Biodegradable Plastic Leachates Slow Growth of Marine Microalgae in Lab Tests

Biodegradable plastics have been marketed as the green answer to one of the ocean’s most stubborn problems, but a new laboratory study from Indonesia suggests that the story is far more complicated. Researchers reporting in the journal Ecotoxicology have found that chemical leachates released by biodegradable polypropylene microplastics can significantly suppress the growth of marine microalgae, the microscopic photosynthetic organisms that anchor entire ocean food webs. The findings, published as an initial toxicity assessment, add to a growing body of evidence that the breakdown products of so-called eco-friendly plastics may carry their own ecological risks, even when the materials themselves are designed to disappear.

The research team, led by Nur Aini Azizah of Brawijaya University and involving scientists from Indonesia’s National Research and Innovation Agency (BRIN), set out to test a question that has received surprisingly little attention: what happens to marine algae when they are exposed to the dissolved chemicals that seep out of degrading biodegradable plastics? While conventional plastics such as polyethylene and polystyrene have been studied extensively for their effects on aquatic life, biodegradable variants have often been assumed to be benign. The new study challenges that assumption with hard numbers.

The experiment focused on three species of marine microalgae that play important roles in coastal ecosystems and aquaculture: Chlorella sp., Tetraselmis sp., and Navicula sp. These organisms were chosen because they are widely used in toxicity testing, reproduce quickly, and respond measurably to chemical stress. Before any biological testing began, the researchers used Fourier Transform Infrared spectroscopy, or FTIR, to confirm the polymer identity of the biodegradable polypropylene material used in their experiments. This analytical step matters because the market for biodegradable plastics is crowded with materials of uncertain composition, and verifying what was actually tested is essential for reproducibility.

Toxicity assays were conducted under controlled laboratory conditions following a standard 96-hour exposure window, a protocol consistent with internationally recognized guidelines for static toxicity testing with microalgae. The researchers measured growth by tracking changes in cell density over time, and for species showing significant inhibition, they also measured chlorophyll-a content, the pigment at the heart of photosynthesis. A drop in chlorophyll-a signals more than slower division; it indicates that the cellular machinery of energy capture itself is being disrupted, which can cascade through growth, reproduction, and ultimately the productivity of entire algal communities.

The first round of testing, a preliminary range-finder phase, revealed something ecologically important: the three algal species did not respond uniformly. Navicula sp., a diatom, showed no clear concentration-dependent inhibition of growth, and the team therefore excluded it from the definitive test. That kind of species-specific variation is a recurring theme in microplastic toxicology. It suggests that the ecological consequences of plastic leachates will not be evenly distributed across marine communities, and that some groups of organisms may be quietly bearing the brunt of chemical exposure while others appear unaffected.

The definitive testing on the two remaining species produced striking results. Chlorella sp. proved to be the most sensitive organism, with a 96-hour median inhibition concentration, or IC50, of just 0.49 milligrams per liter. In practical terms, that means the leachate from biodegradable polypropylene microplastics cut Chlorella growth in half at a concentration of less than half a milligram of leachate per liter of water. Tetraselmis sp. was also affected in a concentration-dependent manner, but it required a far higher dose, with an IC50 of 37.84 milligrams per liter, roughly 77 times greater than the threshold for Chlorella.

That nearly two-order-of-magnitude difference in sensitivity between two green microalgae is one of the most technically interesting aspects of the study. It underscores why single-species toxicity tests, while useful, can be misleading when extrapolated to ecosystems. If regulators were to rely on Tetraselmis alone as a test organism, they might conclude that biodegradable polypropylene leachates pose only a modest hazard. The extreme vulnerability of Chlorella tells a different story. Species sensitivity distributions, which pool responses across many organisms, are increasingly seen as the more defensible basis for environmental risk assessment, and this study provides data points for exactly that kind of framework.

The environmental context of the work is grounded in the Indonesian archipelago, a region where plastic pollution is severe and well documented. Complementary field observations on microplastic occurrence along Indonesian coasts, cited by the authors, underscore the relevance of the laboratory findings. Previous studies have catalogued microplastics in the coastal waters of Surabaya, in mangrove sediments near Jakarta, and in fish from local markets, establishing that plastic fragments are pervasive in Indonesian marine environments. As biodegradable plastics enter those same waters, their fragmentation into microplastics and the subsequent release of leachates becomes a realistic exposure scenario rather than a theoretical one.

The new results also fit into a broader international picture. Earlier research has shown that plastic leachates can impair growth and oxygen production in Prochlorococcus, the ocean’s most abundant photosynthetic bacterium, and that leachates from weathered plastics can be more toxic than those from pristine materials. Other studies have documented leachate effects on coral fertilization, mussel larvae, and benthic invertebrates. What the Indonesian team adds is a specific demonstration that biodegradable polypropylene, a material promoted partly on the assumption that it solves the persistence problem, can generate dissolved chemicals capable of measurable sublethal harm to marine primary producers at low concentrations.

The authors are careful to frame their work as an initial toxicity assessment, and that caution is warranted. The study does not identify the specific chemical compounds responsible for the inhibition, does not examine effects beyond 96 hours, and does not test whether environmentally realistic leachate concentrations overlap with the doses that harmed Chlorella in the laboratory. Long-term, multigenerational studies, chemical characterization of the leachates, and field validation will all be needed before the ecological risk can be quantified. Nevertheless, the central message is difficult to ignore: biodegradability is not the same as harmlessness. As the world races to replace conventional plastics with alternatives, the new findings argue that every substitute material deserves the same rigorous ecotoxicological scrutiny that the originals received, before it is deployed at planetary scale.

Subject of Research: Toxicity of biodegradable polypropylene microplastic leachates to marine microalgae

Article Title: Growth and chlorophyll-a responses of marine microalgae to biodegradable polypropylene microplastic leachates: an initial toxicity assessment

Article References: Growth and chlorophyll-a responses of marine microalgae to biodegradable polypropylene microplastic leachates: an initial toxicity assessment. (n.d.). https://doi.org/10.1007/s10646-026-03157-x

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03157-x

Keywords: biodegradable plastics, polypropylene, microplastics, leachates, marine microalgae, Chlorella, Tetraselmis, Navicula, chlorophyll-a, IC50, ecotoxicology, Indonesia