Hong Kong researchers tracked fluorescent PLA particles into rainbow trout blood and used single-cell sequencing to show that breaking down is not the same as being safe.

Biodegradable plastics are sold on a premise that sounds complete: the material eventually breaks down, so the problem of persistence goes away. The premise is about what happens to the plastic. It says nothing about what happens to the organism in the meantime.

Researchers at City University of Hong Kong have gone looking for the second part. Writing in Environmental Chemistry and Ecotoxicology, they tracked particles of polylactic acid, one of the most widely used biodegradable plastics, into the blood of rainbow trout, and found changes in the balance and activity of several kinds of blood cell, particularly those involved in immune defence.

“Biodegradable plastics are often seen as a solution to plastic pollution, but their ability to break down does not automatically guarantee their safety,” said Wen-Xiong Wang, who led the study. “Our study shows why it is important to consider what happens to these materials as they age in the environment.”

Why blood

Most work on microplastic toxicity examines organs: gut, liver, gill. The choice here was deliberate and different.

“Blood travels throughout the body and connects different organs, making it a promising place to look for early signs of environmental stress,” said Difei Tong, the study’s first author. “Instead of focusing only on damage to individual organs, we wanted to understand how blood cells respond to microplastics and whether these responses could help us detect their potential hazards.”

The logic is sound. Blood is both the route by which anything absorbed travels and the medium in which the immune system operates, so a disturbance there is visible early and reaches everywhere.

Labelled particles and single-cell sequencing

Two techniques carried the study. The first was fluorescent labelling: the PLA particles were tagged so they could be located in tissue under a microscope, which establishes that they reached the blood rather than being inferred to have done so.

The second was single-cell RNA sequencing, which measures gene activity in individual cells rather than averaging across a sample. The distinction matters here. If a tissue contains several cell types and only one responds to an exposure, bulk measurement dilutes the signal to near-invisibility. Single-cell methods keep each cell’s profile separate, so the responding population can be identified and the shift in proportions between types can be seen.

That is what the technique found: changes in the relative abundance of blood cell types and in their activity, concentrated among immune cells, together with evidence that exposure affected how blood cells communicate with one another.

“We found that PLA microplastics were associated with changes in the balance and activity of several types of blood cells, particularly those involved in immune defense,” Tong said. “There’s also evidence suggesting that exposure could affect how blood cells communicate with one another.”

Cell-to-cell signalling is how an immune response is coordinated. A disturbance there is not obviously damage in the way a lesion is, but an immune system whose cells are miscommunicating is not working properly either.

Weathered plastic behaves differently

The finding with the widest implications concerns ageing. The team exposed some particles to simulated sunlight before using them, and the aged particles produced different and more pronounced effects, including on the shape of red blood cells.

“Importantly, when the plastic particles were aged under simulated sunlight, their effects changed,” Tong said. “These findings suggest that the risks of biodegradable plastics may change as the materials weather in the environment.”

This cuts at how such materials are tested. Safety assessments are typically run on pristine material, fresh from manufacture. Nothing in the environment is pristine. Sunlight breaks polymer chains, roughens surfaces, generates reactive fragments and alters surface chemistry, and plastic in water or sediment has usually been weathering for months or years.

If weathered particles are more biologically active than new ones, then the standard test examines the least troubling version of the material. The direction of that error is the unhelpful one.

Red blood cell shape is a case in point. The deformability of a red cell determines whether it can pass through the narrowest capillaries, and a change in shape is a change in function, not merely in appearance.

What the study does not establish

The release reports no numerical results: no concentrations, no exposure durations, no effect sizes, no numbers of fish. Without concentrations in particular it is impossible to say whether the exposures resemble anything a wild trout would encounter or sit far above it, which is the central question in any toxicology study.

Nor does a change in blood cell populations establish that the fish were harmed. The step from a molecular or cellular signal to an effect on survival, growth or reproduction is a real one, and this study does not take it.

What it does offer is a method. Wang frames the contribution that way: “By providing a broader picture of these responses, this approach could help scientists identify potential hazards and contribute to the development of safer biodegradable materials.”

That framing is the honest one. Single-cell sequencing of blood gives a sensitive early readout, which is useful for screening candidate materials and for deciding which to look at more closely. It does not, by itself, tell anyone whether PLA in a river is hurting the fish.

The work was funded by the General Research Fund of the Hong Kong Research Grants Council and the National Science Foundation of China. Wang also holds a five-year Senior Research Fellowship from the Hong Kong Research Grants Council.

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Notes

1. Tong, D., Wang, W.-X. “Single-cell transcriptomic profiling of fish blood reveals responses to pristine and photoaged polylactic acid microplastics.” Environmental Chemistry and Ecotoxicology, 2026. DOI: 10.1016/j.enceco.2026.09.003.
2. KeAi Communications Co., Ltd. “Scientists uncover hidden risks of biodegradable plastics through fish blood.” EurekAlert!, 19 October 2026. https://www.eurekalert.org/news-releases/1148468.