How trace elements affect marine CO2 sinks

Given that the Southern Ocean is rich in nutrients like nitrate and phosphate, one would expect algae to thrive there. But in most regions, there are surprisingly few phytoplankton. It has been known for some time that this poor growth is often due to a serious lack of iron, or in some cases, a lack of manganese. But until recently, no-one knew if this also applied to the southern Weddell Sea. Now, not only have experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and the University of Bremen investigated the abundance of both elements in the remote and inaccessible waters at the 77^th parallel; during the COSMUS expedition (2021), they were the first to assess how the two trace elements affected local algae communities.

What they found: In comparison to their potential level of photosynthesis, the algae grew extremely poorly and therefore transported less carbon to the seafloor than they potentially could have. This finding fits with their poor access to trace elements: “In fact, we found surprisingly low concentrations of iron and manganese,” reports first author Jenna Balaguer, whose doctoral dissertation was supervised by Scarlett Trimborn and received funding in connection with the German Research Foundation (DFG) priority programme “Antarctic Research with Comparative Investigations in Arctic Ice Areas” at the AWI and the University of Bremen. “For some species, both substances seem to be lacking, while others only need more iron.” And this, apparently, can have far-reaching consequences.

This became clear when the group began sampling seawater in the region and adding iron, manganese or both. “We soon saw that the iron wasn’t the only key factor,” says AWI researcher and study co-author Florian Koch. “It was only by combining iron and manganese that we really ramped up algal growth.” But that’s not all: Since the individual species have quite different needs when it comes to trace elements, adding iron and manganese also changed the composition of the biotic community.

This is not only relevant from an ecological standpoint; it also has far-reaching implications for our planet’s carbon budget and therefore the climate balance. After all, phytoplankton have a major influence on carbon transport in the ocean. When the tiny green algae use photosynthesis to produce energy, they not only release large quantities of oxygen; at the same time, they absorb the greenhouse-gas carbon dioxide and incorporate the carbon into their cells. When they die and sink to the ocean floor, they take this carbon with them. Instead of remaining in the atmosphere and causing temperatures to climb further, this biological pump ensures its transport to the ocean’s depths.

In this regard, the processes at work in the study’s survey area are particularly interesting. One quarter of all carbon absorbed by organisms in the Southern Ocean can be attributed to phytoplankton, which can be found south of the 55^th to the 60^th parallel in the Weddell Sea. “Accordingly, for the first time we also evaluated how the lack of iron and manganese there affects carbon export,” says Jenna Balaguer.

In fact, their experiments show that even comparatively small changes in the species composition can have surprisingly pronounced effects on this process: Depending on their size, shape and other qualities, some cells sink to the ocean floor faster and more often than others. For example, supplementing the trace elements led to intensified growth of the alga Phaeocystis antarctica. This sociable species formed larger and more carbon-rich colonies, which, together with local diatoms, sank particularly well. When the team added iron, the carbon export potential was doubled; adding a combination of iron and manganese quadrupled it.

But what does all this mean for the future of the Southern Ocean? According to the research team, it’s still extremely difficult to predict which phytoplankton species will benefit from higher CO₂ concentrations or how much more CO₂ the ocean will be able to absorb. However, what the study clearly shows is that additional iron and manganese from melting ice and sediments could dramatically boost algal growth, accelerating the biological carbon pump in the process. What the actual effects of climate change will be is something that can only be predicted with the aid of models. And these models should reflect the study’s findings, the AWI researchers conclude, since to date, they haven’t included the effects of manganese on the carbon pump in their calculations.