Eumelanin, the brown-black pigment that darkens human skin, and natural organic matter (NOM), the substance that turns rivers, lakes, and soils amber and brown, would seem to have little in common. One is manufactured inside living cells to shield them from ultraviolet light. The other is the leftover residue of dead organisms, broken down over time by sunlight and microbes. Yet a new study from researchers at Ohio State University and Texas A&M University finds that these two materials behave almost identically when they absorb and re-emit light — and the explanation lies in nanoscale architecture so small it had gone largely unnoticed until now.

The study, published in ACS Central Science, proposes that both eumelanin and NOM are built from the same basic optical unit: stacks of just a few flat, ring-shaped molecules layered on top of one another like an extremely short stack of pancakes, only a few nanometers tall. According to the researchers, these “few-layered” stacks are the true light-absorbing and light-emitting components in both substances, regardless of the wildly different chemistry that produced them.

“Synthesizing the molecular building blocks in eumelanin and NOM is akin to two very different approaches for creating a diverse set of words,” said Bern Kohler, professor of chemistry and biochemistry at Ohio State and one of the study’s senior authors, in a university statement. “Melanin synthesis is like giving someone multiple copies of the alphabet and asking them to make words, while NOM synthesis is like using scissors to cut words out of books from a library.”

That distinction matters because it has long puzzled scientists why two materials with such different origin stories would share so many physical traits. Eumelanin is synthesized from a single starting molecule, tyrosine, which living cells convert through a chain of reactions into nanoscale granules. NOM, by contrast, arises from the topdown decay of plant and animal matter and is known to contain thousands of distinct chemical compounds. Despite this divergence, both substances absorb light broadly and featurelessly across the ultraviolet, visible, and near-infrared spectrum; both fluoresce weakly, with the brightness of that glow fading as the molecules are illuminated with longer wavelengths; and both have been noted independently, decade after decade, to share oddly parallel properties — persistent free radicals, sensitivity to pH, and wavelength-dependent emission patterns chief among them. As far back as the 1980s, melanin researcher Tadeusz Sarna referred to humic acids — a major component of NOM — as “melanin-like polymers,” while soil scientist Nicola Senesi described melanins as “humic acid-type polymers,” each apparently sensing a kinship between the fields without fully explaining it.

To get to the bottom of the resemblance, the research team used ultrafast laser spectroscopy and atomic force microscopy to compare synthetic eumelanin against three natural NOM samples drawn from soil and river water. One of the key experimental tricks involved deliberately breaking the synthetic eumelanin apart. By dissolving the pigment in an alkaline solution and filtering out the smallest resulting fragments, the team isolated low molecular weight pieces of eumelanin that, once isolated, started behaving almost exactly like NOM. Their fluorescence brightened more than tenfold, their emission color shifted to match that of river-derived NOM samples, and their light-absorbing behavior under laser excitation became nearly indistinguishable from the natural samples.

Imaging with an atomic force microscope showed why. The disassembled eumelanin fragments and the natural NOM samples both consisted of disk-like structures only one to four nanometers tall — physically consistent with stacks of just two or three flat aromatic molecules bound together. Larger, intact eumelanin particles, by contrast, were aggregates of these tiny stacks bundled together at a much bigger scale, tens to hundreds of nanometers across. The optical behavior of the whole particle, the researchers argue, is essentially just the combined behavior of many of these small stacks acting independently, since energy absorbed by one stack appears to stay locked within that stack rather than spreading to its neighbors.

That restriction on energy movement, technically called excitation energy transfer, turns out to be central to the puzzle. Using a technique called transient spectral hole burning, in which a narrow-bandwidth laser pulse selectively excites only a sliver of the available chromophores, the team showed that both eumelanin and NOM retain a “memory” of which wavelength excited them for an unusually long time, something that would not happen if energy could freely hop between neighboring stacks. The effect, the authors report, had previously been documented in eumelanin but is described here for the first time in NOM.

Meera Madhu, a PhD student in chemistry and biochemistry at Ohio State and a co-author of the study, said the findings extend beyond basic curiosity about pigments. “Eumelanin can absorb the entire solar spectrum, so by finding a way to convert that into energy that can be stored or used in viable ways, it’s going to become one avenue to overcome the energy issues that we have,” she said, pointing to potential applications in solar energy capture, batteries, and bioelectronics.

The researchers suggest the same framework may eventually apply to a broader family of carbon-based nanomaterials, including graphene oxide, carbon dots, and the brown carbon aerosols found in wildfire smoke and urban haze, all of which share the broadband absorption and weak fluorescence that originally drew attention to eumelanin and NOM. The team’s experiments with sodium borohydride, a reducing agent that altered the optical properties of both materials in similar ways, reinforced the idea that charge-transfer interactions between stacked molecules, rather than chemical composition alone, are responsible for shaping how these materials absorb and release light.

“This fundamental understanding of how eumelanin and NOM absorb and respond to light could ultimately help guide rational design of carbon-based materials,” Madhu said. “By bringing together two materials that have so far been studied separately, this work gives researchers a framework they can build on to understand related carbon materials.”

The study’s authors caution that their model remains tentative in places, particularly regarding what happens at ultraviolet wavelengths, where energy transfer between chromophores appears to switch on rather than stay suppressed. Future work, they note, will need to probe individual stacks directly and extend measurements deeper into the ultraviolet to test the limits of the framework. Still, the convergence of two long-separated research traditions onto a single structural explanation marks a notable step toward understanding two of the most common, and most stubbornly mysterious, pigments on Earth.

Endnotes

Senesi, N., Miano, T. M., & Martin, J. P. “Elemental, Functional Infrared and Free Radical Characterization of Humic Acid-Type Fungal Polymers (Melanins).” Biology and Fertility of Soils, 1987, 5, 120–125.

Madhu, M., Ilina, A., Li, H., McKay, G., & Kohler, B. “Common Photoproperties of Eumelanin and Natural Organic Matter Emerge from Ensembles of Few-Layered Nanostructures.” ACS Central Science, 2026, 12, 586–598. https://doi.org/10.1021/acscentsci.5c02304

Woodall, T. “Researchers are unraveling the building blocks of Earth’s pigments.” Ohio State University, via EurekAlert!, June 29, 2026. https://www.eurekalert.org/news-releases/1133940

Sarna, T., et al. “Interaction of Melanin with Oxygen.” Archives of Biochemistry and Biophysics, 1980, 200, 140–148.