The Problem of Pollution in Marine Environment
The Effects of Microplastic Particles on Cells and Tissues of Blue Mussels
Because of human activity, many marine ecosystems are under threat. In particular, it concerns the pollution of marine waters. In their article, Duraisamyand Latha studied the pollution of the marine environment and concluded that pollution reduced the aesthetic and intrinsic value of the marine environment as well as threatened the survival of the marine flora and fauna. In addition, pollution has a direct impact on human health, because people often eat seafood from contaminated regions. Such anthropogenic factor as wrong plastic garbage disposal toxifies the marine environment and kills marine fauna. In their research, Tarzia explored the problem of soil pollution (including pollution by heavy metals) in Naples. Although they investigated the soil pollution, their statements are relevant to the topic of this work because it correlates with the global problem that is the effect of waste on living species. They argued that the problem of anthropogenic pollution of the environment should be faced and resolved as it may affect the assessment of the future risk of pollution and help prevent human-related disasters.
However, the given research highlights the possible harm and consequences of that problem.
Forms of Pollution
Today there are many different marine pollutants, which can be divided into chemical, physical, and bacteriological pollutants. Scientists have been investigating human-made pollutants and their impact on the environment and humans for many years. Weis states that the products of industrial, agricultural, and chemical activity of people are not the only sources of water pollution. In general, the pollution can be divided into two major forms: nature-related pollution like the destructive effect of tsunami and human-related (anthropogenic) pollution like wrong waste disposal. These forms define the source and the root cause of a certain type of pollution.
Natural pollution is the pollution that can happen without human intervention. Natural pollution is not as vast and dangerous as anthropogenic, but it can also play a negative role in the lives of the marine residents.It includes mud and other disaster-related natural pollutions that might serve as a source of natural contaminants like bacteria, viruses, and other pathogens. They also include algae, lignin, yeast, and molds which can trigger the epidemic growth of health-threatening diseases.
A variety of anthropogenic pollutants is quite large, and for each region, the prevalence of a particular one of them is peculiar.
Farmer divided aquatic anthropogenic pollution into the following groups:
- Pollution from the air. Various pesticides and heavy metals such as lead, cadmium, copper, and zinc can enter from the atmosphere into the water.
- Garbage. The most problematic garbage is unnatural and plastic its major representative. Derraik investigated the marine pollution by the plastic debris. In the research, he claimed that the interaction of plastic waste with the marine animals led to their death. He explained that the inhabitants of the sea were caught in the garbage or ate plastic that contains polychlorinated biphenyls, and that led to the extinction of some species.
- Heavy metals. The increasing number of toxic waste disposal like mercury which ultimately settles in the ocean threatens the ocean and increases the toxicity of waters. The action of heavy metals is particularly pernicious because many species of bivalves cannot survive in the conditions of such pollution. The most common heavy metals that contaminate the marine environment are arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc.
- Toxic organic substances. This group includes agricultural pesticides (organochlorine and organophosphorus), as well as some by-products of the industry (organochlorines).
- Oil. The oil spill is probably one of the most striking examples of marine pollution. Because of it, many species die from oxygen starvation or poisoning. In addition, when oil gets on the bird feathers, it prevents them from flying.
- Sewerage. In addition to sewage runoff pollution of marine waters, it can lead to bacterial infection of people who drink the related water. Streams mix the pollution with fresh water making it undrinkable. Moreover, marine taxa feeds and populate there, and it might be crucial for them. The sea environment becomes more polluted and life-threatening thus making species extinct in waters that are not made to produce life, but rather kill it.
- Thermal pollution. Discharge of industrial cooling water can lead to higher sea temperatures, which in turn can lead to the extinction of many species like shellfish that might grow slower in heated waters.
Ubiquitous debris that can be found on the beaches show that they are both a source and a signal of wider dissemination of marine pollution by persistent chemicals because they die due to that pollution (Elliott and Elliott). In their research, Elliott and Elliott investigated the impact of marine pollution through the food chain of sea inhabitants. They considered that many components of pollutants are hydrophobic and absorbed into the plastic, which seabirds are often mistaken for food. If it does not kill them, then it at least greatly poisons them (Elliott and Elliott). Like the previous authors, Ivar do Sul and Costa explored marine pollution, namely the pollution by the microplastics. They state that all living organisms in the marine environment are exposed to microplastics contamination since they are all linked by the same food chain.
Consequently, this research will further investigate the problem of the marine pollution and find new facts that have to be addressed.
Plastics in the Ocean
Plastics as pollutants
Speaking about plastics as pollutants of oceans, their general nature should be discussed. Plastics can be easily recycled by seawater, sunlight, and time going through numerous chemical processes. Those small pieces are harmful to the marine environment. The main process is decomposition or degradation of polymeric materials. Extents of decomposition are determined with methods of polymeric manufacture and chemical content of a dumped plastic. Usually, decomposition of polymeric materials takes a long period, which is why such drastic effects are produced on the marine environment (Andrady). The strongest harm is caused with release of bisphenol A,which is the main water contaminant and basic element of plastic debris (Andrady).
In such a way, processes of polymeric decomposition are divided into following types (Andrady):
- Solar UV-induced photodecomposition reactions
- Thermal reactions such as thermo-oxidation
- Hydrolysis of a polymeric material
- Microbial biodegradation
Among these processes, solar-induced oxidative decomposition is one of the strongest and most widespread (Andrady). The other processes are less fast and do not produce a considerable effect. Nevertheless, slow thermal degradation usually occurs after solar UV-induced processes. Beyond a doubt, persistence of other processes can be observed under various conditions of weathering.
Formation of General Microplastics
First of all, plastic is created using household dust, textile, tire dusts, maritime paint dust, etc. Plastic is commonly recognized as less dangerous than toxic wastes as long as it does not contain toxic elements (Bergmann, Gutow, & Klages). Garbage is cast ashore and tides takes plastic into the water, which is why microplastic gets into the water due to the global natural process rather than human intervention in the marine environment. Moreover, general microplastic is more resistant to chemical reactions caused with water contact. Their influence on general health of some marine species can be identified, but these effects do not present a real threat to the marine biodiversity (Bergmann, Gutow, & Klages). It is becoming increasingly apparent that the main threat is posed with “recycled” microplastic, which is created after defragmentation of larger plastic debris.
Formation of the model Microplastic
As it has been mentioned, model microplastic is formed with fragmentation or degradation of larger plastic debris. Model microplastic undergoes such processes as photo oxidation, mechanical abrasion, and additional chemical processes caused with seawater. The smallest and most dangerous debris of microplastic emerge after exposure to UV-B radiation that causes photo-oxidative decomposition (Norwegian Environment Agency,). However, hydrolysis or biodegradation are less frequent ways of microplastic formation. Again, prominence of decomposition chemical processes depends heavily on weathering and material of plastic. Beyond a doubt, microplastic itself produces much harmful effects, since it contacts not only with seawater but also with organisms of numerous marine species (Norwegian Environment Agency). Regarding that, a nature of model microplastic should be given an account.
The study needs to draw a link between qualities of microplastic debris and organism of a blue mussel.
The Model Microplastic
The origin of the microplastic
The model microplastic origins from larger particles of plastic throughout process of fragmentation. As it has been already mentioned, large plastic debris undergo multiple chemical processes that decompose polymeric material. Thus, being exposed to sunlight and seawater on a regular basis implies favorable conditions for plastic degradation. Processes of fragmentation take different degrees, so that microplastic obtains various forms (Lassen).
Form of the plastic
A current state of knowledge lacks evidence of distinct patterns of decomposition, which is why shaping of microplastic is not attached to a particular theory. The same gap is typical of microplastic size, which may vary from a couple of microns to five millimeters (Lassen). In contrast, the most widespread colors of microfibers are red and blue.
Sources of the microplastic
Chemical composition of the microplastic
The most widespread objects that are decomposed into micro and macro plastic are bottles, soda drink tins, cigarette buds, tire leftovers, empty packages of various products, etc. The majority of this litter is thrown along coastline of oceans (Lassen). Beach area is also the main location where plastic particles are decomposed. Solar light as well as seawater cause chemical reactions with such consistent elements of polymeric materials as bisphenol A. This chemical belongs to a group benzhydryl compounds (Lassen). That implies the fact that this chemical includes carbon radicals, which are easily combinable with salty waters (Lassen). By the same token, exposure to a solar light causes decomposition on a molecular level, which results in formation of new substances harming the environment. Generally speaking, the marine environment “recycles” waste, and that becomes dangerous for it as it transforms polymeric materials into more dangerous substances.
Effects of the microplastic on general environment
Water contamination and increased salinity are the most obvious effects. Besides that, plastic absorption results in contaminating not only water but also natural steams. Thus, microplastic fragmentation enhances water purity. Hence, natural steam, which participates in water life-cycle, includes products of microplastic absorption. Still, the main harm is directed towards the marine environment as seawater spreads chemicals within substantial distances (Norwegian Environment Agency). Extents of the harm are much wider, as long as microplastic particles usually emerge in large amounts within a certain area.
Taking this point into account, the study considers that microplastic particles produce an extremely negative impact on various species, especially on their tissues and cells (Norwegian Environment Agency). A conceptual framework of this phenomenon suggests that marine species contact with microplastic particles throughout ingestion of polymeric fibers (Norwegian Environment Agency). Internal tissues such as gland and digestive tract are the most vulnerable targets for microplastic contamination because of their nature (Norwegian Environment Agency). Microplastic produce much stronger effects within long periods, but even first hours of being exposed to microplastic may demonstrate the evidence of abnormalities among multiple marine species (Norwegian Environment Agency). In such a way, blue mussels have been chosen for the study, since their reaction on exposure of hazardous materials are almost immediate (Norwegian Environment Agency). However, the model organism itself should be described.
The Model Organism
Classification of the organism
Taxomony of a blue mussel in marine environment in a context of this study should start with a statement that blue mussels are sessile suspension feeders. Therefore, blue mussels react on any exposure of hazardous materials relatively fast in comparison with other marine species. Their digestive gland demonstrates an immediate reaction on contamination, as long as it is a vulnerable target for xenobiotic effects. The organism is called as Mytilus edulis. This group is peculiar with its asymmetrical and elongated shell, which is dark-blue in color. A subclass of a blue mussel is pteriomorphia. Although freshwater subclass of a blue shell exists, the current study pays attention to marine-inhabited species as they are the most exposed to contamination with microplastic particles.
Metabolism of the organism
Blue mussels are filter feeders, which is why they can absorb waste. In other words, digestion of food is conducted internally while disposal of waste substances can damage that system. Blue mussels digest food throughout digestive gland and convert it inti heat with use of a standard oxycaloric equivalent -450 kJ/mol-1 while that covers the need of specific elements including the consumption of oxygen (Wang & Widdows, 1993). However, some researchers assume that blue mussels deploy a basic aerobic metabolism since anaerobic heat dissipation requires production of glycolytic flux, which is not typical of such species 1 (Wang & Widdows, 1993). At any rate, digestive mechanism of a blue mussel includes digestive gland tissues that are a direct path for any contaminants. A further change in metabolism is apparent, provided that a blue mussel was exposed to microplastic particles.
The habits of the organism
The basic habit of blue mussels can be described as clumping on rocks that are washed with seawater. They are attached to rocks with byssus. As a consequence, the majority of blue mussels can interact with microplastic particles. Due to sunlight, the toxic effect of microplastics might get even worse due to the radiation. Besides, reproduction of blue mussels is sexual. Male species spread sperm within seawater while female collect the sperm with a special siphon. Thus, contamination of female mussels is also possible during the process of reproduction. This aspect is commonly not considered, as sperm molecules are smaller than microplastic fibers. Still, this assumption requires an independent investigation.
Habitat of the organism
A usual habitat of blue mussels is low and medium intertidal zones, especially in temperate zones. Hence, they are repeatedly seen in the upper layers of seawater, and sunlight might trigger the negative effect of plastic easily. These two factors determine a fast reaction on contamination of digestive tract, especially on a cellular level. The habitat of blue mussels is not closely located to areas of polymeric fragmentation, but inhabiting of seawater surface increases risks of exposure to microplastic particles which spread with water quite fast. Overall, blue mussels occur to be one of the most vulnerable marine species, which is why the study focuses on the effects produced on them. Making certain assumption, however, should be preceded with already investigated aspects of the research problem.
Linking Past Knowledge
A large volume of the literature has been published on the subject of effects produced with microplastic particles on cells and tissues of blues mussels. However, one of the most significant findings are presented with Wright, Thompson, and Galloway because they prove in their article that death of the marine species because of microplastic particle contamination has already reached a population-wide level. It is hard to argue that such evidence is alarming and completely proves harmful effects of microplastic in the marine environment. In a similar way, Lee et argue that size, shape, and abundance of microplastic debris influence effects produced with contamination of marine organisms. To be more precise, Lee claim that micro fibers are the most dangerous elements, as they penetrate tissue and cellular levels of marine species. This finding complies with the assumption that microplastic contamination of the marine environment may result in extinction of entire populations. Plastics is decomposed within a certain time frame, so that a maximum of marine organisms can become exposed to contamination.
Another problem has been highlighted by Farrell and Nelson, who claim in their article that microplastic debris are transported from organism to organism within a food chain. This assumption is quite relevant, since microplastic particles penetrate tissues and cells of organisms that become prey of marine predators. As a consequence, these predators are also affected because they consume already contaminated organisms. That is why the problem should be addressed from the perspective of a larger context, as blue mussels do not inhabit completely isolated marine environment. Hence, Oliveira have traced increase of pyrene metabolites in fish bile. In spite of the fact that most of wish species do not consume mussels as food, their involvement in the overall food chain of the marine environment is apparent. That is why addressing the research problem will open a field for a new, integrated research, as long as the issue of microplastic contamination is a matter of the entire marine environment.
These findings have been partially supported with a study by Setala, Fleming-Lehtinen, and Lehtiniemi. Their article has identified a spread of microplastic fiber debris in population of phytoplankton, which is a key source of food for blue mussels. In such a way, contamination of a food chain begins with its smallest elements. What is more, blue mussels are exposed to contamination from multiple aspects, which is why their cellular reaction on hazardous intervention is relatively fast. Amy Lusher confirms already detected tendency that microplastic fibers are the most dangerous, as they intrude cellular level of blue mussels. In addition, Amy Lusher admits that implications for the entire marine food web are still uninvestigated while the basic causes of the problem can be traced in studying of contaminated mussel digestion on a cellular level. It is becoming increasingly difficult to ignore that presented findings can formulate a conceptual understanding of the research problem. Therefore, expected effects of microplastic contamination should be discussed.
Effect of plastic on gene expression
As we all know, an event that is going on in any of cells of the specific living organism relies on the gene expression. That process involves many specific actions that are done in the cell in order to form the specific protein that further engages in a living cycle. The gene expression starts when the protein cooperates with the specific receptor of the cell that triggers the specific response (Gene Expression). Other proteins trigger mechanisms and signals that involve other proteins that situate inside the cytoplasm to interact. In the new environment, the new protein undergoes the set of specific events and then interacts with DNA and nucleus. The transcription also requires a complex set of interactions and other proteins as the support. After that process, the complementary strain of the RNA is produced (Gene Expression). The RNA has to face many changes that include the removal of unneeded sections of the RNA that are not needed to create the desired protein. After that, the RNA takes the information that was previously encoded in the DNA out of the nucleus and then involves in translation. It interacts with mRNA and tRNA to create the chain of specific amino acids encoded by the previous cycles (Gene Expression). That product also changes the configuration that also includes folding. The final product becomes the new 3D shaped new protein which can continue the process from the beginning. That process is required in order to achieve the proper gene expression, but microplastics can damage that process.
The study by Environment And Human Health, Inc. revealed the negative impact of the previously mentioned Bisphenol A (BPA) on the gene expression (Ehhi.org). Scientists found that there were two effects of the BPA on the endocrine function. One of them, BPA acts as a weak estrogen and binds to the corresponding receptor (Ehhi.org). That might block the binding of other estrogens and damage the estrogen function. As the result, the gene expression will be damaged in the long run. Moreover, it can produce cell signaling and influence the gene expression in that way too (Ehhi.org). Furthermore, BPA also can change the DNA structure because it might add methyl groups to DNA and silence the expression of the BPA (Ehhi.org). The silenced expression will later respond differently to new BPA bindings and might accept low doses of BPA, which might be crucial in the long run for marine species. All in all, it has a harmful effect on the gene expression and RNA and cDNA, as a result.
To study that impact, it is important to use PCR as the source of the sample of the DNA sequence (PCR). It is important to study the effect of the plastic and find any mutation that might occur in DNA. However, this research used the qPCR which is the real time PCR. This option allowed us to track the needed information during the experiment comparing to the ‘in and out’ result of the normal PCR (Real time PCR). That technique works both with the mRNA and DNA while the DNA extraction can be used to apply the qPCR directly, but mRNA has to undergo DNase treatment and reverse transcription to the cDNA and the DNA sequence (Real time PCR). That technique is used to diagnose diseases and mutation which was useful to study the needed effect of the microplastic on the gene expression (Real time PCR). We also used SYBR Green, which is the DNA binding dye in order to track the state of our production. It is based on the concept that SYBR binds to minor groove that is dsDNA so that we can track its fluorescence after all. The fluorescence was bright only for dsDNA.