Pesticides: when oysters pay, over several generations, the price of contamination

Pesticides are everywhere. From agricultural soils to coastlines, all ecosystems are contaminated. With multiple impacts, as recalled at the end of 2023 by a parliamentary commission of inquiry as well as a collective INRAE-Ifremer scientific expertise in 2022: degradation of water quality, decline in biodiversity and in particular of species such as terrestrial invertebrates and aquatic, birds, bats and even amphibians. This reinforces the feeling of helplessness in the face of the use and effects of these pesticides. Theoretically, the Ecophyto plan should however make it possible to reduce the use of phytosanitary products by 50% by 2030. But in practice, the new indicator decided in May 2024 has fueled controversy. According to some scientists, it could cause a purely artificial drop in the monitoring of pesticides used.

Among the species affected by this pollution, we also find mollusks, and in particular populations of cupped oysters (Crassostrea gigas), which constitute one of the main aquaculture resources in the world. And this while oyster production basins are particularly affected by chemical pollution due to their proximity to watersheds. In France, pesticides are regularly detected in shellfish production areas where the renewal of production is partly based on the natural recruitment of larvae, such as in the Marennes Oléron or Arcachon basins. To better understand the effects of coastal contamination by pesticides on invertebrate organisms such as the Pacific oyster, we participated in a long-term research project. And the results are clear: exposure to pesticides affects the reproduction of oysters and some of the deleterious effects are delayed in subsequent generations.

Also read: Mediterranean oysters: the challenges of a sector facing climate change

The new challenges of ecotoxicology

To understand the originality of this research, we must first understand that ecotoxicology, a scientific discipline which studies the behavior and effects of pollutants on ecosystems, today faces several major challenges.

In particular, the embryonic and early life stages constitute critical periods for living things, during which they are more vulnerable to environmental factors. Epigenetics, which studies mechanisms that modify gene expression in a reversible, transmissible and adaptive manner without changing the DNA sequence, appears to be the basis of these mechanisms. However, most of the data available to date in environmental epigenetics come from models based on mammals. More recently, aquatic vertebrates have been studied, but little data is yet available for invertebrate organisms. So we wanted to fix it.

Monitor the effects of pollution on several generations of oysters

Shellfish farming areas, as we have seen, are particularly exposed to pollution. In addition, the oyster, distributed over a wide geographical area, is also a filter-feeding and sessile organism (which lives attached to a substrate and is therefore subject to environmental hazards). Therefore, it can be considered as a bioindicator of water quality. All of these elements make it a model species of choice for marine ecotoxicology studies.

As part of the ANR PESTO project, which took place between 2020 and 2024, we therefore proposed an experimental device that best represents the environmental reality of the contamination of cupped oysters. These were exposed early, during the first 48 hours of their embryo-larval development, to a mixture of 18 pesticides at low concentration (2.85 µg/L in total) representative of the concentrations measured on the coast.

Thanks to the zootechnical installations of the Ifremer marine mollusc platform located in Bouin (85), the animals were then able to be maintained in controlled conditions throughout their entire life cycle, making it possible to study a multitude of physiological and molecular studies to describe the long-term implications of this early exposure.

By repeating this over three generations, it was possible to reveal the multigenerational influence of this exposure, the direct effects of which were nevertheless limited.

Effects delayed over time…

During the first generation of oysters, no alteration was demonstrated using classic toxicity tests: neither the rate of developmental anomalies of the larvae (embryotoxicity) nor the rate of alteration of the d molecule DNA (genotoxicity) did not show a significant increase under the effect of exposure.

Similarly, high-resolution molecular technologies used to measure gene expression levels and DNA methylation after six hours of exposure revealed only slight alterations. Such as, a tendency towards loss of methylation and dysregulation of the expression of few genes, although a part of them are transcription factors involved in the regulation of important developmental processes.

However, notable delayed effects were then measured throughout the life cycle of the Pacific oysters monitored. Swimming performance, measured six days after the end of exposure, was reduced in larvae that had been exposed compared to control individuals (reduced speed and mobility).

Growth, reproduction, diseases…

Two weeks later, the exposed larvae also showed a lower capacity for metamorphosis compared to control individuals, linked to still marked demethylation of the DNA. These results suggest that fewer individuals were able to progress from the larval stage to the spat stage (juvenile oysters).

In adult individuals, a reduced sensitivity to summer mortality phenomena in the environment could be observed, suggesting better tolerance to pathogens. In one-year-old oysters, reproduction was affected and glycogen metabolism, involved in gamete production, was modified in early exposed individuals, increasing their reproductive success.

…and intergenerational effects observed on oysters

It remained to be determined whether early exposure of cupped oysters to pesticides could have consequences for subsequent generations. The research project suggests that this is indeed the case.

Changes in gene expression as well as marked hyper-methylation of DNA began in the second generation of oysters. This could have important implications in terms of adaptation.

Like their parents who suffered early exposure to pesticides, the offspring larvae had a reduced capacity to metamorphose. Evidence suggests that their reproduction could also be affected: the kinetics of gametogenesis is accelerated in these populations, where we also observe a trend towards the feminization of populations.

However, repeated exposure to pesticides over several generations did not seem to amplify the observed effects.

Rethinking ecotoxicity studies

These results are interesting in many ways:

• On the one hand, they demonstrate the existence of delayed and multigenerational effects of exposure to pesticides in Pacific oysters.
• On the other hand, they illustrate the consequences of chemical stress occurring during the early stages of embryo-larval development, even if this stress occurs well below the thresholds considered toxic.

In other words, they show that toxicity studies which only focus on short-term observable effects can be called into question.

The same is true of the analysis methods conventionally used: here, the markers traditionally used (embryotoxicity, genotoxicity) did not provide significant results, while long-term consequences could be highlighted overall. of the life cycle.

It is therefore urgent to deepen ecotoxicity studies by adding a temporal dimension and taking advantage of detailed analysis methods. These assessments must also be based on a better understanding of the mechanisms of transgenerational inheritance of responses to environmental stresses. This is necessary if we wish to correctly assess the risk posed by chemical contamination in the marine environment.

About the authors:
– Rossana Sussarello. Doctor in marine biology, Ifremer.
– Thomas Sol Dourdin. doctor in marine biology, Ifremer.
This article is republished from The Conversation under a Creative Commons license. Read the original article.