Various animal species possess enzymes that enable them to break down PHA plastics
The gutless marine worm Olavius algarvensis (here lying next to some sand grains) is only about two centimetres long and has become so dependent on its bacterial symbionts for nutrition and waste recycling that it has lost both its digestive and excretory systems. Its white appearance comes from the dense layer of bacterial symbionts beneath its skin, which are packed with PHA, the microbial bioplastic at the center of this study. This inconspicuous worm helped scientists discover that animals can digest microbial bioplastics previously thought to be broken down only by microorganisms.
© Alexander Gruhl/ MPI f. marine Mikrobiologie
To the Point
- Intestine-less worm: The tiny marine worm Olavius algarvensis has no intestines or excretory organs; it feeds on bacteria beneath its skin and uses polyhydroxyalkanoates (PHA) as an energy source.
- Distribution of the enzymes: Related PHA-degrading enzymes have been found in over 66 animal species across nine phyla, including marine worms, starfish, earthworms, and sponges.
- Sustainability and research: Understanding the natural degradation of PHAs is important for promoting their sustainable use as an alternative to conventional plastics.
Long before humans discovered biodegradable plastics, microorganisms had already invented their own. Many bacteria and archaea produce natural bioplastics called polyhydroxyalkanoates (PHAs), storing them inside their cells as reserves of carbon and energy.
Until now, scientists thought that only microorganisms themselves could break down these substances. Researchers at the Max Planck Institute for Marine Microbiology in Bremen, Germany, have now overturned that long-standing assumption. In a study published in Nature Ecology & Evolution, they show that animals ranging from marine worms and starfish to terrestrial species including earthworms have enzymes capable of degrading microbial PHAs. The findings reveal a previously overlooked way in which microbial carbon can enter animal food webs.
A worm with symbionts
Hunting for gutless worms on Elba, Italy: Divers collect sediment from the seafloor, where Olavius algarvensis lives buried beneath Mediterranean seagrass meadows.
© Kristina Weinert/ MPI f. marine Mikrobiologie
The story began with an unusual marine worm called Olavius algarvensis. It has neither a mouth nor a gut. Instead, it farms symbiotic bacteria beneath its skin and digests them for food.”One of the worm’s bacterial symbionts stores enormous amounts of carbon as PHA,” says corresponding author Nicole Dubilier, Director at the Max Planck Institute for Marine Microbiology. “We wondered whether the worm had evolved a way to access this rich energy reserve.”
The answer was: Yes. The researchers discovered an enzyme in the worm that breaks down microbial PHAs into small molecules animals can use. High resolution images further showed that the enzyme is produced exactly where the worm digests its symbionts. This suggests that the worm can access the PHA stored by its bacterial partners.
