All organisms, from lowly microbes to higher forms of life (including humans), need iron. Yet there is irony to iron. Despite being the second most abundant element in the Earth, it is not readily available for consumption. Earth owes this irony to the combined effects of geodynamics and biology. The early segregation of iron into the Earth's core relegated iron to “only” the fourth most abundant element in the crust. About 2.3 billion years ago, a complex interplay between photosynthesis and redox changes in Earth's mantle allowed the buildup of free atmospheric oxygen. Today, there is a sufficient supply of photosynthetic oxygen to convert all iron at the Earth's surface and in its surface waters, including seawater, into its ferric [Fe(III)] form. This ferric form is barely soluble, making it hard to access by organisms. Ironically, life itself made iron a ‘trace element’.

Earth historians know this. So, what is so special about iron? It is how it behaves with respect to the “lever” system: small changes in abundance of iron and other trace metals – such as manganese, zinc, copper, and nickel – can amplify perturbations in biogeochemical cycles. Several articles on trace elements in the oceans in this issue of Elements expose these intriguing couplings. In times of global change, these amplifications may have a profound impact on our future.

Let's begin with the tiny marine plants called phytoplankton and their production at the ocean's surface. These plants are the base of the marine food web and, through their photosynthetic activity carry the largest burden for the partitioning of CO2 between the atmosphere and the biosphere: this is the oceans' so-called “biological pump”. Phytoplankton need iron in trace amounts for photosynthesis, respiration, and nitrogen fixation. Phytoplankton thrive best where there is enough dissolved iron in seawater, there being barely any at the surface, (<1 nanomol per litre). This low iron concentration favours phytoplankton in certain areas: for example, those fed by iron from wind-blown dust, the ocean margins, or by upwelling of iron-rich deep water.

Now consider what we humans are doing. We emit almost 10 billion tonnes of carbon per year, in addition to the 90 billion tonnes cycled before humans between the atmosphere and the oceans, and the 60 billion tonnes cycled by terrestrial biota. Fortunately, the ocean takes up some of this additional carbon, but, in so doing, sets in motion a cascade of interacting feedbacks centred on the iron cycle. A lower ocean pH, for example, may increase Fe(III) solubility, but, at the same time, locks dissolved iron into ligand-bound forms that are inaccessible to organisms. Higher global temperatures can lower the capacity of seawater to hold dissolved oxygen, meaning less of the more soluble Fe(II) is oxidised, thereby increasing irons' overall solubility. But conversely, increasing ocean stratification will reduce the upwelling flux of iron into the sunlit zone where most organisms live, resulting in less iron-fueled phytoplankton growth. By turning knobs in multiple ways on this delicate ocean biogeochemical cycle, we potentially amplify effects that, in the coming decades, have important implications for the pathway our planet's marine ecosystems and the services they supply: biodiversity, food supply, and uptake of CO2. We need to understand these amplifying feedbacks and safeguard the boundaries that define a safe operating space for humanity.

It's not only marine life. Land plants feel the irony of iron as well. Iron in plant-available forms is scarce in soils, despite its high abundance in rocks. Plants have, therefore, evolved various mechanisms to obtain iron and other metal micronutrients, often based on the reduction or chelation of these elements. While these strategies seem to suffice in most natural ecosystems, they frequently fail to supply enough iron to agricultural crops. In particular, in calcareous or saline soils, iron and zinc are only sparingly soluble and can become the two nutrients that limit food production. Compounding this, grain that is produced from these crops will also be poor in these micronutrients.

Because plants feed an increasing world population, iron availability has massive societal relevance and is rightly receiving increased attention. Under the UN Sustainable Development Goal called “Zero Hunger”, all people should have access to sufficient and nutritious food all year round, and all forms of malnutrition shall have ended by 2030. To hit these targets, one challenge will be to meet the demand for sufficient calorie-rich food, and so alleviate the malnutrition that currently affects 820 million people. A second, equally formidable, challenge is presented by so-called “hidden hunger”, whereby micronutrient deficiencies affect the health of some two billion people. Iron deficiency (anaemia) leads to a decrease in red blood cells and produces many associated health problems. The victims of hidden hunger live mostly in the developing world but are also increasing in number in the urbanised parts of the developed world.

Limiting the impact of global change and safeguarding food security are two of the largest environmental challenges we face. The geosciences, and especially geochemistry and mineralogy, have a role to play here. We can assist nutrition specialists in developing the means to supply sufficient iron and other metal micronutrients to plants. This will be crucial to satisfying the growing demand for nutritious food. In the oceans, understanding iron's role in the lever principle, and in controlling ecosystems and in the carbon cycle, is essential to future climate models and to avoiding the dangerous tipping points that small shifts in iron availability might trigger. With modern analytical, mineralogical, geochemical, and isotopic tools allied to our fundamental expertise in the complexities of Earth system, we geoscientists are well-poised to take on these challenges.

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