Our planet is littered with plastics. Since the 1950s, over 6 billion tonnes of it have been dumped in landfills, burnt in incinerators or strewn in rivers and oceans. Less than a tenth of it is recycled. When burnt, plastics release planet-warming carbon dioxide and other toxic fumes. Those tossed out in landfills leach out harmful chemicals over time. The ones in the oceans gradually break up into tiny fragments—micro and nano plastics—and circulate through our food chains, posing a major health concern.
Made from byproducts of the fossil fuel industry, plastics are synthetic polymers with tens of thousands of repeating small molecules called monomers. The aromatic organic molecules in plastics contain atoms of carbon, hydrogen and oxygen among others.
Based on the chemical composition of the monomers used, plastics come in many forms. The most produced are polyethylene (used to make plastic bags and most plastic objects we use), polystyrene (used for making single-use cups, plates and takeout boxes) and polypropylene (film-like plastic used for packaging).
Bioremediation with microbes
As recycling and incineration haven’t succeeded in handling all the plastic waste, plastic-eating microbes are emerging as a promising solution. In scientist-speak, this process of using microbes and enzymes to break down environmental pollutants is called bioremediation. Worldwide, scientists have identified nearly a hundred such microbes in landfills, sewage, oceans, wetlands and even the high Arctic.
“Bio-remediation holds immense potential in mitigating the escalating burden of plastic waste,” says Prof Pravindra Kumar, from the Indian Institute of Technology Roorkee, whose research involves finding ways to break down plastics and plasticizers—additives added to plastics to make them less brittle and more flexible—with the help of biology. “Microbes and their enzymes have emerged as the most promising contenders in this regard.”
Unlike dumping in landfills or burning, degrading plastic waste with microbes and enzymes is environmentally friendly and cost-effective. “This mode of clean-up is efficient, feasible and sustainable to tackle environmental pollution,” says Prof Prashant S Phale, from the Indian Institute of Technology, Bombay, who is searching for microbes that digest aromatic compounds, including plastics, in soil, wastewater, agricultural fields and dump sites.
But, can the tiny, invisible microbes handle the ginormous mess humans have created? Yes, but not yet.
Search for better microbes
Plastics are indispensable in our lives because they are sturdy, lightweight, resistant to water and don’t easily wear out. But these properties also make it hard to break the polymer chains. The first step in plastic degradation is to convert the water-resistant polymer molecules into water-loving forms like ketones, aldehydes or alcohols.
However, not many microbes can do that. So scientists use ultraviolet light or heat to oxidise plastic polymers and make them conducive for microbes to grow. Then, the oxidised polymer chains must be broken up into smaller fragments, which can then be digested by the microbes, releasing carbon dioxide and water. As microbes act on plastic, it loses its weight, its mechanical properties and its chemical composition.
As elegant as this process looks, identifying these plastic-degrading microbes can be a long, exhausting search involving laboratory experiments, genetic analysis and serendipity. So far, bacterial species belonging to the genus Pseudomonas, Bacillus and Rhodococcus, and the fungus Aspergillus have proven to degrade some of the commonly-used plastics.
Interestingly, the gut bacteria in some insect larvae, like the greater wax worm (Galleria mellonella), Indian meal moth (Plodia interpunctella), yellow meal moth (Tenebrio molitor), and super worm (Zophobas atratus) are known to digest plastics when the larvae are fed a diet of plastic waste.
In India, scientists have been on the hunt for plastic-eating microbes in recent decades. A 2018 study describes how biologists from Shiv Nadar University, led by Richa Priyadarshini, scouted the Dadri wetlands and identified two strains of bacteria Exiguobacterium that degraded polystyrene. The researchers are now trying to understand the genetics and metabolic activities involved in this process, which can then help scale it up.
A 2019 study by researchers from Pune has identified two mangrove-living strains of Aspergillus terreus and Aspergillus sydowii that can degrade polyethylene. Last year, researchers studying plastic debris in the Andamans identified six bacterial species that could degrade low-density polyethylene.
Phale’s research is involved in isolating bacterial strains that rapidly degrade aromatic compounds, plastics and plasticizers (additives added to plastics). Recently, his group reported the discovery of a new soil bacteria, called Pseudomonas bharatica CSV86T from a petrol station in Bengaluru.
While most bacteria break down simple sugars like glucose, this species prefers degrading aromatic pollutants, including plastics, while also promoting plant growth. Now, they are developing a concoction of bacteria that can work on contaminated sites and agricultural fields.
Enzymes secreted by plastic-digesting bacteria can help degrade plastics. In his work, Kumar is identifying a range of such enzymes that can act on plastics and plasticizers and engineer them to increase their efficiency. Now, researchers in his group are working on a technology to reduce plasticizers in water sources and patent their engineered enzymes for commercial uses. They are also exploring sewage treatment plants to look for new plastic-digesting bacteria.