Long before humans invented plastics, microorganisms were already making biodegradable ones — and new research suggests animals have been eating them for hundreds of millions of years. A study published in Nature Ecology & Evolution by the Max Planck Institute for Marine Microbiology challenges the long-held assumption that only microbes can break down polyhydroxyalkanoates (PHAs), the natural bioplastics that bacteria and archaea store inside their cells as carbon and energy reserves.
The investigation began with an unusual subject: Olavius algarvensis, a marine worm with no mouth or gut that lives off symbiotic bacteria beneath its skin. Researchers wondered whether the worm had evolved a way to access the enormous carbon reserves its bacterial partners store as PHA. It had: the team identified an enzyme in the worm that breaks microbial PHAs into smaller molecules animals can use, and high-resolution imaging showed the enzyme is produced exactly where the worm digests its symbionts.
The discovery then broadened dramatically. When the researchers examined animal genomes, they found related enzymes in more than 66 species spanning nine different phyla. Laboratory tests confirmed that enzymes from very distantly related animals — a sponge, an earthworm and a springtail — could all degrade microbial PHAs. "What started as a discovery in a single marine worm turned out to be a widespread capability shared by animals from very different branches of the tree of life," says first author Caroline Zeidler.
The finding has implications beyond evolutionary biology. PHAs are among the few naturally occurring plastics that are completely biodegradable, and industry already produces them as sustainable alternatives to conventional plastics — used in food packaging, hygiene materials, slow-release agricultural fertilizers, wound dressings and resorbable sutures. If animals throughout the food web can digest these materials, it changes how scientists think about their fate in the environment, and points to a previously unrecognized pathway through which carbon stored by microbes moves into animal food webs.
Microbial PHAs are widespread in soils, sediments and aquatic environments worldwide. "Our study changes our understanding of who can use these microbial carbon stores," says co-corresponding author Maggie Sogin, now at the University of California, Merced. "Animals have probably been feeding on nature's original bioplastic for hundreds of millions of years — we're only discovering it now." How much this process contributes to global carbon cycling remains unknown, but the discovery demonstrates how research on unusual organisms can expose biological processes that have stayed hidden for eons.
