Preventing Plastic Pollution works with scientists from the UK and France to examine the ecological impact of plastic left to degrade in water environments.
With this study, scientists at Ifremer & the CNRS are attempting to further our understanding of how plastic particles interact and affect living organisms, by measuring the physiological response of oysters to the presence of plastic microparticles.
First, a little bit about oysters…
Oysters are a sentinel species along French coasts, with high economic and ecological value. They are an engineer species that performs many services in coastal ecosystems.
Oysters are filter feeders, meaning they filter the surrounding seawater through their gills to feed. Filtering also allows them to capture oxygen and breathe. Oysters can gain nutrition in 2 different ways: either by absorbing dissolved substances present in seawater (organic matter), or by ingesting suspended particles. These particles are retained on the surface of the gills, coated with mucus, and then transported (thanks to the gill cilia) to the mouth to be ingested.
The size of particles they collect in this way varies from a few microns to a few hundred microns. These are primarily microalgae, on which they feed, but can also include plastic particles, which oysters cannot separate from their food. Questions on what effects these plastic particles might have on oysters inevitably arises. Seeking to understand an animal’s response to the constraints of its environment is a scientific discipline called Ecophysiology. To measure this response in a bivalve mollusk, like the oyster, a device called an Ecophysiological bench was built in Ifremer’s laboratory (Brittany, France).
This bench makes it possible to record, different parameters in the oyster’s environment (such as food or dissolved oxygen concentrations), in an automated and regular way over time. These parameters are then used to estimate the main individual biological functions of breathing (which reduces energy expenditure) and food intake (which reflects energy inputs).
In previous studies, this tool made it possible, for example, to measure and understand an oyster’s response to the presence of toxic microalgae or pathogens in their environment (https://doi.org/10.1016/j.toxicon.2017.12.050).
So, what’s on the bench for our experiment?
This ecophysiological bench has 9 identical small tanks (0.5l, each), through which passes a constant flow of seawater, and a mixture of algae (to feed the oysters). Each tank contains a single individual, with the exception of an empty tank, which serves as our control (Fig 1).
The main parameters of seawater, such as temperature, fluorescence, dissolved oxygen, salinity, pH, are measured in real time as the water exits each tank, thanks to a series of solenoid valves. These direct seawater to a set of sensors; all controlled by a human-machine interface, which enable high-frequency visualisation and acquisition of data (Fig 2).
For Preventing Plastic Pollution, Ifremer tested the oyster’s response to the presence of plastic microparticles, including tyre particles and synthetic fibres.
Why tyre microparticles and synthetic microfibers?
These are small enough to be ingested by oysters and are frequently found on European coasts (https://portals.iucn.org/library/sites/library/files/documents/2017-002-Fr.pdf). For example, driving generates 2.9 million tons of tyre particles every year, or 28% of the micrometric sized microplastics released into the ocean. These particles can cause changes in the physiology of the animal that ingested them, due to their physical presence, but also by the release of chemical molecules such as additives, and plasticizers (used in the manufacture of plastics).
What are the effects of these microparticles on the ecophysiology of the oyster?
We were able to show that in response to chemical contaminants released into seawater by pieces of tyre, young oysters (8 months old) reduced their food intake by about half, and their breathing dropped by 16%. Based on these results, the calculation of an index called “growth potential” which provides information on the energy state of the animal, suggests a disturbance of more than half of the energy balance of the animal compared to unexposed oysters in the control tanks.
This energy balance dictates the energy that the animal can invest in its growth and reproduction. These results lead us to study the long-term effects of chemicals released by tyres on oysters, and in particular those that will affect growth and reproduction performance, which may be harmful to the species. The use of this tool reveals the relevance and importance of measuring ecophysiological strategies at the individual scale of a bivalve mollusk, such as oysters, to better explain how environmental factors, including man-made pollution, affect the animal’s energy pathways, and to deduce the repercussions on that animal’s physiology.