Resumen de Microplastics as pollutants of environmental concern: Unveiling the tip of iceberg
Microplastics (MPs) are considered emerging pollutants. Widely distributed in the environment from the equator to the poles, their presence in the marine environment has been reported, ranging from the water column surface to the bottom of the ocean trenches, also in freshwater ecosystems such as rivers, lakes and lagoons, mangrove areas. They have been found in soils, suspended in the air and inside around 700 animal species, endangering food security and the health of a wide variety of organisms including humans the solely responsible for its ubiquity.
The presence of MPs in different ecosystems implies among others: the physical risk when ingested by different species along the trophic chain, from the microscopic plankton to giant mammals such as the blue whale. They cause obstructions along the digestive tract, ulcers, perforations, malnutrition, reproductive problems and even changes in behavior. In addition to their physical presence, the impacts of MPs on the marine environment also lies in their ability to act as transporters of pathogenic and / or invading microorganisms. MPs deposited on the seabed can also cause damage to benthic communities and block gas exchange. They can act as vectors for xenobiotics, including heavy metals and metalloids, organic pollutants and environmental steroids and are able to transport them to distant locations.
Many of these chemicals can be added to the polymer during the manufacturing process and others can be sorbed during their residence time in the marine environment, with the potential risk of being transfer to biota. This complex mixture of chemicals can cause negative toxicological effects at trace levels concentrations.
The main objective of this research, was to determine the chemical composition of primary (pellets) and secondary (fragments) MPs, their associated pollutants (metals) and their ecotoxicological potential. Samples were collected at the Colombian Caribbean, including the cities of Cartagena an industrialized city with the second largest oil refinery, in the country, Coveñas, Puerto Colombia and Riohacha.
For this purpose, the analysis of chemical composition of sampled MPs was carried out by FTIR analysis (Infrared spectroscopy by Fourier Transform) resulting in a predominant polyethylene composition (PE), both in pellets and fragments. The most abundant MPs on the beaches were primary MPs (pellets) with a lower degree of surface degradation, classified as White New polyethylene pellets (WN), followed in quantities by secondary MPs. In the case of mercury content, higher levels of concentrations were found in secondary microplastics MPs, white degraded pellets (WDP) and Black pellets, these contents are related to the degree of surface degradation (SDF) categorized in the pellets examined and the presence of organic adhesions, such as oil residues in pellets categorized as black (Black pellets). The allocation of MPs on beaches allowed the pinpointing of sources, which is the city of Cartagena de Indias, specifically its industrial zone situated in the bay area, that can be assessed as an important “hotspot” of MP pollution in Caribean.
The metals concentrations collected in Cartagena beaches, including total mercury (THg), was quantified using a direct Hg analyzer (DMA-80) and ICP / MS techniques for the other 47 trace elements. Ba, Cr, Rb, Sr, Ce, Zr, Ni, Pb were the most accumulated metals in the pellets categorized as the most degraded (WDP). This is due to the increase of the surface area and a longer residence time in the environment.
The evaluation of the toxicity of sampled MPs was carried out using the model organism the nematode Caenorhabditis elegans (C. elegans). For this evaluation, a wild strain Bristol N2 and the transgenic strains sod-4, mtl-2 and gst-1 were used. Several toxicity endpoints were evaluated, e.g. lethality, growth locomotion and relative expression of genes related to oxidative stress. The results showed that extracts from pellets and fragments were biologically active in the C. elegans and as expected, virgin or (industrial) pellets were less toxic than environmental MPs. The virgin pellets showed also toxic effect; namely the inhibition of locomotion, which is most likely attributed to the presence of plastic additives. The highest toxicity, evidenced as significant changes in the lethality and growth of the nematode, was the result of exposure to MPs from Cartagena´s Bay, a highly polluted ecosystem.
In terms of gene expression, secondary MPs from samples also from the city of Cartagena, overexpressed genes relaed to oxidative stress and metal exposition sod-4, mtl-2 and gst-1 in C. elegans, while pellets collected in the same area significantly induced mtl-2.
In summary, the Caribean coast is significantly polluted by MPs, they originate from extensive industrial activities especially close to the city of Cartagena. The degree of degradation and the residence time of MPs in polluted environment seem to be the most important factors regarding the concentration of metals adsorbed on MPs surface. Metals can be then released from the MP surface and affect organisms, as showed in this study by testing C. elegans.
According to our knowledge, this is the first work focused on sampling of MPs in beaches in Colombia. This is also the first study where the degree of MP degradation, changes of their surface morphology was connected to the adsorption and release of metals from MPs surface. The results of this study are particullarly important, because they highlighted the possibility of MPs to adsorb and transport metals to remote location, release them and consequently affect marine biota. This research establishes also a baseline for the study of microplastic contamination in the Caribbean area.
