6PPD-Quinone and the Power and Potential of Microbial Degradation

As human beings living in the 21st century, we've had our fair share of "once in a lifetime" experiences that conspicuously happened far more than once in our lifetimes. We've seen hundred-year floods and fires happening every few years, and financial crises that were supposed to define a generation somehow just keep popping up. This parallels my experience as an environmental scientist. I have watched a number of toxic, human-made compounds be named the toxin, the chemical that will define our generation and plague the environment for years to come. And that holds true for a while, until the next one comes along

I was midway through graduate school at the University of Washington when I first heard the buzz about the newest contaminant: 6PPD-quinone (6PPD-Q). This chemical was newly identified by a group of scientists across Washington state, including at our own University of Washington, as the cause of massive coho salmon die-offs in the region (Tian et al. 2020).

If you don't live in the Pacific Northwest then you might not know, as I didn't before moving here, how important salmon are in the region. They have a deep cultural and culinary significance to native tribes. My department opens every academic year with a salmon cookout in the courtyard. There are entire labs at the university dedicated to studying them. People here care a lot about salmon. So for those first few weeks, the excitement that we had finally identified the cause of the salmon die-offs was palpable.

While the discovery was promising, it was only the very first step toward tackling the problem. 6PPD, or N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, is a compound applied to tires to protect them from heat and oxidation damage. Driving abrades the tires over time, releasing microscopic particles containing 6PPD. The particles run off roads and into the water, where they are converted into the acutely toxic 6PPD-Q.

In an effort to save our beloved salmon, scientists have since mapped everything from the transformation and fate of 6PPD in the environment, to what reactions may be responsible for converting the chemical to its bioactive form (Shen 2025). This process - determine the effects of a compound in the environment, identify the degradation pathway, then isolate the genes responsible for key transformations - seems to be the established pattern for each new toxic compound that scientists identify as the cause of an environmental problem.

Even the toughest compounds are no match for microorganisms

6PPD-Q is one of the latest compounds to gain popular attention since I joined the bioremediation industry over ten years ago. When I started my undergraduate degree in microbiology, the topical issue was endocrine disruptors. One of our first-year courses involved designing an experiment to test the impact of hormones on zebrafish embryos. Endocrine disruptors were joined by pharmaceuticals and personal care products (PPCPs) later in undergrad and in early grad school. My Masters work studied the degradation of a selection of pharmaceuticals by microbial communities in a nature-based wastewater treatment system (Kargol et al. 2022). In the past few years, attention has turned to PFAS, 6PPD-Q, and the microplastics that were recently reported to present in our blood and brains.

These compounds are persistent, ubiquitous, and have a strong potential to cause human and environmental harm. PFAS even earned the name "forever chemicals" for their apparent inability to degrade under any standard conditions.

Within a few years of identifying these toxic chemical classes, however, scientists have also found microorganisms that degrade at least some members of each of them. Scientists across the world have identified microbes that break down endocrine disruptors, hormones, pharmaceuticals, personal care products, PFAS, and microplastics into non-toxic byproducts. And if a degrader for a pollutant doesn't exist, never fear. Given enough time, an organism somewhere out in the world is sure to evolve the enzymes to break it down. This line of thinking, called Woesian ecological theory, is an important part of how I conceptualize microbiology.

Many scientists track their academic lineage, the line of mentors that takes them back several generations to the founders of their field. And the chains are shorter than you think; we've been doing modern science for a really short time. My lineage goes back to Carl Woese, the man who discovered the third class of life, archaea, and revolutionized how we define microbial species. Woese mentored my advisor's advisor, making him a direct "ancestor" in my tree.

Woese changed the way we talk about microbial evolution. His theories suggest that with the correct selection pressure, microbes will eventually evolve to fill any niche, or specific role in a habitat. This can be expanded to the degradation of human-created pollutants. To understand how his ideas apply to 6PPD-Q, let's take a brief look at the mechanism of microbial evolution.

Microorganisms can evolve to degrade pollutants, even ones they've never seen before

Microbial populations can adapt very quickly to the presence of new chemicals in the environment. This has to do with how fast they grow and reproduce. In the course of just one year, not much changes in the macro world. But in the world of microorganisms, hundreds of generations will have lived, reproduced, and died. Within each of those organisms, random mutations have caused alterations to the DNA, which are then passed on to the next generation.

Most of these alterations to the DNA sequence are neutral, meaning that they don't effect the organism one way or the other. They may be in a part of the DNA that doesn't directly code for a gene, or the change doesn't affect the ability of the gene to build a functional protein.

Impacts occur when the mutation affects the way a protein is folded. Often the mutations are harmful to the organism by preventing it from making a protein it needs to survive. These organisms will die before reproducing and fail to pass on the detrimental genes.

But occasionally, a gene will change in a way that significantly increases an organism's ability to survive. In a system that has experienced pollution, that may mean evolving the ability to break down that pollutant and use it as food or energy. A mutation can cause minor changes to the protein produced by a gene, altering the enzyme's binding site just slightly. The new shape allows it to bind and degrade a human-made pollutant which just happens to be readily available as a food source in the environment.

Suddenly, this microbe has a distinct advantage over others, of both its own species and others. It uses this advantage to grow and reproduce, passing on the new degradation gene to the next generation. The gene continues to change over time, from organism to organism, until it becomes a protein that can easily degrade the pollutant.

Now, what about the rest of the organisms? Will this new super-eater take over the environment and drive all of the other species out? If the pollutant is present consistently, the community structure may change, with the degrader taking on a more dominant role. But if the degrader population grows past a critical level, it will consume all of the pollutant. With their food source and advantage suddenly gone, their population will return to pre-pollution levels.

However, they now retain the gene they used to degrade the pollutant. That means if it shows up again, the microbes will be ready to use it. That also means we can go into an environment where we suspect pollutant degradation is taking place, and identify microorganisms and specific genes responsible for the transformation.

Degrading 6PPD-Q and Beyond

In a world forever cycling through emerging chemicals, Woesian theory offers hope. The evolution process as described above has played out over and over again. For every major class of toxins that has gripped the public consciousness in the past decade, we have discovered degraders.

Degraders of pharmaceuticals and hormones abound in the soil environment, because many of them were isolated from that environment. Degradation genes for dozens of pharmaceuticals have been discovered and quantified, including in my own research. We found genes for BPA degradation and observed transformation of four other pharmaceutical compounds (Kargol et al. 2022) for which genes were later identified.

PFAS degraders have also been discovered in soil, including a number of Pseudomonas species. We now understand the mechanisms and enzymes that microorganisms use to break the strong C-F and C-Cl bounds that contribute to PFAS persistent nature (Ye 2025), and scientists have started using these enzymes to remove PFAS from contaminated water.

And we've discovered dozens of plastic-degrading bacteria (Dhali et al. 2024). While microplastics function slightly different from visible plastic due to their properties, many of the identified genes work on plastic no matter the size. I wouldn't be surprised if there are dozens of other genes out there working on plastics, with hundreds of others only a few generations of genetic change away from creating a giant microbial plastic feast.

As for 6PPD-Q, pathways for degrading the compound were found in the soil environment, and the entire degradation pathway has been mapped out, figuring out what existing genes these organisms are using to degrade this new pollutant (Shen 2025). A fungus has been identified which can trap and degrade >99% of 6PPD-Q into non-toxic byproducts within 7 days (Yu 2025). I have no doubt that bacterial degraders exist as well.

I am not saying that the miracle of microbial evolution should give humans blanket permission to keep dumping chemicals into the environment. We need to focus top-down, on regulation and better treatment solutions to limit the introduction of pollution to the environment in the first place. It also won't solve all of our problems. We're putting so many compounds out there that microbes might not have time or evolutionary incentive to develop degradation genes for all of them. And some materials are so toxic that they kill off microbes before pollutant breakdown can occur.

But what I'm saying is that we're not stuck. Nature has proven itself to be resilient time and time again. We're not accumulating an endless array of forever chemicals permanently throughout all of our ecosystems. It will take time to remediate everything. But it's possible, and it will happen, and microbes will lead the charge.

Have questions about microbial degradation of a specific pollutant? AppliedMicrobio can help - reach out today!

References

1. Dhali, S. L., D. Parida, B. Kumar and K. Bala. 2024. "Recent trends in microbial and enzymatic plastic degradation: a solution for plastic pollution predicaments." Biotechnology for Sustainable Materials 1(1). 10.1186/s44316-024-00011-0.

2. Kargol, A. K., C. Cao, C. A. James and H. L. Gough. 2022. "Wastewater reuse for tree irrigation: Influence on rhizosphere microbial communities." Resources, Environment and Sustainability 9. 10.1016/j.resenv.2022.100063.

3. Lv, S., Y. Li, S. Zhao and Z. Shao. 2024. "Biodegradation of Typical Plastics: From Microbial Diversity to Metabolic Mechanisms." Int J Mol Sci 25(1). 10.3390/ijms25010593.

4. Shen, D., Q. Shi, J. Zhang, N. D. Sy, R. Yates, W. Wang and J. Gan. 2025. "Transformations of 6PPD and 6PPD-quinone in soil under redox-driven conditions: Kinetics, product identification, and environmental implications." Environ Int 200: 109532. 10.1016/j.envint.2025.109532.

5. Tian, Z., H. Zhao, K. T. Peter, M. Gonzalez, J. Wetzel, C. Wu, X. Hu, J. Prat, E. Mudrock, R. Hettinger, A. E. Cortina, R. G. Biswas, F. V. C. Kock, R. Soong, A. Jenne, B. Du, F. Hou, H. He, R. Lundeen, A. Gilbreath, R. Sutton, N. L. Scholz, J. W. Davis, M. C. Dodd, A. Simpson, J. K. McIntyre and E. P. Kolodziej. 2021. "A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon." Science 371(6525): 185-189. doi:10.1126/science.abd6951.

6. Woese, C. R. 2004. "A new biology for a new century." Microbiol Mol Biol Rev 68(2): 173-186. 10.1128/MMBR.68.2.173-186.2004.

7. Ye, D., Z. Wang, X. Qian, K. Ouyang, D. Wu, F. Tang, D. Hrynsphan, T. Savitskaya and J. Chen. 2025. "Biodegradation of per- and polyfluoroalkyl substances: microbes, enzymes and their interactions." Reviews in Environmental Science and Bio/Technology 24(1): 43-62. 10.1007/s11157-025-09721-x.

8. Yu, H., L. Luo, B. Wu, J. He, H. Wang, R. Chen, M. Ji, Q. Yang, G. Zeng, W. Wu and D. Sun. 2025. "Efficient catalytic degradation and detoxification of 6PPD-quinone by the multifunctional enzyme system of phanerochaete chrysosporium." Journal of Hazardous Materials 494: 138634.