“When you look closely at the issue of plastics, the marine environment is emblematic. It plays a central role in international awareness regarding the scope of plastic and microplastic pollution,” emphasizes Cousin, who works in the MARBEC laboratory specializing in the study of marine environments, biodiversity, and the causes of its decline. “However, to grasp all of the issues connected to this pollution, we must adopt a global approach, for plastic is ubiquitous in our life and environment. It interacts with all natural environments and human activities.”

From the environment to health

To illustrate the nature and complexity of these interactions, Cousin takes the example of the “plastisphere.” Within a marine environment, plastics represent a niche conducive to hosting and developing bacterial communities and other micro-organisms. This involves plastics of all sizes, including microplastics (less than 5 millimetres), which are abundantly present.

“The bacterial communities we observe in the plastisphere do indeed come from the sea, where they exist at much lower concentration levels. However, with plastics they find conditions favourable to their development. At the MARBEC laboratory, scientists observed that the presence of certain bacterial groups is up to 200 times greater on plastics than in the water.”

This concentration has consequences on both the environment and health. The presence of pathogenic bacteria, as well as the increasing spread of antibiotic resistance, are two heightened exposure and risk factors cited by Cousin.

Interdisciplinarity and international negotiations

To protect the health of animals, human beings, and the environment, all of the effects of plastic must be taken into account, namely by following its entire life cycle from raw material extraction (99% petroleum) to waste. It is also important to consider, aside from the plastisphere, the chemical products associated with plastics. There are over 16,000 chemical substances present in plastics, with toxicity being proven for one quarter of them, and unknown for two-thirds².

This is the task that the Intergovernmental Negotiating Committee (INC) on Plastic Pollution set out for itself when the United Nations Environment Assembly adopted the resolution in 2022: to develop a legally binding international instrument to halt plastic pollution.

The effort to craft this global accord was led by the INC, with participation from numerous scientists and research laboratories, who notably met as part of the Scientists’ Coalition for an Effective Plastics Treaty, which includes over 400 scientists from 70 countries. All research fields connected to plastic pollution are represented. Cousin, a member of the coalition, points out that “the role of researchers in the negotiation process is to provide the data and scientific analyses needed to build tenable scenarios, and to elucidate the decisions made by delegations. It is also essential to explaining the risks and limitations of the technological innovations that are presented as solutions.”

While the first rounds of negotiation have not led to an agreement, Cousin nevertheless has noted “a significant evolution in the number of countries expressing high ambitions for the treaty.” He estimates that this number rose from sixty countries before CIN 4 in 2024, to over one hundred today, an encouraging sign for the working sessions to come.

Exploring new research focuses: the toxicity of biodegradable plastics

Biodegradable plastics feature prominently among the solutions proposed for reducing plastic pollution. Their adoption nevertheless raises questions. While the characteristics of these plastics appear beneficial for the environment, do they have toxic effects nonetheless?

The BIOMIC project seeks to answer this question by studying the effects of biodegradable microplastics on fish physiology and microbiota. It is based on chronic long-term exposure in marine environments to microplastics from three types of plastics: PLA, a common biodegradable plastic; PHBV, which is biodegradable in a marine environment; and a conventional plastic, polystyrene. This approach differs from standard “acute toxicity” tests, conducted each time a new chemical substance is brought to market in order to measure the effects of short-term exposure in accordance with specific standards. Cousin believes that such tests do not “look at the problem of toxicity in its entirety.”

The exposure and individual analysis phase for the BIOMIC project is currently being assessed. The model species of fish being studied has a short life cycle, with only a few months from the egg to becoming a reproducing adult, thereby allowing for rapid study of the effects on all stages of life, including growth, behaviour, reproduction, and survival.
At the same time, research is being conducted to determine if ingesting these microplastics leads to dysbiosis (microbiotic imbalance), or to the microbiota adapting to plastic degradation. “Dysbiosis is a toxic effect of plastics that is found in all organisms, including humans.”

Finally, scientists will study the causal links between microbiota modifications and impacts observed on an individual level. This will allow them to determine the effects of biodegradable microplastics on physiological function, essential knowledge for maintaining the population in its environment.

¹ Source : OECD (2024), Policy Scenarios for Eliminating Plastic Pollution by 2040, OECD Publications, Paris. https://www.oecd.org/en/publications/policy-scenarios-for-eliminating-plastic-pollution-by-2040_76400890-en.html
² Source : Wagner M. et al. (2025). State of the science on plastic chemicals - Identifying and addressing chemicals and polymers of concern, PlastChem, Zenodo https://doi.org/10.5281/zenodo.17208791