Tackling Plastic Pollution: Mangrove-derived Bacteria in Degrading Plastic

Plastics have become one of the most renowned culprits of recalcitrant contaminants within our world. Notorious for its sparse bio-degradability, a single plastic bottle used can remain undecomposed for hundreds of years. Upon feeding into a waste sorting machine, it jammed up machines, eating into hefty sums for overhauls. Eventually, most plastics end up in awry, hopeless landfills, encroaching upon and occupying our territories for centuries. But, what if the mangroves can turn the tide?

The Molecular Problem of Plastic

Prior to understanding the whole conundrum, we shall strip everything back to the molecules of plastics. Well, they are made up of gigantic, enormously lengthy artificial chains of molecules called synthetic polymers. Let’s take the most strikingly relatable instance of plastics, say, polyethylene. In its simplest form, ethene is a simple gas that breaks down naturally, thanks to its unstable double bond. Yet, things soon turn sour when billions of ethene monomers are joined together synthetically into polyethylene. All of a sudden, all the double bonds get hydrogenated into recalcitrantly stable single C-C bonds, hampering it from being attacked and degraded by most external agents in nature. This is exactly what makes it hard to decompose, and that’s the whole point of plastic solution.

Biodegradable Pwlastic

Throughout the 20th century, breakthroughs in the invention of biodegradable plastic shed light on a new door. Nevertheless, none ever comes off enough to fully supersede its traditional un biodegradable counterparts. Why might this be the issue? While heavily marketed as bio-compatible, plant-based and environmentally friendly, it’s far from a silver bullet. Contrary to the belief of many, most biodegradable plastic decomposes quick enough only under heavily controlled conditions within specialised setups in the industry. Place it within your home compost or simply ditch it aside in the garage, and you will likely find the plastic bags unaltered even after years. Additionally, the disproportionately high cost and flimsiness of such bags render it less adopted by many.

Now, let’s shift the viewpoint. Perhaps the solution to our world’s plastic problem does not lie in biodegradable plastic after all, but in finding ways to degrade the current backlog of plastic. A group of researchers in Southeast Asia have been delving into this, and they have identified that untapped bacterial communities within mangroves have the potential to degrade several plastics, including polypropylene and polyethylene. 

Trapping Plastic In Mangrove Sediments

The complex root systems of mangroves are able to physically trap substantial amounts of marine plastic debris. Pneumatophores, 30-40cm high vertically-erected roots of Avicennia species, trap debris through their dense, upward-extending structure. On the other hand, prop roots of Rhizophora species produce anchor-like structures that trap debris between their arching supports, preventing waste from escaping to the open sea. The prevalence of plastic increases as it approaches the river mouth. And, studies have suggested a direct relationship between root abundance and trapped plastic litter. Mangroves retain microplastics 2.24 times better than tidal flat areas, consolidating its pivotal role in seawater purification. The physical properties of plastic items influence retention as well. 

Flexible items such as plastic bags and sachets that become entangled in root networks are unlikely to be washed out again. Conversely, buoyant rigid items like plastic bottles can float away on receding tides. Smaller microplastics (0-1000 μm) accumulate more readily in mangroves relative to larger particles (1000-5000 μm). In terms of geographical distribution, an analysis done in Philippine forests has shown that landward zones host the greatest proportion of macroplastics, with land-derived items like plastic bags dominating the samples. On the contrary, seaward zones trap mainly fishing gears as well as materials for boat repairs. Middle zones retain plastic that escapes root filtering at both landward and seaward areas, acting as a sink for litter of both land and sea origins. By and large, the accumulation of plastic increases with distance from shore.

A global meta-analysis has revealed microplastic abundance averaging 23.73 ± 8.80 items m⁻² in mangroves, comparable to the highest levels recorded on beaches. Yet, this is often overlooked owing to reduced accessibility of sophisticated terrain within mangroves relative to sandy beaches. Plastic accumulation within mangroves can take a toll on its ecosystem, with evidence demonstrating its repercussions towards the respiratory system of local animals. Plastics can also suffocate mangrove seedlings and accelerate the colonisation of disease-causing microbes. Nevertheless, things are not all that void and bleak. A promising news is that some researchers have found that microbes residing within mangroves play a role in degrading polypropylene and polyethylene. The roots of mangroves have provided a scaffold for relatively stable conditions, resulting in the highest enrichment of potential plastic-degrading microorganisms. As such, plastic-degrading bacteria including Bacillus, Pseudomonas, Rhodococcus, Alcanivorax, and Gordonia are at higher functional concentration in mangroves than its surrounding seawater. When microplastics are trapped in mangroves sediments, they become part of a new niche, selectively enriching taxa with plastic-degradation functions. This forms a new microecosystem in which we call the plasticsphere.

Biological Mechanisms of Plastic Degradation by Microbes

The first step of plastic degradation by bacteria in mangroves begins with bacterial colonisation, which is indicated by SEM imaging revealing surface pitting, cracking and groves on plastic debris. Confocal laser scanning microscopy and atomic force microscopy can confirm the presence of biofilms formed by mangrove-derived microbes as well. Following this, extracellular enzymes secreted from microbial biofilms convert long-chain polymers into oligomers and oxidise them under the influence of oxygen to form oxygen-containing functional groups such as carbonyl, carboxyl and hydroxyl. Such oxidation processes increase the surface hydrophilicity of plastic sheets. Plastic-degrading enzymes including alkane hydroxylase, alcohol dehydrogenase and aldehyde dehydrogenase are identified through shotgun metagenomics of mangrove sediment. Laccase and lipase drive polyethylene and polypropylene degradation by mangrove isolates. Following depolymerisation, oligomers are assimilated through intracellular metabolic mechanisms, predominantly β-oxidation and the citric acid cycle. Ultimately, they will be mineralised to CO₂, CH₄, and H₂O. Whole genome sequencing of Serratia marcescens E1-2, an effective polyethylene degrader within mangroves, has revealed multiple enzymes and metabolic pathways pertinent to this function. Liquid chromatography-mass spectrometry (LC-MS) analysis shows that the alcohol, ethers and acids produced from plastic degradation will however be assimilated by the microbes intraprocess.

Figure 1: Extracellular enzymes implicated in plastic depolymerisation across bacterial isolates in mangroves

PE = polyethylene

LDPE = low-density polyethylene

PP = polypropylene

Enzyme	Targeted polymer	Mechanism & evidence	Examples on microbial source
Laccase	PE, LDPE, PP	Oxidative formation of carbonyl and hydroxyl groups; 24% LDPE weight loss over 105 days	Bacillus sp., Pseudomonas sp., Aspergillus terreus
Lipase	PE, PP, LDPE	Surface erosion and decreased hydrophobicity; co-secreted with laccase in mangrove isolates	Pseudomonas aeruginosa, Bacillus tropicus, Paenibacillus sp.
Alkane hydroxylase, aldehyde dehydrogenase and alcohol dehydrogenase	PP		Xanthomonadacea, Nocardioidaceae, and unclassified Gaiellales
Esterase	PE, PP, LDPE	Part of a synergistic enzyme pair that introduces C–O functional groups on PE	Paenibacillus sp., Aeromonas sp., Dermacoccus sp.

Technical and Environmental Challenges

Nonetheless, mangrove microbial degradation of plastics is hurdled by intrinsically slow kinetics and the potential toxicity of byproducts. Admittedly, various studies have recorded degradation metrics not exceeding 12% for pertreated plastics within laboratories, and the figures will likely be lower in situ. Quite apart from that, the aforementioned polymers remain in a transitional state of microplastics and nanoplastics for extensive periods post-degradation, indicating that the decomposition process itself can elicit secondary pollution. Microplastics easily bind to other toxic chemicals or metals. Such substances act as vectors for toxic substances besides entering our food chain, resulting in bio-accumulation. A cloud hangs over when a study published by the World Wide Fund (WWF) indicates that we could be ingesting roughly 5 grams of microplastics per week - about the weight of a credit card. In lieu of nipping the pollution issue in the bud, we can somehow end up with another form of pollution - microplastics as well as nanoplastics, and this can be even more intractable than traditional plastic. In other words, we might be jumping from the frying pan into the fire.

Future Scalability and Applications

After all, we will very well get over all the roadblocks mentioned if only we are able to continually deploy the mangrove-derived plastic-degrading bacteria or its enzymes in large scale within closed, controlled industrial setups.

A. Controlled bioreactors

This looks into how mangrove-derived plastic-degrading bacteria can be confined within industrial setups to yield a concentrated cohort for optimum effectiveness. As plastics are fed into the reactors, we are able to keep any microplastics and nanoplastics isolated within, minimising the unintended consequences of microplastics leaching during plastic decomposition. At this point, a laboratory-scale bioreactor developed to degrade oxobiodegradable plastics using Phanerochaete chrysosporium has demonstrated amelioration in plastic degradation, which may bring some hope in the future possibility of utilising reactors for bioremediation involving plastics. The microbe deployed within this study is a fungi, and noteworthy is that there remains a significant dearth within experiments in keeping with mangrove-derived bacteria as far as this application is concerned.

B. Genetic engineering

With recent advancements in CRISPR technology, we might one day be able to identify and extract the genes responsible for extracellular enzymes effectively degrading plastics. We can then incorporate such genes into E. coli for mass production, deploying pretty much the same method as in how we scale up the production of synthetic insulin via genetically engineered E. coli. Additionally, we might be able to develop more desirable strains with enhanced enzyme efficiency.

C. Enzyme immobilisation

Switching gears, enzyme immobilisation is also another pathway we can pursue. This directly staves off the main operational weaknesses of free enzymes in terms of stability and reusability. Research has demonstrated that PETase nanoflowers give 3.5-fold higher terephthalic acid (TPA), indicating a more efficient output compared to free enzymes. Most importantly, immobilised enzymes can achieve >90% depolymerisation of untreated amorphous post-consumer PET and nearly full degradation in just 6 days.

Conclusion

For decades, plastics have been inundating landfills owing to their high molecular weight and dense chemical bonds, which render them recalcitrant to natural decomposers. While alternative materials such as biodegradable plastics offer us with a hopeful substitute, they have stagnated under commercial pressure - mainly put down to high production costs, excessive flimsiness which hampers their convenience and an inability to truly degrade in the wild. In the meanwhile, the increasing discovery of plastic-degrading bacteria and enzymes within mangrove ecosystems in recent years is silently shifting the global conversation. Admittedly, transforming this biological breakthrough into a global solution is not without its hurdles. The biochemical challenges of optimising enzymatic efficiency and possible leaching of harmful by-products are holding things back. Moving beyond, we can however very well overcome them through technological advancements. Genetic engineering and enzyme immobilisation has already equipped us with the tools to manipulate microbes and its enzymes. If scientists can successfully incorporate highly concentrated mangrove-derived bacteria or enzymes into closed-loop bio-recycling systems, we may finally be able to manage plastic waste effectively. Along the way, nature has somehow provided us with the resources to solve one of the world’s greatest crises. Our future now depends on how efficiently we can upscale it.

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This article was prepared by Low Yi Xian (Tunku Abdul Rahman University).

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