Feeding Oceans with Iron

Feeding Oceans with Iron All organisms need nutrients to live and grow, and those living in the open ocean are no exception. Small phytoplankton drift on the water surface, converting sunlight, water and atmospheric carbon dioxide into food and oxygen. In these small organisms, as in other photosynthetic organisms such as land plants, photosynthesis requires nutrients. On land, nitrogen and phosphorus are usually the limiting nutrients, but in many parts of the open ocean these nutrients are generally available. What is missing are other trace nutrients, and iron is one of the key players in this system. Adding small amounts of iron to the ocean surface can trigger a phytoplankton population explosion visible from space. Such additions occur naturally, for example through dust carried by winds from the Sahara Desert or ash released from a volcanic eruption. When dust or ash clouds settle on the ocean surface, they trigger large phytoplankton blooms that remove significant amounts of carbon dioxide from the atmosphere. Iron fertilization is a Carbon Dioxide Removal (CDR) technique that mimics this natural system by artificially adding iron to the ocean surface to promote phytoplankton growth.

What is Phytoplankton?

Derived from the Greek words phyto (plant) and plankton (made to circulate or drift), phytoplankton are microscopic organisms that live in both saltwater and freshwater environments. Some phytoplankton are bacteria, some are protists and most are single-celled plants. Common types include cyanobacteria, silica-shelled diatoms, dinoflagellates, green algae and chalk-coated coccolithophores. Like land plants, phytoplankton have chlorophyll to capture sunlight and use photosynthesis to convert it to chemical energy. They consume carbon dioxide and release oxygen. All phytoplankton photosynthesize, but some gain additional energy by consuming other organisms. Phytoplankton growth depends on the availability of carbon dioxide, sunlight and nutrients. Phytoplankton, like land plants, require various levels of nutrients such as nitrate, phosphate, silicate and calcium depending on the species. Some phytoplankton can fix nitrogen and grow in areas where nitrate concentrations are low. Very low iron concentrations over vast areas of the ocean limit phytoplankton growth. For this reason, phytoplankton require trace amounts of iron. Other factors such as water temperature and salinity, water depth, wind and the types of predators feeding on them affect phytoplankton growth rates. When conditions are favorable, phytoplankton populations can enter a rapid reproduction phase, a phenomenon known as a bloom. "Blooms" in the ocean can cover hundreds of square kilometers and are easily visible in satellite images. A bloom can last several weeks, but individual phytoplankton rarely live longer than a few days. The Importance of Phytoplankton: Food Web Phytoplankton form the base of the aquatic food web as primary producers, feeding everything from microscopic, animal-like zooplankton to whales weighing tonnes. Small fish and invertebrates also feed on plant-like organisms and are subsequently consumed by larger animals. However, phytoplankton can also be harbingers of disease and death. Some phytoplankton species produce powerful biotoxins, making them responsible for "red tides" or harmful algal blooms. These toxic blooms can sicken and even kill marine life and humans who consume contaminated seafood. Phytoplankton can also cause mass death in other ways. After a large bloom, dead phytoplankton sink to the ocean or lake floor. Bacteria that decompose phytoplankton consume oxygen in the water, suffocating animal life; the result is a dead zone.

Climate and the Carbon Cycle

Through photosynthesis, phytoplankton consume carbon dioxide on a scale equivalent to forests and other land plants. Some of this carbon is transported to the deep ocean when phytoplankton die, and is transferred to different layers of the ocean as phytoplankton are eaten by other creatures that themselves reproduce, produce waste and die. Although samples from the ocean are needed for some studies, satellites are critical for observing phytoplankton on a global scale and their role in climate change. Individual phytoplankton are small, but when billions of them "bloom," high concentrations of chlorophyll and other light-absorbing pigments change the way the surface reflects light. Water can appear greenish, reddish or brownish. The chalky scales covering coccolithophores color the water milk white or bright blue. Scientists use these changes in ocean color, observed in satellite images, to estimate chlorophyll concentration in the ocean and phytoplankton biomass.

Global Distribution

Phytoplankton thrive along coastlines and continental shelves, across the equator in the Pacific and Atlantic Oceans and at high latitudes. Winds play a strong role in phytoplankton distribution, as they drive currents that cause nutrient-rich deep waters to be drawn to the surface. Along the equator, sustained by the convergence of trade winds from the east, and along the western coasts of several continents, these regions are among the most productive ocean ecosystems. By contrast, phytoplankton are sparse in remote ocean gyres due to nutrient limitations.

Long-term Changes in Phytoplankton

Because phytoplankton are so important for ocean biology and climate, any change in their productivity can have a significant impact on biodiversity, fisheries and human food supply and the rate of global warming. Many ocean chemistry and biology models predict that phytoplankton productivity will decline as ocean surface temperatures rise in response to increasing atmospheric greenhouse gases. As surface waters warm, the water column becomes increasingly stratified, meaning lower productivity. In recent decades, scientists have begun searching satellite observations for this trend, and early studies show a small decline in global phytoplankton productivity. For example, ocean scientists have documented an increase in the area of subtropical ocean gyres (the least productive ocean areas) over the past decade. These nutrient-poor "marine deserts" are expanding due to rising ocean surface temperatures. Phytoplankton populations can explode and grow within days or weeks. This satellite image pair shows a bloom that formed east of New Zealand between 11 October and 25 October 2009. (Source: NASA imagery by Robert Simmon and Jesse Allen based on MODIS data.)  

Iron and Iron Oxide

It was noted at the beginning of this article that iron has critical importance for phytoplankton populations. As ancient ore deposits show, the oceans were full of iron three billion years ago. When life first evolved, iron was abundant and this metal became part of a long list of basic cellular functions. Animals need iron to carry oxygen in their blood, break down sugar and other nutrients for energy, and plants need it to transfer electrons during photosynthesis and make chlorophyll. Phytoplankton use iron to "fix" nitrogen into a usable form. Most of us know rust as a reddish-brown flaky coating on metal and don't think much more about it, but rust is a term commonly used for the corrosion and oxidation of alloys such as iron and steel. Technically, rust is Hydrated Iron (III) Oxide, also known as iron oxide (Fe₂O₃), which forms when iron, oxygen and water react together. If a piece of iron is exposed to water and oxygen long enough, rusting is inevitable, though this process can take days, weeks, months or even years depending on the intensity of exposure. Because iron reacts readily with oxygen, rust is very common. However, despite being the fourth most abundant element in the earth's crust, iron is virtually absent in the modern ocean. The process of iron disappearing from the seas began more than 2.4 billion years ago when cyanobacteria evolved and began to consume carbon dioxide and release oxygen. This is because dissolved iron (present in the oceans) rapidly combines with new abundant oxygen atoms to form iron oxides such as hematite, a common mineral containing a form of the element known as iron (III). Most phytoplankton and other living organisms cannot use iron in this form. For this, a different form that dissolves more easily and is absorbed by cells is needed—iron (II). Only certain types of minerals produce dust rich in soluble forms of iron, including iron (II), which diatoms can easily digest. For example, iron-containing clay minerals, as understood from experiments on dusts from around the world including the Sahara Desert in Africa, Chinese loess (ash-colored soil) and Saudi Arabian coastal sand, yield iron (II) more easily than hematite. Various studies have determined that winds blowing from the Sahara are one of the most important sources of iron dust in the ocean and provide more than 70 percent of the dissolved iron in the Atlantic. However, there are also different routes for iron (II) to reach the oceans, including rivers, hydrothermal vents, volcanoes and glacial outburst plains.

The Importance of Iron Fertilization

Because iron is a micronutrient, phytoplankton need only trace amounts to have a major impact. Ash from the 2008 eruption of Kasatochi in the Aleutian Islands created an algal bloom estimated to have removed 10 million tonnes of carbon from the atmosphere. Soot from the 2019-2020 Australian bushfires triggered a bloom that may have removed 150 to 300 million tonnes of carbon between New Zealand and South America. Ice core records also show a connection between iron-rich dust and changes in climate. During times when large amounts of dust settled over the ocean, global temperatures fell and an estimated 60 billion tonnes of carbon was removed from the atmosphere during these events. Many scientists argue that iron dust has at least partly contributed to the development of past ice ages. If relatively small amounts of iron can be added to the ocean surface to effectively remove large amounts of carbon dioxide from the atmosphere, iron fertilization has the potential to play a very important role in reducing the additional effects associated with climate change. However, this will only work if the carbon removed from the atmosphere sinks to the depths of the ocean where it will remain locked for at least a century. This would buy time for a complete transition from fossil fuels to renewable energy sources.

Potential Effects

In the 1990s and early 2000s, a series of experiments tested iron fertilization in the open ocean. These tests consistently found that continuous iron additions led to phytoplankton blooms, but how much of this carbon sank to the depths was not always measured and phytoplankton could not use all the iron for growth before the minerals sank. Researchers documented changes in phytoplankton communities, with diatoms becoming more abundant than many other phytoplankton types. These algae can be up to 1000 times larger than cyanobacteria and allow them to absorb more carbon dioxide through photosynthesis. Diatoms produce silica-based glass-like shells that add weight, and when they die, this increases the likelihood that they sink faster than other smaller phytoplankton. Fast growth rates and losses in deep seas are a good sign for potentially removing carbon from the atmosphere and keeping it in the deeper ocean. However, some diatoms release toxins that could contribute to harmful algal blooms, although this has not occurred after any field experiments. Additionally, iron fertilization has the potential to change how and where nutrients are allocated in the marine ecosystem. Until experiments are conducted to test these potential consequences and determine how much carbon can be retained in the ocean depths, iron fertilization should not be used as a method to slow climate change. Early iron fertilization experiments met with resistance due to many "unknowns." Nevertheless, scientists are returning to this idea as a single CDR tool that should be on the table in fighting climate change. Currently, they are trying to establish codes of conduct so that research can be carried out transparently to better understand both the intended and unintended consequences of adding iron to the ocean surface. New technologies using autonomous platforms and sensors are now available that allow scientists to fully investigate iron's potential to eliminate atmospheric carbon and track the subsequent movement of this carbon through the ocean. Because iron fertilization would be relatively inexpensive, it could be an important part of a range of CDR activities aimed at removing excessive amounts of carbon dioxide from our atmosphere. However, it should not be forgotten that such approaches do not replace the need for sudden and major reductions in the use of fossil fuels that produce carbon dioxide in the first place. In any case, despite involving some risks, iron fertilization of the oceans is an important and effective tool that can be used to remove large amounts of carbon dioxide from the atmosphere in the short term. Sources • https://www.whoi.edu/oceanus/feature/fertilizing-the-ocean-with-iron/ • https://interestingengineering.com/science/fertilizing-the-oceans-with-iron-could-help-remove-a-gigaton-of-carbon-dioxide-per-year • https://www.ncheurope.com/en/what-is-rust •https://news.google.com/stories/CAAqNggKIjBDQklTSGpvSmMzUnZjbmt0TXpZd1NoRUtEd2lWZ0tUY0JSSE81U0lmeHlHVVpDZ0FQAQ?hl=en-US&gl=US&ceid=US%3Aen • https://earthobservatory.nasa.gov/features/Phytoplankton •https://oceanservice.noaa.gov/facts/plankton.html#:~:text=Phytoplankton%20are%20microscopic%20plants%2C%20but,carbon%20dioxide%20and%20produce%20oxygen. • https://www.science.org/content/article/draw-down-carbon-and-cool-planet-ocean-fertilization-gets-another-look • https://www.smithsonianmag.com/science-nature/complicated-role-iron-ocean-health-and-climate-change-180973893/ • https://www.whoi.edu/know-your-ocean/ocean-topics/climate-ocean/ocean-based-climate-solutions/iron-fertilization/ • Images: www.pixabay.org / shutterstock Compiled and Translated by: B. Serhat Cengiz