As students, we are taught that plants take in carbon dioxide to produce organic compounds and release the oxygen that aerobic organisms—including humans—need to live. Arguably less well known is that plants, most notably phytoplankton, and microbes in the ocean produce just as much oxygen as their land-based counterparts, in effect fueling every other breath we take.
On land, autotrophic plants and microbes pull nutrients from the ground, carbon dioxide from the air, and energy from the Sun to make their own food in a process called primary production. The same process occurs at sea, but in the open ocean, autotrophs near the surface can’t draw nutrients directly from seafloor rocks or sediments. And because they live far from river outflows, they often rely on wind-blown aerosols, such as mineral dust, for their daily nutritional needs.
Studying the extent to which nutrients in wind-blown aerosols fuel primary production in the open ocean is an active area of research. This information is vital to our understanding of foundational biological activity across much of Earth’s surface as well as of how this activity influences other marine life that phytoplankton support. It is also essential to our understanding of global climate because a fraction of the carbon taken up by marine biota in photosynthesis ends up being sequestered in the deep ocean. However, difficulties in quantifying atmosphere-to-ocean nutrient exchanges in nature, and in the lab, pose major challenges to this understanding.
An international group named RUSTED (Reducing Uncertainty in Soluble aerosol Trace Element Deposition) is now addressing these challenges to ensure that aerosol nutrient data from different research efforts can be compared and interpreted effectively to shed light on the macroscopic impacts of microscopic marine organisms.
Micronutrients Control Marine Biological Activity
Like on land, primary producers in the ocean have adapted to flourish at an optimal range of light and temperature and with the right balance of nutrients, both micro and macro. Oxygen-generating marine microbes require inorganic minerals (known as micronutrients) and organic vitamins in relatively small quantities to maintain healthy growth and reproductive capacities. Plants require micronutrients, particularly iron, for energy production and biochemical catalysis, ultimately aiding the assimilation of macronutrients, namely, carbon, nitrogen, and phosphorus. Some nutrients, especially metals such as copper and cadmium, are beneficial at low levels but become detrimental to the health of phytoplankton and other marine plants (and organisms like fish that feed on them) at levels exceeding toxicity thresholds.
The availability of micronutrients exerts a fundamental control on marine biological activity, from primary producers all the way up to apex species.
The availability of micronutrients thus exerts a fundamental control on marine biological activity, from primary producers all the way up to apex species, and it supports marine ecosystem services (e.g., carbon cycling, sustainable fisheries, marine pH regulation, and oxygen production) across spatial and temporal scales. However, in large areas of the ocean, primary production is limited by the scarcity of one or more essential nutrients. Changing atmospheric supplies and transport of micronutrients to the open ocean can therefore relieve or exacerbate nutrient limitation. So it is important to understand where, when, and why these changes may happen.
Nature itself occasionally demonstrates the effects of changes in nutrient supply to ocean-dwelling autotrophs. Large natural aerosol emission events, such as dust storms, wildfires, and volcanic eruptions, sometimes produce large quantities of nutrient-bearing airborne particles sufficient to trigger algal blooms that have been observed from space. These events often redistribute nutrients over vast distances from terrestrial source regions to remote ocean (or land) ecosystems. However, merely sprinkling these particles over the ocean does not ensure that ocean autotrophs are well nourished.
The Problems with Studying Aerosol Solubility
Rivers, sediments, and groundwater can all deliver micronutrient trace elements to nearshore marine ecosystems. Internal ocean processes such as upwelling and other vertical or lateral advections can also redistribute nutrients from deeper waters into the near-surface sunlit zone, where photosynthetic organisms live. However, in the open ocean, far from land, atmospheric deposition is often the most significant source of trace elements.
Direct nutrient inputs from above can relieve marine phytoplankton stress, but these supplies are highly episodic. Furthermore, not all of the aerosol nutrients delivered dissolve in seawater and are readily accessible, or bioavailable, for marine primary producers to use.
The solubility of aerosol-bound trace nutrients refers to the portion that dissolves upon contact with receiving waters and is often expressed as a percentage of the total concentration delivered. The factors that control aerosol nutrient solubility are complex and include the aerosol emission source; the mineralogy, particle size, and pH buffering capacity of the aerosol material; atmospheric transport pathways; atmospheric processing, acidity, and relative humidity; interactions of the aerosol material with organic material both in the atmosphere and in seawater; and the physical and chemical conditions of the seawater where the material is deposited.
Many, if not all, of these (often competing) factors have nonlinear influences on nutrient solubility, meaning it is extremely challenging to distill the complex interplay among them. This complexity also means that different combinations of factors determine solubility in different regions of the ocean. It is therefore essential to quantify aerosol solubilities in different ocean basins to look for trends in aerosol solubility and assess the full impact of changing atmospheric deposition on marine productivity.
The current lack of standardized experimental approaches to assess aerosol iron solubility contributes to the wide range of reported measurements.
Quantifying the solubility of aerosol-carried iron is especially important because it’s a vital element in numerous biochemical reactions, notably photosynthesis, and colimitation with other trace metals may also occur. But determining aerosol iron’s solubility and, potentially, bioavailability is not straightforward. In situ measurements are not feasible because we don’t have sensors or analytical systems capable of overcoming contamination issues and reliably detecting the extremely low aerosol concentrations found over the open ocean. Instead, researchers must estimate iron’s solubility using laboratory-based techniques.
Lab methods used to assess the solubility of iron and other metals in aerosols involve collecting aerosol particles on filters in situ, then leaching the soluble portions from those particles. However, there are no community-accepted, standardized protocols for such leaching experiments, and numerous protocols with various leaching solutions are used, all of which produce different results.
The current lack of standardized experimental approaches to assess aerosol iron solubility contributes to the wide range of reported measurements. With this variability, it is very difficult to assess whether differences in the data reflect natural variability in solubility or stem from different experimental conditions. It is also challenging to describe solubility relationships mathematically and to associate laboratory measurements with specific environmental processes.
As a result, it is fundamentally difficult for global models to reproduce micronutrient concentrations in the atmosphere or deposited in surface waters accurately, even for iron, the element for which the most data exist. It is also difficult for these models, which are evaluated against measurement data, to predict with confidence the responses of marine ecosystems to atmospheric deposition in the present and under different past and future climate scenarios.
These problems are especially acute for areas of the ocean that receive very high or very low aerosol inputs, although total amounts of metals and their soluble fractions remain poorly constrained in most ocean regions. Improving the determination of these key fluxes is a priority for several international research programs, including the Surface Ocean-Lower Atmosphere Study (SOLAS), NASA’s just-launched Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite, the World Meteorological Organization’s Global Atmosphere Watch program, and GEOTRACES.
Fires, Dust, Climate, and Fish
High-quality solubility data are necessary to validate model projections of the biological responses to changes in nutrient-bearing aerosol deposition under future climate warming scenarios. Recent research has shown the potential for large deposition events to trigger large-scale ecosystem responses. Several studies have demonstrated, for example, that ash and dust generated by extreme wildfires have triggered massive phytoplankton blooms in key oceanic carbon sink regions across the globe from the Southern Ocean, to the Arctic Ocean, to the coastal waters off California.
Evaluating the contribution and variability of nutrient-bearing aerosol deposition in oceanic ecosystems is a critical component of understanding how our planet will respond to changes in this deposition.
As extreme weather events such as wildfires are predicted to become more intense and more frequent, evaluating the contribution and spatiotemporal variability of nutrient-bearing aerosol deposition in oceanic ecosystems is a critical component of understanding how our planet will respond to changes in this deposition.
Primary producers are not the only organisms affected by atmospheric deposition; larger marine organisms, including important fishery species, are as well. Another recent study linked the migration of Atlantic skipjack tuna to the annual peak in atmospheric dust loading off the coast of West Africa. As the spatial distribution of such predator species is thought to be partly driven by the external supply of iron to microorganisms at the base of marine food webs, changes in aerosol nutrient supplies could lead to unforeseen impacts. For example, a sudden abundance or shortage of commercially valuable fish might fuel conflicts over fishing rights in disputed waters.
RUSTED’s Metal Work
The scientific literature reveals a huge range in solubilities reported for aerosol iron—from far less than 1% to nearly 100%—as well as for other essential nutrient elements, such as nickel, copper, and zinc. No single factor is responsible for this entire range. Some of it is accounted for by natural variability resulting, for example, from differences in aerosol sources or the environmental conditions the aerosol experienced during its lifetime.
Although scientists have little control over natural external conditions or the types of aerosols sampled on filters, many agree on the need to design experimental approaches to reduce uncertainty associated with the collection and measurement of soluble aerosol iron and other metals.
To address this challenge and ensure the quality of soluble aerosol trace element data, ocean and atmospheric scientists from around the world teamed up with numerical modelers in late 2022 to form the RUSTED working group under the auspices of the Scientific Committee on Oceanic Research. RUSTED’s goal is to provide clear guidance and recommendations for standardized aerosol leaching protocols, as well as advice on how to use these data for modeling studies.
Since its inception and through several meetings so far, the RUSTED working group has been reviewing and comparing laboratory methods for assessing aerosol metal solubilities from the past 30 years. RUSTED members aim to complete and publish this review by the end of 2024, which will enable them to draft standardized instructions for some of the most frequently used methods to improve confidence in and the comparability of different research groups’ results.
Better guidance is needed to advise researchers not only on selecting the most appropriate methods for studies but also on how experimental data can be used to improve model simulations of environmental processes.
However, the RUSTED team will stop short of recommending a single common protocol because it is important that historical data gathered using diverse approaches are not discarded and because different protocols may be more or less appropriate depending on the research question under investigation. Currently, different methods are used as analogues representing different environmental processes, such as those occurring during atmospheric transport, delivery to the sea surface in precipitation, or dissolution upon contact with seawater.
Better guidance is needed to advise researchers not only on selecting the most appropriate methods for studies but also on how experimental data can be used to improve model simulations of environmental processes involving aerosol nutrients and the impacts that humans are having on natural biogeochemical cycles.
In addition to these resources, RUSTED is gathering new and existing data on aerosol metal solubility into a single, easily accessible database. And the group is working to establish a multidisciplinary network of scientists interested in the effects of aerosol nutrient deposition on changes in current and past climates, human health, fisheries, and more. This network is being built through sessions and events hosted by RUSTED at scientific meetings, including an early-career workshop in India cohosted with SOLAS that will take place later in 2024.
In sum, the outputs of RUSTED will provide a foundation and framework for future work related to the deposition, dissolution, and biogeochemical cycling of atmospherically supplied nutrients in the ocean. This work will inform the story of aerosols and the ocean’s primary producers, which is far more than one of scientific curiosity. It is one of interconnectedness, of how microscopic particles shape the macroscopic world, and of how our planet’s (and our) health is intrinsically linked to the tiny marine organisms that contribute every other breath we take.
Author Information
Rachel Shelley ([email protected]), University of East Anglia, Norwich, U.K.; Morgane M. G. Perron, Laboratoire des Sciences de l’Environnement Marin, Plouzané, France; Douglas S. Hamilton, North Carolina State University, Raleigh; and Akinori Ito, Japan Agency for Marine-Earth Science and Technology, Yokohama
