchers quantified the iron bioavailability of meltwater from various glaciers and groundwater from South Georgia by determining the iron uptake rates of a typical iron-limited Antarctic phytoplankton community subsequent to adding the iron sources under investigation in experiments on board. They then linked these results to geochemical processes such as the chemical speciation of iron, as well as the redox and weathering processes at the sampling sites.
As Dr Scarlett Trimborn, AWI Senior Scientist, Ecological Chemistry, and co-author of the publication, explains: "The Southern Ocean holds tremendous potential to store or release carbon. Our study demonstrates the complexity of iron bioavailability for phytoplankton and the crucial significance of further research on this topic. Only then will we be able to better model how primary production and CO₂ uptake will change in the future. This is particularly vital in view of global warming, with glaciers melting rapidly and permafrost thawing."
Original publication:
Stimpfle, J., Koch, F., Ebner, B., Völkner, C., Zitoun, R., Sukekava, C., Sander, S., Henkel, S., Bundy, R., Ruacho, A., Kasten, S., Trimborn, S.: Glacially derived iron is more bioavailable to Antarctic phytoplankton than other sources. Communications Earth and Environment (2026). DOI:https://doi.org/10.1038/s43247-025-03092-5