Are mussels a bio-indicator of marine pollution?

© Diana Scheerschmidt / Veolia-Allemagne

For the 6 months of the 2019 Microplastics Mission, Tara and her crew are sailing up European rivers to thoroughly study their waters. Our objective is to understand the origin of plastics flowing into the sea, and analyze their impact on marine biodiversity. For this purpose, the scientific team relies on surface nets towed behind the schooner or her tender, as well as valuable collaborations with filter feeders, mollusks, and more specifically, mussels.

Plastics: who colonizes what?

To understand how plastics and biodiversity interact, a protocol was implemented. Coordinated by Dr Anne-Leila Meistertzheim, toxicologist and co-founder of Plastic@Sea, in collaboration with the Microbial Oceanography Laboratory (Laboratoire d’Océanographie Microbienne – LOMIC, OOB), this study has several dimensions. First of all, it aims to compare the biodiversity developing on plastic materials by identifying organisms (plant or animal) colonizing plastics in rivers and presenting different typologies across regions in terms of temperature, salinity gradient, flow, etc. To facilitate the comparison, plastics with identical composition are used along a salinity gradient – in other words, from sea water to fresh water – in every river studied.

In practice, one month before Tara’s arrival, 5 types of plastics are placed in fish traps offshore, at the mouth of the estuary, and also upstream and downstream from a densely populated major city, such as London, Hamburg, Rotterdam, Rouen, etc. Microorganisms colonizing the 5 types of plastics will then be compared at European level, in terms of species (genome identification using DNA) and metabolism (identification of molecules produced by the organisms using metabolomics). 

Moules-Romy Hentinger© Romy Hentinger / Fondation Tara Océan

Mussels – champions of filtration

To test a new bio-indicator for microplastic contamination in the marine environment, mussels are also placed in the fish traps, offshore and in estuaries. Capable of filtering 20-25 liters of water per day, these organisms absorb various pollutants in their tissues, in particular persistent organic pollutants (POPs) and microplastics.

In response to this bioaccumulation of toxic substances, mussels adapt, developing various biological processes to overcome this stress factor. As a result, they differentially distribute their energy between growth, breeding, energy reserve and stress response.

Their response is measurable both at molecular and individual levels. Shells are collected at sea, not far from where they will be placed and studied along the river, on a site with the greatest potential for diffuse pollution. After marking the mussels using a non-toxic fluorescent dye (calcein) that penetrates their shell to physically set a “time zero”, these organisms are left on site for a month. A few control individuals are also frozen for comparison. By comparing the measurements carried out on mussels and plastics, this method will show if these mollusks can faithfully reflect plastic pollution in the marine environment, and therefore become a bio-indicator of this pollution.

Measures and dissections

After a month, when the schooner arrives, the fish traps are removed from the water and the mussels are dissected. Control samples are also collected and dissected on site. By separating mussel meat from their shell, scientists measure the concentration of microplastics bio-accumulated in mussels, assess the pollutant content (POPs) in their tissues and compare it to that of plastics.

In the long run, the overall health status of mussels exposed to plastics will be analyzed at various levels of physiological integration: at the molecular level using the expression of genes that code for various biological functions (response to stress, energy, reproduction, etc.), and at the level of the organism, using measurement of shell growth (performed in collaboration with the Benthic Ecogeochemistry Laboratory – LECOB, OOB).

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