Micronutrients with a climate effect

To examine the phytoplankton, the scientists traveled in the Weddell Sea with the research ship Polarstern. © AWI

Green “global players” in sight: What factors influence the important carbon dioxide storage by sea algae in the Southern Ocean? A study now shows an intensive effect of the trace element supply: If the phytoplankton receive more iron and manganese at the same time, their species composition changes in an important way: The community can then bind more CO2 and form sticky clumps much more effectively, which after sinking carbon deposited on the seabed. The scientists say that ice melting caused by climate change could now increase the supply of trace elements in the Southern Ocean. Significant effects on the sea algae can therefore be expected.

Carbon dioxide is released through various processes and bound again through others: it was humans who unbalanced our planet's natural carbon cycle through the enormous release of CO2. In order to be able to assess how this will develop further and what feedback effects could arise from the consequences of global warming, it is important to understand how the system works. It is clear that the oceans play a central role in the absorption and long-term storage of CO2. Specifically, the tiny sea algae - the phytoplankton - are the key elements: They absorb CO2 dissolved in the water and, as part of photosynthesis, convert it into bound carbon, which forms the majority of their biomass. It can then decrease later and thus lead to long-term storage of carbon.

Not optimally “fertilized”

One of the world's oceans is particularly important: in the Southern Ocean, these processes take place particularly intensively due to certain factors. It is estimated that this ocean absorbs around 40 percent of the carbon dioxide that we humans release. That's why it's so important to understand which aspects play a role there. Researchers at the Alfred Wegener Institute (AWI) in Bremerhaven have been dedicating themselves to this research topic for some time. They were able to show in more detail why algae growth in the Southern Ocean is still below expectations: the organisms are richly supplied with the main nutrients nitrate and phosphate. But there is a lack of trace elements that are just as vital for the algae as certain vitamins or minerals for us.

For a long time, the focus was on the lack of the micronutrient iron. But in an earlier study, the researchers were able to show that a manganese deficiency can also critically limit phytoplankton growth. Now a team of researchers from (AWI) and the University of Bremen has examined the amounts of both elements in the waters in the southern Weddell Sea. They also explored in more detail what influence both trace metals have on the local algae communities.

Basically, it turned out: “We actually found surprisingly low concentrations of iron and manganese,” reports first author Jenna Balaguer from AWI. The researchers were then able to prove through laboratory experiments that this has a significantly negative effect on the growth and photosynthesis performance of the algae. But there were important peculiarities: “For some phytoplankters, both substances seem to be very scarce, while others only need iron.”

Critical factor manganese

What the different needs lead to was shown in further experiments in which the scientists filled seawater from the region into containers and then added either iron or manganese or both. “It was only by combining iron and manganese that we were able to really stimulate the growth of the algae,” says Florian Koch. In detail, however, it became apparent that since the individual species have such different requirements when it comes to the supply of trace elements, the composition of the algae community also changed significantly with the different additions.

This in turn was associated with a surprisingly significant effect, the team found: depending on their characteristics, some types of algae sink to the seabed more intensively than others. Specifically, the addition of both trace elements led to strong growth of the algae Phaeocystis antarctica. This species forms particularly large and carbon-rich colonies, which then clump together with diatoms and ultimately sink particularly well. If the research team enriched the water only with iron, the export potential for carbon doubled. A combination of iron and manganese, however, caused it to increase fourfold.

According to the team, the study results can now have important significance for assessing future developments. As the ice melts to be expected, significant amounts of iron and manganese could end up in the sea, thereby leading to the observed effects on an enormous scale. Therefore, the influences of the changed micronutrient supply should now be integrated into models of the future development of the carbon cycle. In this regard, however, there is also a need for further research in order to better understand the complex connections.

Source: Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: Current Biology, doi: 10.2139/ssrn.4342993

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