Microorganisms developed biodegradable plastics long before humans began producing them. Many bacteria and archaea naturally create compounds called polyhydroxyalkanoates (PHAs), storing these natural biopolymers inside their cells as reserves of carbon and energy.
Scientists have long believed that microorganisms were the only organisms capable of breaking down these natural plastics. However, a new study from the Max Planck Institute for Marine Microbiology in Bremen, Germany, challenges that assumption. Published in Nature Ecology & Evolution, the research shows that a wide range of animals—including marine worms, starfish, earthworms, and other terrestrial species—possess enzymes that can break down microbial PHAs. The discovery reveals a previously unknown pathway through which carbon stored by microbes may enter animal food webs.
Gutless Marine Worm Reveals Hidden PHA-Degrading Ability
The investigation began with the rare marine worm Olavius algarvensis. Unlike most animals, this gutless worm has no mouth or digestive tract. Instead, it depends on symbiotic bacteria living beneath its skin. These bacteria provide the worm with nutrients.
“One of the symbiotic bacteria of this worm stores huge amounts of carbon as PHA,” said corresponding author Nicole Dubilier, director of the Max Planck Institute for Marine Microbiology. “We wondered whether the worm had evolved a way to access this abundant energy reserve.”
The answer was yes. Researchers identified enzymes in the worm that break down microbial PHAs into smaller molecules that animals can use. High-resolution images also showed that the enzymes are produced in the same location where the worm digests its bacterial partners. This finding suggests that O. algarvensis can use the PHA stored inside its symbiotic bacteria.
The discovery was not limited to a single marine species. When researchers analyzed animal genomes more broadly, they found related enzymes in more than 66 species across nine different phyla. Laboratory experiments showed that enzymes from distantly related animals—including sponges, earthworms, and springtails—can also degrade microbial PHAs.
“This was a real surprise,” said lead author Caroline Zeidler of the Max Planck Institute for Marine Microbiology. “What began as a discovery in a single marine worm turned out to be a widespread ability shared by animals from very different branches of the tree of life.”
What Are PHA Bioplastics?
PHAs serve as carbon and energy stores for microorganisms, but they are also used to manufacture biodegradable plastics. During industrial production, bacteria are grown in large fermentation tanks and fed carbon-rich materials such as sugar, starch, and vegetable oil. Under the right conditions, the bacteria accumulate large quantities of PHA, which can then be extracted and processed into plastic materials.
PHA bioplastics are moldable, relatively water-resistant, and stable enough for many everyday applications. They are used in food packaging and sanitary products. In agriculture, fertilizers can be enclosed in PHA beads so their contents are released gradually as the plastic breaks down. Medical applications include wound dressings, drug-delivery systems, and absorbable implants and sutures that slowly degrade inside the body.
PHA is especially notable because it supports a biological circular economy. Microorganisms produce it, and biological processes can break it down. Although PHA currently represents only a small share of the global bioplastics market, its use may expand as demand grows for bio-based and biodegradable materials.
The latest research adds another important dimension to the natural life cycle of PHA, showing that many aquatic and terrestrial animals also possess enzymes capable of degrading these microbial bioplastics.
Natural Bioplastics Are Widespread in the Environment
Microbial PHAs occur naturally in soils, sediments, and aquatic environments worldwide. Microorganisms produce them when they have more carbon available than they immediately need, storing the excess for future use. PHA is also one of the few naturally occurring plastics that can be completely biodegraded.
Interest in PHAs is increasing because they are being developed as a sustainable alternative to conventional petroleum-based plastics. As production expands, scientists are studying how these natural biopolymers behave and break down in the environment.
The findings from Dubilier and her colleagues suggest that animals, as well as microorganisms, may contribute to the breakdown of natural bioplastics. More importantly, the research shows that animals can access microbial carbon reserves that scientists previously believed were unavailable to them.
“Our study changes our understanding of who can exploit these microbial carbon stores,” said co-corresponding author Maggie Soggin, who conducted much of the research at the Max Planck Institute for Marine Microbiology and is now an assistant professor at the University of California, Merced. “Animals have probably been consuming naturally occurring bioplastics for hundreds of millions of years, and we are only now discovering it.”
A New Pathway in the Carbon Cycle
Scientists do not yet know how widespread this process is in natural ecosystems or how significantly it contributes to the global carbon cycle. Even so, the discovery offers a new perspective on the relationship between microorganisms and animals.
It also demonstrates how studying rare organisms can reveal biological processes that may have existed for hundreds of millions of years but remained hidden from science. The ability of animals to break down microbial PHAs could help researchers better understand carbon transfer through ecosystems and the environmental fate of biodegradable plastics.
Source: www.sciencedaily.com