The zebra mussel, an invasive freshwater species -

© F Lamiot - Wikimedia CC BY-SA

  • Aquatic environments are unfortunately the receptacle of a large number of contaminating substances emitted by human activities, according to our partner The Conversation.

  • A European project is therefore studying, in artificial rivers, the effects of a mixture of five molecules (paracetamol, irbesartan, diclofenac, naproxen and carbamazepine) on different species.

  • The analysis of this phenomenon was carried out by Alain Geffard, professor of Exact and Natural Sciences at the University of Reims Champagne-Ardenne.

The services provided by aquatic ecosystems are of capital importance, which leads to a strong societal expectation regarding the maintenance of their quality.

However, these environments are the receptacle for a large number of contaminating substances emitted by human activities.

The effluents from wastewater treatment plants turn out to be a major and chronic source of pollution, in particular by so-called “emerging” molecules, a category which includes drugs and whose impacts on the environment are very poorly understood.

To better understand the possible effects of these contaminating molecules, a part of the European cooperation project INTERREG DIADeM (for Development of an integrated approach for the diagnosis of water quality in the Meuse) has attempted to study the effects of a mixture of five molecules - paracetamol (analgesic), irbesartan (antihypertensive), diclofenac (anti-inflammatory), naproxen (anti-inflammatory) and carbamazepine (neuroleptic) - on different species, and especially on a bivalve mollusk widely studied in aquatic ecotoxicology: the zebra mussel or dreissene (

Dreissena polymorpha

).

A cocktail of drugs toxic to the organisms of our rivers?

Aerial view of the 12 artificial rivers © A. Geffard / The Conversation

The artificial rivers seen up close © A. Geffard / The Conversation

Taking into account the complexity of an aquatic ecosystem while controlling the contaminating factor is impossible in rivers given the presence of many other molecules (hydrocarbons, metals, pesticides, etc.).

The project consortium therefore carried out, between October 2017 and October 2018, an original experiment by creating artificial rivers (mesocosms) in order to test a range of concentrations representative of median environmental concentrations or those of highly contaminated rivers.

Present on a platform of the National Institute for Industrial Environment and Risks (INERIS), these devices make it possible, after a 6-month ecosystem reconstruction phase, to study the effects of molecules in a context of chronic exposure (up to 6 months) to aquatic organisms.

With the exception of paracetamol, the molecules studied are found in the soft tissues of dreissenes but only for exposure to the highest concentrations (condition C) and this, from eight weeks of exposure until the end of experimentation, 23 weeks.

Concentration of Carbamazepine, Diclofenac, Irbesartan and Naproxen (that of paracetamol is below the detection limit) in the soft tissues of dreissene after 8, 16 or 23 weeks of exposure to condition C (see table 1) © A. Geffard / The Conversation

These results highlight the ability of contaminants to accumulate in aquatic organisms.

But can they interact with the constituent biomolecules of living things and disrupt physiological functions?

To find out, the responses of several markers of biological processes such as reproduction, energy metabolism, immunity or even DNA integrity have been observed during exposure to these contaminants.

Reproduction inhibited, digestion stimulated

After 8 weeks of exposure, the organisms are normally in a period of maximum maturity, when they emit gametes (eggs and sperm) into the medium.

This pattern, classic of the reproduction cycle, is indeed found in male individuals exposed to control and A conditions, with 80% of individuals having emitted their gametes.

For organisms exposed to the highest drug concentrations, on the other hand, there is a delay in the cycle with a large percentage of individuals not having emitted gametes (pre-spawning), in particular for condition C.

Percentage of male individuals at the different stages of their reproduction (maturation, pre-spawning or post-spawning) © A. Geffard / The Conversation

While these results suggest that the drugs used have a direct effect on the dreissene reproduction process, they also seem to show a reallocation of energy towards defense mechanisms to the detriment of reproduction.

They could also reveal the consequences of a less favorable living environment in terms of food availability (mainly phytoplankton), which can also be impacted by such contaminations.

From an energy point of view, after 16 weeks of exposure, an increase in the activity of lipase - a digestive enzyme - is observed in dreissenes exposed to the 3 concentrations.

Therefore, two hypotheses are possible: either the drug cocktail has a direct influence on the enzyme (induction), or an indirect influence via an effect on the food resource.

The mollusk could thus increase its digestive capacities in order to optimize the assimilation of nutrients resulting from the digestion of food that has become rarer or of lower quality.

Lipase activity in dreissenes exposed in the mesocosm for 8 and 16 weeks to a mixture of drugs (see Table 1) © A. Geffard / The Conversation

Pollutants reduce immunity

At the same time, the project was also interested in cells ensuring immunity in invertebrates: hemocytes.

The integrity of the DNA of these cells was assessed by the so-called “comet” test: if the DNA is fragmented, we observe a comet-like appearance, with the DNA intact in the head and the fragments in the tail. of the "comet" ("tail DNA").

At 8 and 23 weeks of exposure, an increase in the percentage of DNA in the comet tail is observed, in particular in organisms exposed to condition C. This reflects damage to the DNA and therefore genotoxicity of the environment. on the hemocytes of these organisms.

In a complementary manner, the capacity of phagocytosis (internalization of foreign bodies to destroy them) of hemocytes can be measured using beads.

The average number of beads phagocytosed by each hemocyte thus decreases after 8 weeks in dreissenes exposed to conditions B and C, highlighting an effect of the contamination on the activity of these cells and therefore on their defense capacity in the event of the presence of micro- organisms in the water.

DNA integrity (% tail DNA, comet test) of dreissene hemocytes exposed for 8 and 23 weeks to a mixture of drugs in the mesocosomes (see table 1) © A. Geffard / The Conversation

Figure 5: number of beads phagocytosed by dreissene hemocytes exposed for 8 weeks to a mixture of drugs in the mesocosomes (see table 1) © A. Geffard / The Conversation

A model of ecological monitoring

All of these results provide a better understanding of the direct and indirect ecotoxic effects of drug contamination on a model organism.

Such effects appear for the highest concentrations, representative of those observed in many regions of the world whose rivers are subject to strong anthropogenic pressures.

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The study also underlines the interest of mesocosms and their conditions similar to those of natural ecosystems for evaluating the effects of drugs on an aquatic environment.

Such a tool gives a glimpse of the ecotoxic consequences on an environment exposed for several months to a cocktail of contaminating molecules.

In short, to approach a certain environmental realism.

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This analysis was written by Alain Geffard, professor of Exact and Natural Sciences at the University of Reims Champagne-Ardenne.

The author would like to thank the people who contributed to this work: M. Palos Ladeiro, O. Dedourge-Geffard, M. Bonnard (University of Reims Champagne Ardenne);

S. Joachim, JM Porcher (INERIS);

K. Nott S. Ronkart (SWDE);

C. Robert (CERgroupe).


The original article was published on The Conversation website.

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