A tiny marine worm with no mouth, gut, or anus has helped overturn a long-standing assumption about how animals obtain carbon from microbes. Researchers at the Max Planck Institute for Marine Microbiology in Bremen, Germany, report that animals can produce enzymes capable of breaking down polyhydroxyalkanoates, or PHAs, natural microbial plastics once thought to be digestible only by microorganisms. The discovery, published in Nature Ecology & Evolution, suggests that animals ranging from marine worms and starfish to earthworms and springtails may have been exploiting these biological carbon reserves for hundreds of millions of years.
PHAs are biodegradable polymers manufactured by many bacteria and archaea. Inside microbial cells, they function much like fat stores in animals: when carbon is abundant but other nutrients are limited, microorganisms convert the surplus into dense intracellular granules that can later be consumed for energy and growth. Chemically, PHAs are long chains made from hydroxyalkanoate building blocks linked by ester bonds. Their structure gives them plastic-like properties, while also making them susceptible to enzymatic hydrolysis under the right biological conditions. Because PHAs are produced naturally in soils, sediments, and aquatic ecosystems, they represent one of the most widespread forms of microbial carbon storage on Earth.
The investigation began with Olavius algarvensis, a slender marine worm found in shallow Mediterranean sediments. The animal is only about two centimetres long, yet it has abandoned several organs that most animals consider essential. It lacks a mouth and digestive tract, and it has no conventional excretory system. Instead, the worm lives in close association with bacterial symbionts positioned beneath its skin. These bacteria provide nutrients, while the worm supplies them with a protected habitat and access to compounds from the surrounding environment. The relationship is so intimate that the worm’s pale appearance comes from the dense layer of symbiotic cells packed throughout its body wall.
One of the worm’s bacterial partners stores unusually large quantities of PHA. That observation raised a fundamental question: when O. algarvensis digests its symbionts, can it also access the carbon locked inside their microbial plastic granules? The researchers identified an animal enzyme that can degrade PHA into smaller molecules. These products can then be metabolized through ordinary biochemical pathways, allowing the worm to recover carbon and energy from a compound that had previously been considered largely unavailable to animals. In effect, the worm appears to possess a molecular tool for opening a microbial energy reserve.
High-resolution imaging provided important evidence about how this process works inside the animal. The PHA-degrading enzyme was found precisely where the worm’s symbiotic bacteria are digested. Its location is significant because enzymes often reveal their biological function through the cellular compartments in which they operate. By producing the enzyme at the interface where bacterial cells are broken down, the worm may be able to digest both the microbial biomass and the PHA granules stored within its symbionts. The finding points to a coordinated nutritional strategy rather than an incidental ability to break down an environmental polymer.
The researchers next searched animal genomes to determine whether the enzyme was unique to this unusual worm. It was not. Related genes appeared in more than 66 animal species belonging to nine different phyla, a distribution that spans distant branches of the animal evolutionary tree. Laboratory tests strengthened the genomic evidence: enzymes from animals as different as a sponge, an earthworm, and a springtail were able to degrade microbial PHAs. Such phylogenetic breadth suggests that the capacity may be far more common than previously recognized, although the precise biochemical activity and ecological importance of each enzyme may vary between species.
The discovery could change how scientists think about carbon movement through ecosystems. Microbial PHA granules have generally been treated as a resource that cycles primarily within microbial communities. Bacteria produce the polymers, store them, and later use them when carbon is scarce; other microorganisms may also contribute to their decomposition. If animals can directly digest PHAs, however, microbial carbon can move into animal food webs without first being completely converted into simpler compounds by another microorganism. Animals that consume bacteria, feed on sediments, graze on biofilms, or live in close partnership with microbes may therefore gain access to a hidden source of energy.
The finding also adds an unexpected dimension to the environmental story of biodegradable plastics. PHAs are increasingly manufactured as alternatives to petroleum-based plastics because they are bio-based and can be broken down by biological processes. Industrial bacteria are grown in fermentation tanks and supplied with carbon-rich materials such as sugars, starches, or plant oils. Under carefully controlled conditions, the cells accumulate PHA, which is then extracted and processed into materials for packaging, hygiene products, agricultural coatings, and medical applications. PHA-based compounds are also being investigated for drug delivery, wound dressings, resorbable sutures, and implants designed to gradually disappear inside the body.
Although PHAs are marketed as biodegradable, their breakdown depends on environmental conditions, microbial communities, polymer chemistry, and the availability of appropriate enzymes. The new study indicates that animals may join microorganisms as active participants in this process. That does not mean animals will rapidly eliminate discarded PHA products in every ecosystem, nor does it establish how much animal-mediated degradation occurs outside the laboratory. Instead, it reveals an overlooked biological possibility that now requires ecological testing. Researchers will need to determine which animals consume PHA directly, whether the enzymes are produced continuously or only under particular nutritional conditions, and how much carbon animals recover from the polymers.
The broader lesson is that evolutionary innovation can remain hidden in organisms that attract little attention. Olavius algarvensis evolved an extreme dependence on its bacterial partners, and that dependence led scientists to a metabolic capability that appears across much of the animal kingdom. The work shows that microbial chemistry is not necessarily confined to microbes: animals can acquire, modify, and exploit compounds produced by their microscopic partners. As researchers continue to examine the molecular exchange between animals and microorganisms, natural systems may reveal many more examples of biological materials being used in ways that conventional textbooks never anticipated.
Subject of Research: Animals
Article Title: Animal degradation of microbial storage polyhydroxyalkanoates
News Publication Date: 13-Aug-2026
Web References: https://doi.org/10.1038/s41559-026-03153-8
References: Nature Ecology & Evolution, DOI: 10.1038/s41559-026-03153-8
Image Credits: Alexander Gruhl / Max Planck Institute for Marine Microbiology
Keywords
polyhydroxyalkanoates, PHA, biodegradable plastics, microbial carbon, animal enzymes, symbiosis, marine worms, Olavius algarvensis, carbon cycling, microbiology, animal evolution, bioplastics
