Chemical quality of rivers revealed by small freshwater shrimp

Gammarus, an excellent biological model

A majority of biodiversity is made up of invertebrates, which represent almost 90% of the living beings in the biosphere. For example, the biotic index, one of the most commonly used tools to monitor rivers in France, is based on a description of these species. The index can characterise a disturbance in an aquatic environment (reduction in population or disappearance of certain species), but has difficulties determining the causes. Another technique used by Cemagref researchers is based on determining the impact of chemical substances directly in the cells and tissues of animals, and Gammarus is the right biological model for this type of approach.

Why? Because the small, freshwater shrimp is an abundant and very common species in most French rivers. Contrary to fish, it is easy to catch using a simple net. Its diet is very basic, Gammarus eats exclusively the remains of animals and plants. All the above advantages simplify the tests, both in the laboratory and in the field (individuals are caged), that are required to assess the pollution in rivers.

Development of three Gammarus biological markers

Human activities release many environmentally toxic substances to rivers. Among them are insecticides, pesticides and endocrine disruptors that, even at low concentrations, modify the metabolism of Gammarus cells and tissues. Depending on what they attack, these substances are called neurotoxic if they affect the nervous system, endocrine disruptors if they affect the hormonal system and genotoxic if they attack the DNA of the exposed animal.

Today, Cemagref researchers are attempting to develop a biological marker for each technique employed by these contaminants. They have notably developed a marker for neurotoxicity that measures AChE or acetylcholinesterase, an enzyme involved in neurotransmission. Experiments have shown that two insecticides commonly found in aquatic environments cause a 40% drop in AChE activity, with consequences for the quantity of food ingested by the animal.

Concerning markers for endocrine disruptors, an increase in the production of vitellogenin in male fish was noted in the 1990s. This protein is normally synthesised by the liver of females to store the nutrient reserves required for the future development of embryos.

Today, researchers measure synthesis of the protein in male Gammarus shrimp and study its impact on the viability of their young. Other substances impact the DNA of Gammarus. A very visual test, called the comet test, measures the damage done to the DNA. Tests carried out on rivers reveal damage of up to 20%. Pollutants attack three different targets, but in the end, the result is the same, the overall dynamics of populations are threatened.

Studies are now being expanded to other species of Gammarus in order to offer generic tools for monitoring water quality.

The comet test in practical terms

The comet test measures the percentage of damaged DNA in the contaminated organism. After withdrawing Gammarus cells, the DNA nuclei are isolated and subjected to an electric field in an electrophoresis apparatus

If the DNA is not damaged by the tested pollutant, it forms a compact sphere. However, if the DNA is damaged, it forms a comet with a "nucleus" and a "tail" corresponding to the damaged DNA.

In Cemagref labs in Lyon, Émilie Lacaze uses Gammarus spermatozoids because they are more sensitive. She measured damage levels of up to 20% that can result in developmental problems in the young produced using the spermatozoids. Over the long term, this situation may impact on food supplies for species higher up in the food chain, e.g. trout which eat great quantities of the small shrimp.