The benefits of adding more iron to the oceans — known as iron fertilisation — would be limited, at least on a global scale, according to a new study published in the 18 February 2020 issue of the Proceedings of the National Academy of Sciences (1). In other words, there is ‘just enough’ iron in the ocean.

Oceans play a vital role in sequestering carbon dioxide (CO2) from the atmosphere. This happens in two ways: 1) diffusion from the atmosphere when the concentration of CO2 in the atmosphere is less than the concentration in the ocean; 2) algae and phytoplankton absorb carbon dioxide through photosynthesis.

Since iron is a key ingredient that promotes phytoplankton growth, this has led scientists to propose a form of geoengineering known as iron fertilisation: seeding the world’s oceans with more iron to grow more phytoplankton and stimulate algae blooms, which would, in theory, absorb more CO2 from the atmosphere.

The idea was first explored back in the 1990s. But now, scientists are rethinking the idea and carefully considering the potential implications on ecosystems. And it seems the oceans might already be performing their own sophisticated balancing act.

Most of the iron used by phytoplankton comes from dust that settles onto the ocean’s surface. But this iron is relatively short-lived since it quickly sinks deep into the ocean, eventually settling on the seafloor where it is no longer useful.

“The fundamental problem is, marine microbes require iron to grow, but iron doesn’t hang around. Its concentration in the ocean is so minuscule that it’s a treasured resource,” said explained lead author Dr Jonathan Lauderdale of Massachusettes Institute of Technology (MIT) in a statement.

To overcome this, phytoplankton naturally produce compounds that bind to iron to keep it in the surface ocean. Scientists have known for a while that these so-called ‘ligands’ — which typically come from organic waste products, dead cells, or siderophores — exist but they still don’t know much about them. The researchers from MIT’s Department of Earth, Atmospheric and Planetary Sciences wanted to find out more about the role these important molecules might play in regulating the ocean’s capacity to promote phytoplankton growth — and of course, more CO2 absorption.

The researchers used computer simulations to study the interactions between phytoplankton, iron, and other nutrients in the ocean – and at the same time, the ocean’s capacity to store CO2, and discovered that finely-tuned interactions lead to a delicate balance of nutrients in the ocean.

This ‘reinforcing cycle’ locally matches the availability of iron and other nutrients in the ocean and the required stocks of dissolved iron are maintained by association with the ligands produced by biological processes.

Simply put, the world’s oceans seem to have just the right amount of ligands, which maintain the right amount of available iron to maximize the growth of phytoplankton, that optimally consume macronutrients – which creates a “self-reinforcing and self-sustainable balance of resources”.

And this is true, even when huge regional differences in iron content exist: for example, some oceans, like the North Atlantic, are high in iron because they are close to dusty continents, but low in macronutrients; whereas, the Southern Ocean has a relatively high concentration of macronutrients but low iron content.

“According to our framework, iron fertilization cannot have a significant overall effect on the amount of carbon in the ocean because the total amount of iron that microbes need is already just right,” Lauderdale said. Plus, introducing more iron could have unintended effects, he added: “We have to consider the whole ocean as this interconnected system”.

“Something like 75 per cent of production north of the Southern Ocean is fueled by nutrients from the Southern Ocean, and the northern oceans are where most fisheries are and where many ecosystem benefits for people occur. Before we dump loads of iron and drawdown nutrients in the Southern Ocean, we should consider unintended consequences downstream that potentially make the environmental situation a lot worse”.

Another recent study showed that, up to now, the percentage of CO2 uptake by oceans is stable. In other words, the more CO2 humans produce, the more the oceans absorb. But at some point, will the oceans become saturated?

Moreover, differences in CO2 uptake among regions were also documented, suggesting complex feedback systems, just like the one described by Lauderdale and colleagues, are likely at play. So, instead of finding ways to bandage the existing problem with complex geoengineering solutions, such as iron fertilisation, the most logical solution seems to be reducing CO2 emissions.

(1) Lauderdale, J.M. et al. Microbial feedbacks optimize ocean iron availability. Proceedings of the National Academy of Sciences (2020). DOI: 10.1073/pnas.1917277117

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