Nanoscale materials, which are about 1/10,000th the diameter of a human hair, are everywhere. Naturally occurring nanomaterials include things like volcanic ash, ocean spray, and the proteins and protein clusters that help your body function. They exist alongside incidental nanomaterials created as a result of human activities, like burning diesel, mining, and starting a fire. Recently, the amazing chemistry at the nanoscale, including high reactivity and unique optical and electronic properties, has produced technology with specific benefits to society using human-made engineered nanomaterials (ENMs). The ability for scientists to control the size, shape, and composition of ENMs allows for the design of new materials with targeted reactivity, driving breakthroughs in functional materials for energy, medicine, and agriculture. Many of these ENMs may accidentally end up in the environment, or be purposefully released in specific applications, with an estimated 300,000 metric tonnes of nanoparticle-containing products disposed annually with potential release of particles into the environment [1] (although this is just a fraction of the amount of plastic that reaches the environment annually through mis-managed waste, estimated as 8 million tonnes / year). For example, in the agricultural sector, ENMs can be used to more effectively deliver nutrients to plants to encourage growth without the need for harmful pesticides.[2] While nanotechnology has many exciting avenues to improve aspects of daily life and society, we are still in the early stages of understanding how they move in biological systems and interact within the environment. As environmental applications of ENMs grow, an understanding of their biochemical interactions must lead the way to progress beyond observations towards mechanistic understanding. 

One large difference between the bulk materials (that we can physically see) and nanoscale materials is that ENM have a high surface area to volume ratio, due to their incredibly small size. Their small size means that ENM are in the size range of many biomolecules, such as proteins and their complexes (e.g. lipoprotein complexes), which can adsorb to the surface and form a coating known as the corona (Figure 1). This protein corona creates a new biological identity for the particle, altering an organism's response to ENMs and changing the ENMs’ reactivity within the environment. 

Figure 1. Size comparison of various naturally occurring biological nanomaterials (top) and engineered or incidental nanomaterials (bottom). Biological materials at the nanoscale include viruses, lipoproteins, protein complexes, proteins, and DNA, while examples of incidental or engineered nanomaterials include metal nanospheres, exhaust particles, carbon nanotubes, and fullerenes.  Figure not to scale (Created with BioRender.com).

While the interaction of proteins with surfaces has been studied for decades, the role of surface curvature in defining these interactions and the fact that nanoscale particles have different binding affinities than macroscale surfaces of similar composition emerged more recently, with the term “corona” having been coined in 2007.[3, 4] Approaches are under development to use the protein corona to increase efficacy of treatments in the biomedical context.[5] There is commonly a lag between studies on human health and environmental health when it comes to toxicity and environmental science. Similarly, “eco-corona” studies are just emerging. These early studies reveal the complexities of the increased biodiversity in the environment and the broad range of proteins, metabolites, and other organic matter. Just as the protein corona mediates ENM reactivity, transport, and toxicity in humans, the  eco-corona plays a role in the fate and transport of ENMs in the environment, including organismal uptake and trophic transfer. Since an eco-corona can form outside an organism, within it, or change as the EMN is transferred between organisms, the evolution of the eco-corona over its lifetime within an ecosystem is ripe for investigation, especially given the importance of the identity of proteins within the eco-corona for organismal response. 

Figure 2. The protein corona has been primarily studied within the biomedical context to improve nano-based drugs, treatments, and diagnostics. The environmental corona, on the other hand, consists primarily of natural organic matter. Environmental proteins and other bio-molecules are included in the complexity of the eco-corona. Like the protein corona in humans, the proteins in the eco-corona with a biological identity. Figure not to scale (reprinted from [6]).

Our manuscript[6] presents our vision for the future of eco-corona research to close the gap between our understanding of clinical implications of ENMs and the environmental fate and consequences of ENM release into the environment. Although we know that large quantities of ENMs are released into the environment, the formation and interactions of the corona in the complex wider environment remains poorly understood. With a molecular level understanding of ENM reactivity, we can design safer ENMs, better model ENM transport in an ecosystem, and limit the release of novel environmental contaminants or exploit these interactions for environmental benefit.  

Studies of the eco-corona are in their nascent stages. Looking forward, we think researchers in the field will need to consider the following as they design studies and advance the field.

1. New tools are needed to study the eco-corona, both experimental and computational: The chemical richness of the environment is much broader than the human environment of clinical studies; with the multitude of species and environmental pollutants, the number of permutations for corona compositions far exceeds that of studies using human or bovine blood products. The wide array of species contributing proteins, metabolites, and nucleic acids in addition to varying physical factors provides many exciting avenues of research on the eco-corona and demonstrates the knowledge gaps still present in understanding the formation and impact of the corona (Figure 3). New methods are necessary to characterise the entirety of the corona to include metabolites, nucleic acids, and environmental pollutants in addition to proteins. With increased use of open and FAIR (Findable, Accessible, Interoperable and Re-usable) principles in data management and enhanced data availability in the future,[7] we hope that better predictive models can be generated for corona behaviour and composition, as well as for prediction of the role of acquired or designed corona-driven ENM environmental fate.
2. Studies need to address the challenge of biodiversity: It is increasingly important to begin investigating more environmentally relevant ENMs and follow realistic environmental lifetime conditions, widening the number of organisms studied to account for species-specific corona variability. 
3. Focus on environmentally relevant ENMs: We call for a concerted effort in the analysis of more environmentally relevant ENMs, since those studied thus far are primarily those of clinical interest. These would include particles used in pigments and colorants such as titania, nanoscale plastic debris from post-consumer plastics and zinc ENMs from cosmetics. As with all materials and chemicals in the environment, ENMs are exposed to a vast range of environments such as fresh water, terrestrial soils, high and low UV exposures and mechanical abrasions as the particles are transported, all of which age the particles. 
4. Finally, there are exciting developments ahead in understanding and exploiting protein exchange within the corona: As the ENM traverses the environment, the proteins within the corona exchange, react, and age. Few studies have addressed the latter, but this is especially important. For example, UV radiation, known to react with ENMs to change their environmental fate, may alter the corona as well. Moreover, throughout the lifetime of an ENM, proteins within the corona can serve as biomarkers, providing a history or map of the ENM’s travels within an organism or as it traverses across various species and through the food chain within an ecosystem. 

Figure 3. ENMs take on eco-coronas with different biomolecules dependent upon where they are released, surrounding organisms, and molecular diversity of the area. As new tools are developed to tackle these challenges, the field will rapidly advance toward understanding of the importance of biomolecules in directing the fate and impact of nanomaterials in the environment. Figure not to scale (reprinted from [6]).

While the protein corona has been widely discussed and studied in the past decade, the field of the “environmental bimolecular corona” is still very much in its infancy, but is rapidly advancing. Can proteins be used as biomarkers to provide a history of a particle and the organisms it has encountered in its journey through the environment? Can the corona be used to enhance the efficacy of disease fighting in plants? And, how do we better design safe nanotechnologies, or even those that enhance the health of our planet? We are looking forward to strategic studies that embrace the complexity of the corona and the wide ranging conditions in which ENMs can be found. With this groundwork, we believe that the eco-corona features will be widely useful.

References

[1]  Keller, A.A.; Lazareva, A. Predicted Releases of Engineered Nanomaterials: From Global to Regional to Local, Environ. Sci. Technol. Lett. 2014, 1, 1, 65–70; https://doi.org/10.1021/ez400106t

[2]  Spielman-Sun, E.; Avellan, A.; Bland, G.D.; Clement, E.T.; Tappero, R.V.; Acerbo, A.S.; Lowry, G.V. Protein coating composition targets nanoparticles to leaf stomata and trichomes, Nanoscale, 2020,12, 3630-3636; https://doi.org/10.1039/C9NR08100C

[3]  Cedervall, T.; Lynch, I.; Lindman, S.; Berggård, T.; Thulin, E.; Nilsson, H.; Dawson, K. A.; Linse, S. Understanding the Nanoparticle–Protein Corona Using Methods To Quantify Exchange Rates and Affinities of Proteins for Nanoparticles, Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 2050– 2055; https://doi.org/10.1073/pnas.0608582104

[4]  Klein, J. Probing the interactions of proteins and nanoparticles, Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 7, 2029-2030; https://doi.org/10.1073/pnas.0611610104

[5]  Ke PC, Lin S, Parak WJ, Davis TP, Caruso F. A. A Decade of the Protein Corona, ACS Nano 2017, 11, 12, 11773–11776;  https://doi.org/10.1021/acsnano.7b08008

[6]  Wheeler, K.E.; Chetwynd, A.J.; Fahy, K.M.; Hong, B.S.; Tochihuitl, J.A.; Foster, L.A.; Lynch, I. Environmental dimensions of the protein corona. Nat Nanotechnol. 2021, 16, 6, 617-629. https://doi.org/10.1038/s41565-021-00924-1. 

[7]  Ammar, A.; Bonaretti, S.; Winckers, L.; Quik, J.; Bakker, M.; Maier, D.; Lynch, I.; van Rijn, J.; Willighagen, E. A Semi-Automated Workflow for FAIR Maturity Indicators in the Life Sciences, Nanomaterials 2020, 10, 10, 2068; https://doi.org/10.3390/nano10102068