Bacteria armed with the M5 motif on their PETase enzyme can feast on plastic, a trait now seen thriving across the world’s oceans.
Credit: 2025 KAUST
A newly discovered enzyme motif reveals how ocean microbes are evolving to digest plastic, potentially aiding future cleanup efforts.
Hidden in the depths of the ocean, scientists have discovered marine bacteria equipped with enzymes that can consume plastic, their evolution shaped by humanity’s discarded waste.
According to a global study by researchers at KAUST, these microscopic recyclers are not only abundant but also genetically adapted to break down polyethylene terephthalate (PET), the tough plastic used in products such as beverage bottles and fabrics.
The key to their ability lies in a distinctive structural feature of the PET-degrading enzyme, called PETase. This identifying mark, known as the M5 motif, serves as a molecular signature of the enzyme’s plastic-eating power.
“The M5 motif acts like a fingerprint that tells us when a PETase is likely to be functional, able to break down PET plastic,” explains Carlos Duarte, a marine ecologist and co-leader of the study. “Its discovery helps us understand how these enzymes evolved from other hydrocarbon-degrading enzymes,” he says. “In the ocean, where carbon is scarce, microbes seem to have fine-tuned these enzymes to make use of this new, human-made carbon source: plastic.”
From Indestructible Plastic to Microbial Feast
For years, scientists believed that PET could not be naturally broken down. That view began to change in 2016, when researchers identified a bacterium living in a Japanese recycling facility that was thriving on plastic waste. This organism had developed an enzyme, known as a PETase, that could dismantle PET into its basic building blocks.
However, scientists were still unsure whether ocean-dwelling microbes had evolved similar enzymes.
Through a combination of AI-driven structural modeling, extensive genetic analysis, and laboratory testing, Duarte and his team discovered that a specific feature called the M5 motif distinguishes genuine PET-degrading microbes from those that only resemble them. Marine bacteria possessing the complete motif were able to efficiently break down PET in the lab. Further gene expression studies showed that M5-PETase genes are highly active throughout the oceans, particularly in regions heavily polluted with plastic.
To chart the global spread of these enzymes, the team analyzed more than 400 ocean samples from across the seven seas, finding functional versions with the M5 motif in nearly 80 percent of the waters tested — from rubbish-rich surface gyres to nutrient-starved depths two kilometers down. In the latter, the ability to snack on synthetic carbon may confer a crucial survival advantage, according to Intikhab Alam, a senior bioinformatics researcher who co-led the study.
A Slow Natural Response to Human Pollution
Ecologically, the rise of these enzymes signals an early microbial response to humanity’s planetary littering.
Duarte warns that nature’s cleanup crew works far too slowly to rescue the seas. “By the time plastics reach the deep sea, the risks to marine life and human consumers have already been inflicted,” he says.
On land, however, the discovery could fast-track industrial enzyme design for closed-loop recycling. “The range of PET-degrading enzymes spontaneously evolved in the deep sea provides models to be optimized in the lab for use in efficiently degrading plastics in treatment plants and, eventually, at home,” Duarte notes.
To that end, the M5 motif now offers the blueprint, pinpointing the structural tweaks that matter in real-world conditions, not just in a test tube. If scientists can harness those tweaks, then — as the world gropes for ways to tidy its plastic mess — they may find unlikely allies in the abyss: bacteria that already turn waste into lunch.
The Life of Earth
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