Our climate is continuing to warm at an unprecedented rate. The ocean is a large buffer of temperature, carbon dioxide is absorbed into the ocean which helps mitigate atmospheric warming. The sea surface temperature is predicted to increase by nearly 4 degrees Celsius by 2100. As the temperature warms, more plastics enter aquatic systems. Substantial quantities are released from melting ice. There has also been a significant increase in plastics since the 1990s, with about 4.8-12.7 tonnes of plastic dumped into the ocean and 19–23 Mt of microplastics (smaller than 5 mm) entering aquatic ecosystems per year. As warming and microplastic pollution increase, it is important to understand how their effects work together and separately.
Plastics have physical and toxicological effects on marine organisms. Ocean warming and microplastics have been found to increase the fecundity and the growth rate of the water flea, as well as inhibit the growth of a sea urchin. Phytoplankton are at the base of the food web and are critical primary producers. They respond rapidly to environmental stresses as they have a fast growth rate and short life cycle. It is important to understand the impacts of ocean warming and microplastics on phytoplankton because their interaction can affect the whole food web, as well as the biogeochemical cycles.
This study by Xuan Hou et al. investigates the effects of ocean warming and microplastic pollution on Chaetoceros gracilis (C. gracilis), a dominant genera of diatoms. Diatoms are a major class of phytoplankton and contribute 40% of marine primary productivity. Diatoms in particular are crucial to the biological carbon pump because of their silicified cell walls. Their ability to sequester carbon is regulated by the Si:C ratio of the cells. The long-term response of C. gracilis helps us better understand biogeochemical cycles in the ocean.
How can these effects be measured?
This study investigated the effects on C. Gracilis over 100 generations. Four treatments were used. They were cultured in artificial seawater and maintained under neon light conditions at 14/10 h light/dark cycles. The control was cultured at 24 degrees C, the ocean surface temperature. The MP treatment is cultured at 24 degrees C and treated with 2-3 μm polystyrene (ps), which is a dominant type of microplastic. The OW treatment is cultured at 28 degrees C to account for the predicted 4 degrees C warming. The MP + OW treatment is cultured at 28 degrees C and treated with ps. The measurements include growth rate, Chl a, photosynthetic efficiency, biogenic silicate (bSi), carbon, nitrogen, and silica assimilation, carbohydrate, protein, and lipid content, measurement of pore size, gene response, metabolism response, and a statistical analysis.
What happened under ocean warming and microplastic pollution?
They found that microplastics and ocean warming lead to decreased photosynthesis efficiency and carbon sequestration. Under MP and MP + OW treatments, chlorophyll a content decreases, which results in an inhibition of photosynthesis. There is also a loosening of the space between the thylakoid membranes with MP + OW that is correlated with the decrease in photosynthesis efficiency. As a biological adjustment strategy to the decrease in photosynthesis, algal size decreases and there is a metabolic change. Silification under warming significantly decreases, but significantly increases when exposed to mp’s. Because these two factors are antagonistic, there is no significant change in silicification under MP + OW. MP and MP + OW significantly decrease carbon assimilation rates, while MP, OW, and MP + OW significantly inhibit nitrogen assimilation rates. There are also genomic and metabolic changes. Under MP + OW there is a significant reduction in the genes encoding the expression of NRT, NR, and NIR, and this inhibits the transformation of nitrate into ammonium into glutamic acids. Glutamic acid is a type of amino acid that is crucial for the building of proteins. Cells adjust to the decrease in nitrogen by allocating into flexible forms of carbohydrates instead of proteins. The amount of proteins decreases while the amount of carbohydrates increases, with no change in lipids. This decreases the Si:C ratio, which results in a decrease in carbon sequestered.
How can we use this information?
It is clear that microplastics and warming have significant effects on phytoplankton and the resulting biogeochemical cycles. Since warming and microplastic pollution result in decreased carbon questions, the negative feedback cycle of climate change is exacerbated. This affects how we evaluate the responses of marine ecosystems to warming and climate change. It is critical to look at how ocean warming and microplastic pollution work synergistically, antagonistically, and independently.
Critical Review
This was a thorough study. Many factors are measured over a long period of time. The use of 100 generations is important to understand the long-term effects, particularly given the short lifespan of phytoplankton. However, this study did not specify how long the lifetime of each individual in the differing treatments, which may have been interesting. However, it goes into great depth on the mechanics of how they are affected. There is enough information in the methods for this study that could be replicated. The results are well explained and a cause-and-effect relationship is established, although it is written in a bit of a complex manner and it is not totally clear how this is a cause-and-effect relationship at first glance. There is a technical depth and language that is difficult to parse through and is likely written for specialists in the field.
Further Action
This paper brings up a lot of questions. Namely, what are the greater scale effects? What are the trophic cascade effects of microplastic presence in phytoplankton? Do microplastics accumulate as you move up the trophic ladder? And what can we do to help? Is decreasing the number of plastics used and inputted into oceans the most critical thing to avoid microplastic pollution? Or could we use plastics that degrade less quickly into microplastics to increase the likelihood it will be able to be cleaned up on a large scale? Can we clean up microplastics using plastic-eating bacteria? Or could there be negative unforeseen consequences to using this on a large scale, such as the breakdown of plastics being used? And most importantly, how can we get people to care and take action? This does not just affect one small species in the ocean, the whole ecosystem is connected and this affects us as well. We are inhaling, ingesting, and absorbing microplastics every day, which we still have an inadequate understanding of. However, we do know that microplastics can cause toxicity through oxidative stress, inflammatory lesions, and accumulation in organs, resulting in metabolic disturbances, neurotoxicity, and increased cancer risk (Rahnman). And lastly, how can we remain hopeful about this issue while taking action, instead of falling into nihilism? While avoiding microplastics completely is likely unavoidable, there are easy ways to decrease consumption. This includes things such as decreasing our use of plastic, washing one’s hands before eating, and avoiding foods with high microplastic accumulations such as commercial fish, and avoiding spending too much time next to printers. Doing these things and making an intentional effort to take care of our bodies by eating healthy, sleeping, and exercising will all help us stay healthy and improve our quality of life. And by decreasing our plastic use and cleaning up plastic and microplastic pollution, we can take care of our ecosystem, and ultimately better the health of the whole planet.
Works Cited
Hou, Xuan et al. “Warming and Microplastic Pollution Shape the Carbon and Nitrogen Cycles of Algae.” Journal of hazardous materials. 447 (2023): n. pag. Web.
Rahman, Arifur et al. “Potential Human Health Risks Due to Environmental Exposure to Nano- and Microplastics and Knowledge Gaps: A Scoping Review.” The science of the total environment. 757 (2021): n. pag. Web.