Oysters with a plastic flavor in your plates for your celebrations!by Léa Bascoul, Vincent Dumontet and Sophia Lestringuez
Published by the November 8, 2021 on 5:52 PM
For over half a century, global plastic production has increased exponentially. Plastics represent 50% to 80% of marine litter. Microplastics (MP) are derived from the degradation of larger plastic articles or directly produced as microparticles (< 5 mm). Oceans have a “funnel” effect concerning MP drained from rivers and streams, they concentrate pollution from fresh waters in the oceans. In pearl farming structures and equipment, the use of plastics is a source of pollution to the detriment of the lagoon ecosystem. Due to their ubiquitous and small dimensions, MP are likely to be ingested by many organisms and in particular by the oyster, Pinctada margaritifera (Figure 1), which is a species of ecological and commercial interest. So, the objective of the study is to assess the impact of these microplastics on the physiology of this oyster species from the pearl lagoon of Ahe atoll, in the waters of French Polynesia (Figure 2).
Figure 1: Pinctada margaritifera
Figure 2: Ahe atoll localisation
In order to assess the effects of these molecules, IFREMER from French Polynesia, recreated microcosms [1] where oysters are exposed to polystyrene microbeads. Indeed, polystyrene is one of the most widely used plastic polymers and the most found in marine ecosystems.
After two weeks of depuration, oysters, were divided into three treatments. For each, 24 oysters were exposed to polystyrene solutions for two months. These solutions were a mixture of polystyrene microbeads of different diameters (6 and 10 μm) at equal weight. A control was carried out without exposure to polystyrene. Each tank, kept homogeneous, was exposed for 12 hours a day to light and the physico-chemical parameters were kept constant. The microplastic solution was injected continuously into the tanks associated with a mixed diet of two microalgae (Tisochrysis lutea and Chaetoceros gracilis).
Figure 3: Experimental exposure device of oysters to microplastics
During the experiment (Figure 3), various parameters, like oxygen consumption, were measured to assess the different energy expenditure of the oyster. Expenses related to food (ingestion rate and assimilation efficiency), to growth (the scope for growth defined from the energy budget and the shell growth rate) as well as those allocated to reproduction (characterization gonad [2] size and analysis of gametogenesis [3]) are quantified in each experimental group and compared to the control. The measurements were carried out on eight oysters per treatment after one month and then at the end of the experiment.
This experiment made it possible to demonstrate that at low MP concentrations, pearl oysters suffer adverse consequences for the sustainability of their populations. In fact, the metabolic responses of a stressed organism can lead to an increase in an individual's energy costs, forcing the organism to sacrifice some of its energy normally allocated to functions, such as growth or reproduction.
In this classical and standardized experiment, the oysters exhibited a significant dose-dependent reduction in the “scope for growth” without however modifying the shell deposit rate. On the other hand, although the ingestion rates of the microalgae were not affected by the microplastics, for an equal volume of filtered water the assimilation of the microalgae is disturbed by the presence of the microplastics and the energy supplied by food is lower. This is either linked to interference in assimilation processes in the digestive tract or to indirect competition between algae and microplastics.
The fact that metabolic (respiration) rates are not impacted suggests that the only possibility for the oyster to cope with a decrease in available energy is to modify its gametogenesis and / or its reproductive effort. However, reproduction in P. margaritifera is continuous and requires a lot of energy. The results of the study show that reproductive effort is not affected but that gametogenesis is strongly impacted. Thus “small holes” in the gonadal tubules [4] are observed in oysters exposed to microplastics. They could therefore be the start of the phagocytosis [5] of germ cells initiated to recover energy (Figure 4).
Figure 4: Impact on P. margaritifera metabolism
In a few words, the pearl oyster exposed to MP for a period of two months maintains its metabolism and its vital functions to the detriment of its reproduction, and therefore the sustainability of its future population.
This study could also serve as a model for the MSFD [6] Technical Group Marine Litter to assess the effects of microplastic litter in biota (D10C3), because the effects observed on Pinctada margaritifera in the lagoon of French Polynesia must be the same as on other filter feeding bivalves in Europe. The European Technical Group Marine Litter could thus propose a regulation of waste at the European level and not only at the level of the lagoon of French Polynesia.
The experiment results should support decision-making on plastic waste management measures in Polynesian lagoons, especially in atolls that have decades of pearl culture history without waste regulation or clean-up. This work would provide the necessary information to local authorities to regulate pearl equipment and establish a waste management policy such as the collection and recycling of this equipment.
The aim of the study was to measure the impact of microplastics on the metabolism of Pinctada margaritifera. The team of researchers therefore sought to artificially recreate different levels of pollution in the laboratory. This methodology is suitable because it aims to recreate the pollution conditions possibly encountered in natural environments. To measure these different levels of pollution, the experiment is carried out at different concentrations. As for the various parameters measured to assess energy losses, they make it possible to identify, in a non-invasive manner, the functions (growth and reproduction) "sacrificed" to cope with the loss of energy. They also allow us to see if this loss of energy had an effect on the metabolism of oysters. The parameters monitored therefore seem consistent for this study and could be used for similar studies on other bivalve filter-feeding organisms. Experiments using this protocol could be considered in fresh waters with other types of bivalves such as the mussel pearl (Margaritifera margaritifera). Also, this study can be evaluated with a direct approach on oysters from pearl farms and over several generations.
In several years, oyster populations on a global scale could be impacted by pollution from microplastics … so, if there are no more oysters, what will we eat with our white wine?
[1] smaller version of ecosystem
[2] an animal organ in which gametes (a haploid germ cell, such as a spermatozoon or ovum) are produced
[3] gamete formation process
[4] organs of gametes excretion
[5] cellular defense process, destructive function of phagocytes
[6] Marine Strategy Framework Directive
This post is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
