Scientists from the US measured the relative amounts of ‘bioreactive’ iron in four sediment cores from the bottom of the Atlantic. They showed for the first time that the further dust is blown from the Sahara, the more iron in it becomes bioreactive through chemical processes in the atmosphere. These results have important implications for our understanding of the growth-promoting effect of iron on oceanic phytoplankton, terrestrial ecosystems, and carbon cycling, including under global change.

Iron is a micronutrient indispensable for life, enabling processes such as respiration, photosynthesis, and DNA synthesis. Iron availability is often a limiting resource in today’s oceans, which means that increasing the flow of iron into them can increase the amount of carbon fixed by phytoplankton, with consequences for the global climate.

Iron ends up in oceans and terrestrial ecosystems through rivers, melting glaciers, hydrothermal activity, and especially wind. But not all its chemical forms are ‘bioreactive’, that is, available for organisms to take up from their environment.

The core of the matter

Owens and colleagues measured the amounts of bioreactive and total iron in drill cores from the bottom of the Atlantic Ocean, collected by the International Ocean Discovery Program (IODP) and its earlier versions. IODP aims to improve our understanding of changing climate and oceanic conditions, geological processes, and the origin of life. The researchers selected four cores, based on their distance from the so-called Sahara-Sahel Dust Corridor. The latter ranges from Mauritania to Chad and is known to be an important source of dust-bound iron for downwind areas.

The two cores closest to this corridor were collected approximately 200km and 500km west of northwestern Mauritania, a third in the mid-Atlantic, and the fourth approximately 500km to the east of Florida. The authors studied the upper 60 to 200 meters of these cores, reflecting deposits over to the last 120,000 years – the time since the previous interglacial.

They measured the total iron concentrations along these cores, as well as concentrations of iron isotopes with a plasma-mass spectrometer. These isotope data were consistent with dust from the Sahara.

They then used a suite of chemical reactions to reveal the fractions of total iron present in the sediments in the form of iron carbonate, goethite, hematite, magnetite, and pyrite. The iron in these minerals, while not bioreactive, likely formed from more bioreactive forms through geochemical processes on the seafloor.

“Rather than focusing on the total iron content as previous studies had done, we measured iron that can dissolve easily in the ocean, and which can be accessed by marine organisms for their metabolic pathways,” said Owens.

“Only a fraction of total iron in sediment is bioavailable, but that fraction could change during transport of the iron away from its original source. We aimed to explore those relationships.”

Blowing in the wind

The results showed that the proportion of bioreactive iron was lower in the westernmost cores than in the easternmost ones. This implied that a correspondingly greater proportion of bioreactive iron had been lost from the dust and presumably been used by organisms in the water column, so that it had never reached the sediments at the bottom.

“Our results suggest that during long-distance atmospheric transport, the mineral properties of originally non-bioreactive dust-bound iron change, making it more bioreactive. This iron then gets taken up by phytoplankton, before it can reach the bottom,” said Dr Timothy Lyons, a professor at the University of California at Riverside and the study’s final author.

“We conclude that dust that reaches regions like the Amazonian basin and the Bahamas may contain iron that is particularly soluble and available to life, thanks to the great distance from North Africa, and thus a longer exposure to atmospheric chemical processes,” said Lyons.

“The transported iron seems to be stimulating biological processes much in the same way that iron fertilization can impact life in the oceans and on continents. This study is a proof of concept confirming that iron-bound dust can have a major impact on life at vast distances from its source.”

As the climate crisis alters familiar growing conditions, we urgently need to find ways of protecting the world against famines. Currently, our food system is heavily dependent on pesticides—but these pesticides grow less effective as pests develop resistance to them, have a substantial carbon footprint, and can damage biodiversity. Induced resistance, using plants’ immune systems to build up their strength and fight pests, could be critical to reducing our reliance on pesticides and developing sustainable agriculture.

Food for thought

Right now, crops are mostly protected using pesticides and breeding for resistance genes, although there is a significant risk that pests will out-evolve plants intended to resist them. Induced resistance enhances abilities a plant already has to provide more sustainable and potentially broader-spectrum protection: defending against several pathogens and pests, not just one.

Induced resistance can take several different forms—for instance, plants releasing compounds which attract herbivores’ predators—but the best known and most widespread is defense priming. Defense priming takes place when part of a plant experiences a stress, and this weakly activates defense mechanisms which then activate fully when the plant undergoes another attack. Intriguingly, this priming seems to last so long that it can appear in the next generation of plants, potentially transmitted via epigenetic mechanisms.

However, induced resistance usually doesn’t offer complete protection, so must be combined with other measures. It also needs to be carefully calibrated to ensure that it doesn’t leave a plant open to other threats and doesn’t compromise growth by causing the plant to allocate too many resources to defense.

“Induced resistance is the result of a complex network of developmental and environmental pathways in the plant,” explained Mauch-Mani. “So safe and efficient exploitation of induced resistance is not as straightforward as the introgression of a single gene or spraying a single pesticide. We will need case-by-case evaluation of the optimal growth conditions, crop germplasm, and agricultural practices to capitalize on induced resistance’s multifaceted benefits.”

Seeds of the future

Once implemented, induced resistance could do more than just ward off pests. Some of the defense compounds that plants produce in response to induced resistance are linked to health benefits or higher-quality nutrition, meaning that we could benefit not just from avoiding pesticides but from eating healthier food. Induced resistance is also faster than traditional breeding, offering a quicker way to adapt to changing climatic conditions. It’s harder for evolving pests to evade, and it has the potential to offer broad-spectrum protection.

Combined with integrated pest management that uses pests’ natural enemies as crop protection, we could use induced resistance to cut pesticides to a bare minimum, making agriculture more sustainable. We could also secure much longer-lasting crop protection, once we develop a better understanding of the epigenetic mechanisms that transmit defense priming to a new generation.

To make induced resistance part of farmers’ and food scientists’ toolkits, the researchers said we urgently need more research that covers more real-world circumstances. We also need to understand how induced resistance performs under less controlled conditions and support the development of methods that can be scaled up to field trials and then into full-scale agriculture. The researchers also called for legislative support to establish quality standards, protecting producers and consumers.

“We strongly believe that fundamental research into induced resistance will be critical for the transition towards a truly sustainable food supply,” said Mauch-Mani. “However, there is an urgent need for better communication between discovery-focused research and other stakeholders who have the expertise to translate discovery into application.

“Governments need to create a research environment and funding climate that allows for more efficient knowledge exchange between scientists, policymakers, and industry. Like the biology underpinning it, successful exploitation of induced resistance relies on a multifaceted effort.”