Polycyclic Aromatic Hydrocarbons (PAHs)
Polycyclic Aromatic Hydrocarbons (PAHs) are a class of environmental pollutants that arise from a diverse range of processes, including the incomplete combustion and pyrolysis of organic matter. These chemical compounds are characterized by their unique structure, consisting of two or more fused benzene rings, and possess low water solubility. As a consequence, PAHs tend to associate predominantly with particulate matter found in soils and sediments, forming a significant environmental concern.
The sources of PAHs encompass a combination of natural events and human-driven activities. Volcanic eruptions and forest fires release PAHs into the atmosphere, contributing to their ubiquitous presence in the environment. However, human-related activities carry out a leading role in PAH contamination. Incomplete combustion of fossil fuels in various contexts, such as vehicles, wood burning, and heating systems, significantly contributes to the release of PAHs into the air. These airborne PAHs can readily adhere to particulate matter, such as soot, pollens, dust, and fly-ash, facilitating their transportation to and deposition on soil surfaces. Subsequently, these contaminants can enter water bodies through processes of both dry and wet deposition, exacerbating their environmental impact.
Impact of PAHs on Health and the Environment
The presence of PAHs in the environment poses significant health risks to both humans and wildlife. These compounds are known to be teratogenic, mutagenic, and carcinogenic, making them hazardous to human health. In fact, certain PAHs have been classified as probable human carcinogens by the International Agency for Research on Cancer. Notably, compounds like benzo[a]pyrene, naphthalene, chrysene, benzo[a]anthracene, benzo[k]fluoranthene, and benzo[b]fluoranthene are identified as carcinogens with the potential to cause cancer in humans.
Humans can be exposed to PAHs through various pathways. Inhalation of airborne PAHs is of particular concern, especially in regions near industrial areas and high traffic zones. These locations tend to have elevated levels of PAHs in the air due to emissions from industrial processes and vehicular exhaust. Additionally, exposure can occur through the ingestion of contaminated food and water, where PAHs have entered the food chain from contaminated soil and sediments. Moreover, direct dermal contact with PAH-contaminated surfaces can also lead to exposure. The bioaccumulation of PAHs in mammals can result in adverse effects on reproduction, development, and immune function, highlighting the need to address these pollutants.
PAHs have long-lasting effects on the affected individuals and their offspring once they enter the body. Mice fed high levels of a PAH during gestation have difficulty reproducing, as do their offspring. These descendants also show higher rates of congenital malformations and lower body weights. Animal studies have also shown that PAHs can have harmful effects on the skin, body fluids, and the ability to fight diseases after both short- and long-term exposure.
Our Solution: The Pseudodetox Project
Bacteria play a crucial role in improving soil fertility as they have the ability to solubilize phosphate, fix atmospheric nitrogen, and produce indole acetic acid (IAA). Among the multitude of bacterial genera tested, the Pseudomonas genus has shown the most promising results for these three criteria. Additionally, Pseudomonas bacteria possess the capability to degrade polycyclic aromatic hydrocarbons found in polluted soils by using three major degradation pathways via angular and lateral deoxygenation or monooxygenation as the upper catabolic pathway.
We have focused our research on Pseudomonas putida, specifically strain KT2440, which has been genetically modified to produce and release rhamnolipids. Rhamnolipids are biosurfactants that act by solubilizing PAHs, facilitating their uptake by bacteria, and enhancing their degradation. A recent study showed that the addition of sophorolipids, which are the most widely produced and sold biosurfactants on the market, increases the biodegradation ratio from 21.4% to 91.7% within 48 h. Our project named Pseudodetox aims to genetically modify P. putida to make it capable of producing both rhamnolipids and sophorolipids at a lower cost, thereby enabling effective soil decontamination and using P. putida strain KT2440 as an organic fertilizer.
Sophorolipids synthesis pathway
Sophorolipids are aliphatic compounds biosynthesized by yeasts such as Starmerella bombicola. To produce sophorolipids in P. putida, we have drawn inspiration from the biosynthetic pathway of S. bombicola. The metabolic pathway leading to the production of sophorolipids consists of just four enzymes and uses oleic acid or cis-vaccenoyl acid as precursors. These precursors are naturally present in P. putida KT2440, making it possible to create the metabolic pathway in this bacterium.
- - Cytochrome P450 CYP52 M1
- - UDP-glucosyltransferase A1 (UGTA1)
- - UDP-glucosyltransferase B1 (UGTB1)
- - Lactonase (or Lactone esterase)
References
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- Claire Froger, Nicolas Saby, Claudy Jolivet, Line Boulonne, Giovanni Caria, et al. Spatial variations, origins, and risk assessments of polycyclic aromatic hydrocarbons in French soils. 2021. ⟨hal-03200468⟩
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- Chengyi Luo, Xin Hu, Mutai Bao, Xiaojun Sun, Fengshu Li, Yiming Li, Wenxiu Liu, Yan Yang, Efficient biodegradation of phenanthrene using Pseudomonas stutzeri LSH-PAH1 with the addition of sophorolipids: Alleviation of biotoxicity and cometabolism studies. Environmental Pollution, Volume 301, 2022, 119011, ISSN 0269-7491, DOI: 10.1016/j.envpol.2022.119011.
- Ciesielska, Katarzyna, Bing Li, Sara Groeneboer, Inge Noëlle Adrienne Van Bogaert, Yao-Chen Lin, Wim K. Soetaert, Yves van de Peer and Bart Devreese. “SILAC-based proteome analysis of Starmerella bombicola sophorolipid production.” Journal of proteome research 12 10 (2013): 4376-92.