STEM I is our course where we lead an independent research project for the first half of the year. We learn to research effectively and then apply that knowledge to a topic of interest to us. This has already taught me many things about working on a long-term project and doing experiments. My project is in the fields of marine biology and environmental sciences. More information can be found below.
Microplastic pollution in oceans and other bodies of water is drastically increasing over the past few decades and it impacts aspects of organisms living in these environments such as respiration and growth, but there is presently not much research done on these impacts on organisms at the bottom of the food chain. Therefore, the goal of this project was to study how microplastics impact respiration and growth rates using freshwater clams as a model system. Microplastics were first generated using a synthetic fabric made of polyester and spandex. Two sets of experiments were run with three groups of four clams were exposed to 0%, 0.05%, and 0.5% concentrations of microplastics which were mixed into the sediment within the aquarium. The respiration rate and growth of the clams was tracked over a 15-day window on days 1, 5, 10, and 15. After 15 days of exposure, the clams exposed to microplastics had respiration and growth rates equal to those in the control group. Clams that were exposed to the higher concentration of microplastics experienced an equal change in respiration rate and growth than those in the group with the smaller amount of microplastics. The equal change in respiration rate and growth after exposure to microplastics can be attributed to the fact that no microplastics were ingested by the clams. These results are unable to reveal anything about the implications of microplastic pollution in the short or long term.
How do microplastics affect the growth and respiration rates of freshwater clams?
If microplastic concentration increases then the respiration and growth rates of freshwater clams will decrease because they will ingest the plastic particles and they will accumulate in the organisms.
Microplastics, small plastic fragments less than 5mm in size, are accumulating in oceans and other bodies of
water around the world (Wright, Thompson & Galloway, 2013). The most common types of plastics found in water are polyethylene (PE), polypropylene
(PP), and polyvinylchloride (PVC). Microplastics can be categorized into two types, primary and secondary.
Primary microplastics are those that are created at the small size, these are commonly used in cosmetics and
textiles. Secondary microplastics are those that break down from larger pieces of plastic.
They are mainly sourced on land from wastewater treatment plants (WWTP) and the breakdown of other plastics (Li, Liu & Paul Chen, 2018).
Microplastics have been proven to have a multitude of effects on organisms in polluted environments. There have been reports of microplastics being ingested by organisms which can cause a plethora of negative effects and has been observed in all trophic levels (Li et al., 2018). Chemical and biological effects can occur when toxins are adhered to the surface of the plastic when they are ingested. One type of toxin commonly found on microplastics are called persistent organic pollutants (POP) (Andrady, 2011).
Despite the abundance of microplastic pollution, there are scarce scientific studies evaluating the impact of microplastics on organisms in aquatic environments. There is a lack of research into what implications microplastics will have on aquatic fauna, especially those at the bottom of the food chain (Wright et al., 2013). One of those organisms is Corbicula fluminea, or the freshwater clam. Since C. fluminea is a filter-feeding bivalve, they are more at risk to the ingestion of microplastics. They will ingest particles that accumulate on their gills, so any microplastics present in the water or sediment surrounding them will be likely to be consumed. Also, because of their feeding habits, they will be more at risk to microplastic ingestion (Xu et al., 2017).
The microplastics used in the experiment were manually created through a process of cutting fabric into 1-5mm sized pieces.
The fabric was composed of 88% polyester and 12% spandex. The microplastic concentrations were generated in three different
amounts, 0, 0.05, and 0.05%. The sand and the microplastics were weighed out and then the percent concentrations were calculated.
Then the correct amounts of microplastics were mixed into the sediment that was then placed within the containers for each clam.
The first quality of the clams that was measured was their growth. On each day of testing, both their height and width were measured using a caliper. The respiration rate was calculated as the amount of dissolved oxygen consumed by the clams in an hour. In order to measure this, the containers with the clams had to be taken out of the aquarium. Using a dissolved oxygen probe the initial dissolved oxygen concentration was measured and recorded. The containers were then sealed with the clam inside them and left for an hour. After that time was up, the process of measuring dissolved oxygen was repeated and recorded once again.
Once the testing process was complete, the clams were checked to see if they had actually ingested microplastics. This was done by using a fluorescent microscope, that was constructed for the purposes of the experiment. The clams were opened up, and their organs were dyed using the Nile Red dye. They were then placed under the microscope and images were taken to identify if there were microplastics present.
The growth and respiration rates were compared over the course of the experiment using their means and standard deviations. Also, an unpaired, two-tailed, student’s t-test was applied to determine if the results of this study were statistically significant.
After the experimentation process was complete, it was identified that the two experiment sets were
different. The only result where the difference was not prevalent was with the height of the clams in both groups over time.
The comparison of the two control groups in regards to respiration rate resulted in a p-value of 0.012. Additionally, there
was a significant difference with the control groups in regards to width, which had a p-value of 0.034. This difference led
to the two experimental groups being handled separately in the analysis.
There were also multiple clam deaths over the course of the testing process. Any organisms that died were removed from the tank, and they were no longer accounted for in the data. On Day 10 of the first experiment’s control group testing, one clam died. Also in the first experiment set, two clams died on day 10 for the 0.5% concentration as well as one more on day 15. In the set of repeat experiments, only one clam died which was in the control group on day 15.
Overall, there was no statistically significant variance in any of the data over time. The growth and respiration rate stayed relatively stagnant in respect to the control groups. There was also no presence of microplastics in any of the clams after experimentation was over, so they did not ingest any particles over the 15 days. This data rejects the proposed hypothesis that microplastics decrease the respiration and growth rate of freshwater clams.
In this study, the changes in the respiration rates and growth of freshwater clams after being exposed
to microplastics were investigated. The results of this study were unable to prove the original hypothesis that microplastics
have a negative impact on the respiration and growth rates of freshwater clams, specifically C. fluminea. There could be a few
causes for the inconclusive results. The first thing to consider is that microplastics can come in different shapes in sizes.
It is possible that the ones used in this study were the wrong shape or size. Clams are size-selective feeders and the microplastics
they were exposed to were large in terms of microplastic size (Wright et al., 2013). They may have been less inclined to ingest
particles of these sizes, which would explain why no microplastics were present in the clams after the experiments were over.
Secondly, the microplastics were mixed into the sediment rather than being suspended in the water. Filter-feeders may be more
likely to ingest particles that are floating in the water than those in the sediment. A final aspect of the testing process that
could have led to the results was the fact that it took place over 15 days. This short of a time frame may not be long enough for
any negative effects of microplastics to take place.
The present errors in the experiment may have been the reason why there were no significant results. There is a lot of research proving that microplastics have negative effects, which contradicts many other studies done in this area. One study by Oliveira (2018) exposed clams to both microplastics and mercury over a short window similar to this project, and they witnessed significant change over that time. If this research was done over a longer period of time with a larger sample size like many other studies, then there is a chance that actual results would have been acquired.
Since there were no significant changes observed in this study, nothing can be generalized about the impacts of microplastic pollution in both the short and long term. Something that was revealed by this study is that microplastics that are on the larger side will have less of a chance of being ingested by smaller organisms. 1 mm large microplastics will not have any effects on freshwater clams that are around 12-20 mm in width over a time frame of 15 days. This does not mean that clams will never be negatively impacted by microplastics because they will. Those microplastics will be the fragments of larger pieces of waste that break off and then are ingested. There can also still be larger effects on the food chain as a whole. They can enter either through the process of ingestion or adsorption and can spread all the way up to humans. Van Cauwenberghe & Janssen (2014) identified that people who consume seafood regularly will ingest around 11,000 microplastics a year. Despite the results of this study, microplastics still pose a danger to the environment and to marine organisms.