Plastic degradation by marine bacteria

Plastic degradation in marine bacteria describes when certain pelagic bacteria break down polymers and use them as a primary source of carbon for energy. Polymers such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) are incredibly useful for their durability and relatively low cost of production, however it is their persistence and difficulty to be properly disposed of that is leading to pollution of the environment and disruption of natural processes. It is estimated that each year there are 9-14 million metric tons of plastic that are entering the ocean due to inefficient solutions for their disposal. The biochemical pathways that allow for certain microbes to break down these polymers into less harmful byproducts has been a topic of study to develop a suitable anti-pollutant.

Adaptive pressures With the increasing presence of plastics in the environment, certain species of bacteria have evolved to degrade plastics into harmless by-products. Since the 20th century, microbes have evolved to degrade plastics, as the global production of plastics steadily increased from 2 million metric tons to 380 million metric tons per year. A study performed in 2021, led by Jan Zrimec of the National Institute of Biology in Slovenia, was able to isolate 30,000 non-redundant enzyme homologues from more than 200 million genes in DNA samples obtained from the environment capable of degrading 10 different types of plastics. The results showcased the impact of plastic pollution on the microbial environment and the tendency of microbes to adapt to a rapidly changing situation. A strong correlation can be seen between the microbial potential to degrade plastics and the content of global plastic pollution. Of the 30,000 enzyme homologues isolated, 12,000 were found in samples from the ocean. Region-specific analyses show that plastic degrading enzymes were found in high concentrations in deeper areas of the ocean where plastic pollution was more common.

Metabolism

Diversity With over 5000 grades of plastic polymers and variations in coatings such as flame retardants and pigments, diverse plastic polymer substrates suggest the existence of very heterogenous metabolic processes in plastic degradation. Dynamic ocean conditions ranging in humidity, temperature, UV irradiation, pH, wind, and waves, create varied growth conditions for bacteria and increase the possibility of diversified plastic degradation metabolisms.

Mechanisms As a developing topic, few studies have characterized the metabolic and biochemical mechanisms involved in the degradation of plastic by marine microbes. A limited number of plastic degradation pathways in marine microbes have been extensively studied. Although several metabolic processes in plastic degradation have been well-documented, these processes are likely not representative of the microbial population capable of plastic degradation. Additionally, reaction times of plastic biodegradation metabolisms are poorly understood and are estimated to range between 1–400 hours in the marine environment.

Polyethylene (PE)

Bacteria capable of polyethylene degradation have been described to utilize oxygenase to initiate biodegradation. The formation of alcohol groups through oxygenase makes polyethylene more labile for degradation. The hydrophilic properties of polyethylene polymers increase as the material experiences degradation and oxidation, which causes polyethylene to become less recalcitrant. Lipases, esterase, endopeptidases, and other extracellular enzymes then further degrade the polyethylene polymers. The role of laccase in polyethylene degradation by Rhodococcus ruber is well-documented as an important enzyme for biodegradation. Alkane hydroxylase is thought to play a similar role in pseudomonas species capable of polyethylene degradation. Once enzymes degrade polyethylene polymers into oligomers, microbial cells uptake the molecules through either Major Facilitator Superfamily proteins or ATP binding cassettes. The polyethylene oligomers are converted into Acetyl-CoA and succinyl-CoA and enter the tricarboxylic acid cycle, and eventually the respiratory chain to produce ATP.

Polyethylene Terephthalate (PET) Aromatic rings in the structure of polyethylene terephthalate pose challenges for microbial biodegradation. Despite the challenge of degrading aromatic rings, several microbes are documented to use polyethylene terephthalate as a sole energy and carbon source. Microbes that utilize PET degradation first adhere to the substrate surface and release enzymes such as hydrolases and cutinases. Following the activity of enzymes like MHETase, molecules from PET degradation are taken up by active transport into bacterial cells. As demonstrated in Figure 2, transported PET molecules consist of terephthalic acid and ethylene glycol, monomers of PET. Certain microbes can use these monomers as their primary carbon source leading to PET degradation. The most relevant pathways related to the degradation of the monomer ethylene glycol are the acetaldehyde/ethanol pathway and the glyoxylic acid pathway. The glyoxylic acid pathway is commonly studied in the Pseudomonas genus as many species such as P. putida are known to be highly capable of degrading polyethylene in aquatic environments. Following a series of chemical processes, the terephthalic acid is converted into 4-carboxy-2-hydroxymuconic, which is a precursor molecule to the TCA cycle and eventually converted into pyruvate and oxaloacetate. Several bacterial species in Betaproteobacteria, Myxococcota (formerly included in Deltaproteobacteria), and Gammaproteobacteria are capable of PET Biodegradation.

Polystyrene (PS) Polystyrene consists of molecules with both strong hydrophobicity and a high molecular weight. Bacteria that are capable of degrading this molecule are documented to release monooxygenases to initiate the oxidization of polystyrene molecules. Following the monooxygenase step, the polystyrene molecule is transformed into phenylacetic acid during the upper pathway of styrene metabolism. Phenylacetic acid is first converted into phenylacetyl-coA, and later acetyl-CoA and succinyl-CoA after a series of enzymatic reactions. Acetyl-CoA and succinyl-CoA then enter the tricarboxylic acid cycle. Several Rhodococcus ruber strains are capable of polystyrene biodegradation.

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