From the Deepwater Horizon oil spill to the train derailment in East Palestine, Ohio, our environment is constantly exposed to pollution caused directly or indirectly by humans. Chemicals like polycyclic aromatic hydrocarbons (PAHs), which are found in oil and plastics, and pollute terrestrial, aquatic and atmospheric environments, are pervasive and difficult to remediate, leaving scientists to identify innovative solutions. One such solution involves using microbes to degrade chemical pollutants, including PAHs. Indeed, it is well-established that bacteria can break down PAHs to help clean up the environment. However, there is a group of PAH degraders scientists know little about: the co-metabolizers.

Polycyclic Aromatic Hydrocarbons in the Ocean

PAHs are toxic, carcinogenic pollutants that can enter marine environments through a variety of direct sources, such as industrial wastewater and oil spills, and indirect sources, such as terrestrial runoff and atmospheric deposition. Once in the environment, these pollutants are environmental and health hazards affecting both animals and humans. In humans, PAHs can cause cancer, cardiovascular problems and liver and kidney damage.

PAH compounds consist of fused benzene rings; their chemical structure contributes to their recalcitrance, ultimately decreasing the bioavailability and increasing the stability of these compounds. To further complicate PAH removal from the environment, PAH degradation intermediates (i.e., dihydrodiols and epoxides) are actually more toxic than their parent compound. PAHs are categorized as low- or high-molecular weight, with toxicity increasing with the number of aromatic rings. While many bacteria have been shown to degrade both low- and high-molecular weight PAHs, most studies only investigate bacterial sole-metabolism of PAHs—the ability to use PAHs as a sole carbon and energy source.

Hidden Players of PAH Degradation

However, there are bacteria that are not sole-metabolizers of PAHs but are still able to break these compounds down via co-metabolism. In this process, PAH degradation occurs only in the presence of a more labile carbon substrate (i.e., a substrate that the microbe can more easily metabolize, such as methanol, acetate or glucose).

Notably, co-metabolism PAH degraders are prevalent in the ocean. For example, marine Alphaproteobacteria, such as those in the Roseobacteraceae family, include many reported co-metabolism PAH degraders. This family is significant because it can make up to 25% of marine microbial communities and play a major role in biogeochemical cycling. Bacteria in this family are known for their ability to degrade plant-derived aromatic compounds, which may explain their ability to also degrade PAHs. 

Because of their prevalence, these microbes offer high potential for bioremediation applications, broadening the pool of microbes that can be used for this purpose. Indeed, the ocean is not a carbonless environment—its many labile carbon substrates may satisfy the co-metabolism requirement. Understanding the resiliency of marine microbial communities in the face of PAH stress may help to revise future bioremediation technologies and treatments. 

A Need to Understand PAH Co-Metabolism 

Still, compared to sole-metabolism of PAHs, co-metabolism is less understood. Sole-metabolism has been investigated in a variety of genera, including Pseudomonas spp., Mycobacterium spp. and Sphingomonas spp. Mycobacterium vanbaalenii PYR-1 was one of the first organisms found to sole-metabolize PAHs, and the extensive study of its pathway led to the discovery of other sole-metabolism PAH degraders. Common methods to investigate PAH degradation by bacteria include isolate screening using PAH as a sole carbon source or searching for genetic biomarkers in genomes or bioinformatic data. The protein-coding genes pahA and pahE are common biomarkers used to identify potential PAH degraders. 

Yet, bacteria able to co-metabolize PAHs do not appear to have these biomarkers and aren’t captured by studies that screen for degradation with PAHs as the sole carbon source. While the carbon source requirements for this co-metabolism to occur are unknown, the availability of microbial necromass—dead organic matter made of extracellular compounds exuded from microbes—could potentially serve as a sufficient carbon source. With few studies investigating co-metabolism of PAHs, there is little data about how widespread this phenomenon is among marine bacteria.

Initial evidence suggests that this co-metabolic ability is not constrained to any particular taxa, but further study is needed to determine the true diversity of PAH degraders that employ co-metabolism. Ultimately, additional research into which carbon sources allow for co-metabolism, and the breadth of co-metabolizing organisms, could allow for industrial application or biostimulation of co-metabolism PAH degraders in PAH-contaminated environments.

Plastics Policy Implications

There’s been a recent increase in proposed legislation in Congress to combat microplastic pollution. It is important to consider not just plastics themselves, but also the additives and adsorbed pollutants. PAHs are plastic additives and can readily adsorb to microplastics, thus any policy involving the regulation of plastics and microplastics is also a policy that involves PAHs. Communicating with legislators about marine pollution and co-contamination of pollutants could strengthen arguments in favor of evidence-based legislation to help address microplastic pollution. In addition, advocating for scientific funding could help novel pollutant degradation mechanisms come to light—including co-metabolism of bacterial pollutant degraders.
 

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Jillian Walton received ASM's 2023 Brad Fenwick Fellowship for the Advancement of Civic Science, designed to recognize and support early career scientists with an interest and aptitude for bridging the worlds of science and the broader society through policy and advocacy. Bradley W. (Brad) Fenwick, DVM, Ph.D., was an award-winning scientist, veterinarian and leader in science policy. He was a passionate advocate for science and for cultivating the public’s appreciation of its power to improve society, and this fellowship was created to honor his memory. Applications for the 2024 Fenwick Fellowship close at 11:59 p.m. ET on May 5, 2024.



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