Microplastic Effects on Zostera and the Possible Utility of Zostera as a Bioremediation Crop for Reduction of Microplastics in Coastal Ecosystems

Abstract 

This paper reviews the etiology of microplastic (MP) accumulation in Zostera bed ecosystems and the possible effects this has on the plant, the associated epiphytes, and the ecosystem. Recent studies have shown that seagrasses, specifically the family of eelgrasses known as Zostera, and their associated epiphytes are uniquely efficient at trapping marine microplastics (MPs) in the sediment and on the leaves of the grass. This paper reviews the mechanisms behind this process, showing they are both mechanical and biological in nature. This paper also reviews the potential negative higher trophic effects that could be caused by the higher concentration of MPs in eelgrass beds relative to the surrounding water. These findings can help inform the possibilities of using Zostera for bioremediation of MPs to highlight their possible repercussions and benefits. 

Introduction 

Over 14 million tons of microplastics, pieces of plastic less than 5mm (MPs), have accumulated in the world's oceans to date. An additional 1.5 million tons enter the ocean annually [1, 2], translating to 50 to 75 trillion pieces of MPs in our oceans in total, with more being added each year. While MPs are ubiquitous to every aquatic ecosystem [3], they are disproportionately prevalent in coastal environments [4]. 

Along the California coastline, this is attributed to high urbanization in former wetland habitats [4]. Rain picks up plastic wastes from urban environments and breaks them down into millions of MPs; they are then flushed into estuaries, wetlands, and beaches as runoff [5]. Consequently, marine MPs pose many risks for coastal life as they are a pervasive vector for carcinogens and toxic heavy metals (used in the plastic production process) that accumulate in the tissues of marine organisms [6, 7]. These MPs and associated carcinogens also work their way back into terrestrial environments through consumption of contaminated organisms by humans and other terrestrial animals. 

A focal organism in this environmental issue is the eelgrass, a marine plant whose beds were once widespread along the California coastline but has since shrunk by 90% over the last century due to coastal urbanization [8]. Eelgrass beds are among the most productive ecosystems in the world and serve a wide variety of key ecological functions, including refuge for endangered species, habitat for benthic organisms, water purification, and carbon sinking [9]. The remaining beds of the most common genus, Zostera, concentrate within bays and estuaries that receive an influx of inland freshwater; as a result they collect a large portion of MP-contaminated urban runoff. This paper will elucidate the mechanisms in which MPs get trapped in Zostera beds, as well as examine the ecological impacts this has. These findings will help to discuss Zostera restoration for its possible utility as a bioremediation crop for removal of MPs from water.

Microplastic Trap 

Recent findings suggest that eelgrass beds are remarkably effective at trapping and sequestering MPs [10, 11]. The mechanisms for this are both mechanical and biological in nature. To determine the mechanical mechanism, de Los Santos et al. (2021) used Zostera in flow-through chambers to observe the way that MPs flow through the eelgrass. The researchers found that Zostera foliage slowed down the flow of water and caused the MPs to either become lodged in the sediment (sedimentation) or stuck on the leaves. Furthermore, they found that the higher the foliage density of the eelgrass, the more MP particles were trapped [12]. de Smit et al. (2021) found similar results and further elaborated the effects of near-bed turbulence created by seagrass structures in this process [13]. Additionally, another study examined the biological mechanisms of MP entrapment of Zostera by collecting samples from Huiquan Bay [10]. They found that MPs can adhere to eelgrass leaves to contribute to the amassing of microorganisms into biofilm. This biofilm forms a viscous white floc and helps to further capture MPs and promote sedimentation of the particles. The researchers were also able to isolate two epiphytic bacteria (Vibrio and Exiguobacterium) which lowered suspended MPs by roughly 95%. These studies together show that both the eelgrass foliage and the associated epiphytic communities help to sediment MPs as well as adhere them to the surface of the leaf. 

The one caveat to this entrapment of MPs is that Zostera, like land plants, has a dieback of leaves in the winter months. As the seagrass loses its leaves in the autumn like land plants, these leaves build up and wash up to the shore. To examine what happens to MPs after leaf loss, researchers observed the effects of these loose seagrass leaves on MP entrapment in coastal waters by looking at the Neptune grass (Posidonia oceanica) [14].  The researchers collected these loose leaf wracks and aegagropila (natural aggregates of vegetal fibers intertwined by seawater motion) in transects along the coast. Then, they recorded plastic content by weight in petri dishes to determine entrapped plastic composition. They determined these loose leaf wracks were also effective at trapping plastics. The mechanism for this lies in the formation of lignocellulosic debris made by decomposing eelgrass leaves that trap these plastics. This finding provides evidence that eelgrass leaves maintain the MPs trapped after they die and can even collect more MPs as they aggregate into aegagropila. The researchers also concluded that this is a novel function in the eelgrass coastal ecosystem, as no other plant in the ecosystem performs this function. 

Higher Trophic Effects 

Zostera’s MP sinking abilities have raised questions as to the impacts this property has on the wider ecosystem. Horn et al. (2019) studied MP ingestion in a Pacific Coast crustacean species commonly known as Pacific mole crabs (Emerita analoga) [15]. The researchers decided to use Pacific mole crabs because they make up 84% of the biomass on California sandy beaches [15, 16]. Therefore, they are a main food source for other organisms like birds and coastal fish, causing them to be a vector of MPs in many marine food webs. The purpose of this study was to determine if MP concentrations in the sediments they lived in determined how many MPs they ingested. By sampling beaches with varying MP concentrations, they found that crabs from every sampled beach contained MPs. However, the number of ingested MPs did not depend on the MP concentration in the sediment. This finding indicates that regardless of how many MPs eelgrass removes or sediments, the mole crabs will ingest roughly the same amount of MPs. However, it is possible that eelgrass (through mechanisms of either leaf entrapment, sedimentation, or leaf wrack trapping) can prevent MPs from reaching the sandy beach water line, decreasing the amount of MPs mole crabs are exposed to. 

Even if eelgrass MP sinking has limited effects on mole crabs, there is a concern for the organisms living in the sediments of eelgrass beds as well as those that predate on eelgrass. A study done at the Lake of Bizerte in Tunisia found that molluscs living in eelgrass beds had similar concentrations of MPs in their body as the surrounding eelgrass bed water, indicating a very high transfer rate of MPs to molluscs [17]. MPs have also been shown to decrease oyster reproduction, which could decrease the water filtration abilities of the eelgrass bed ecosystem because oysters and other bivalves are the most productive filter feeders in the system [18]. Another study used turtle grass (Thalassia testudinum) to study predator ingestion of MPs. Turtle grass is very similar to eelgrass in terms of MP entrapment, sedimentation abilities, and similar vibrant epiphytic community [19]. They found conclusive evidence that the main eelgrass predator of the area, parrotfish, preferred to consume turtle grass leaves with higher epiphytic encrustment. Since higher turtle grass encrustment also correlates with higher MP content on the leaf, this means that the parrotfish were preferring to consume leaves with more MPs. The increased MP ingestion by these organisms could lead to a decrease in overall ecological health as well as contribute to higher MP transfer rates to the wider ecosystem. 

Other Considerations 

Other considerations should be taken into account when examining Zostera’s use in bioremediation of MPs. Here, we highlight two studies that pertain to the effects of MPs on eelgrass health, including its epiphytic community and how that epiphytic community can possibly facilitate the degradation of MPs. In the first study, researchers wanted to quantify the respiration effects of MPs on the common eelgrass (Z. marina) and its epiphytes. Samples were collected from Northern Zealand, Denmark and placed in aerated aquariums with a 14hr daylight cycle. MPs were then added to the flow through chambers. By using fluorescence imaging to observe chlorophyll activity, photosynthetic ability was measured with and without MP treatment. This same imaging was also done on epiphytic communities isolated from the leaves with and without MP treatment. It was determined that MPs had limited short-term effect on photosynthesis, respiration, and carbonate utilization [20], which supports the utilization of Zostera in phytoremediation of MPs. However, it is noted in a review by Gerstenbacher et al. (2022) that very few studies have been conducted that look directly at the epiphytic community of the plant. Therefore, no solid conclusions about the possible negative effects to the eelgrass microbiome can be made [21].

The second study by Viel et al. (2023) examined the mechanisms for which MPs are degraded metabolically by bacteria, fungi, and microalgae, all of which include members of the diverse epiphytic community of eelgrass blades. The processes of biological degradation resulted from the colonization and biofilm formation of microorganisms on the polymer surface. This induces biodegradation via surface erosion and abiotic hydrolysis of plastic functional groups [22]. Since MP adherence to eelgrass blades promotes biofilm growth around particles (as mentioned earlier), this suggests that MPs within eelgrass beds are being degraded at an accelerated rate compared to MPs suspended in surrounding water.

Conclusion 

In this paper, we discussed the known effects MPs have on Zostera plants as well as the wider ecosystem. While it has been shown that Zostera can be used as a potential phytoremediation crop to entrap  MPs in the natural environment, the potential risks to higher trophic levels leaves doubt for its future utility for the MP problem. More research on whether free floating MP is more plentiful and harmful in coastal areas without the eelgrass beds is needed to fully determine the latter’s efficacy of trapping microplastics in the natural environment. However, reviews such as Masiá et al. (2020) introduce the possibilities of using seagrasses as wastewater treatment tools in specialized treatment plants [23]. A treatment plant such as this would not contain organisms for consumption to the higher food chain, while the epiphytic community of the eelgrass would also facilitate quicker degradation of MPs. Other methods of microplastic removal from the ocean should be investigated. However, mitigation is still the best measure for the reduction of MPs from the marine environment, as preventing the plastic from entering the water in the first place will cut down on the vast majority of MPs. Regardless of use in the MP problem, eelgrass population rehabilitation should stay relevant due to its immense ecological role.

About the Author: Andrew Matayoshi

Andrew Matayoshi is a biological sciences major at UC Davis with an interest in marine ecology. He is particularly interested in researching coastal invasive species as well as coastal anthropogenic stressors that affect some of the most productive ecosystems in the world such as eelgrass beds. Andrew is not only passionate about research, but also finding implementable solutions to the big problems facing our coast which prompted him to write this piece. Solutions are needed now more than ever which is why Andrew helps run a startup dedicated to kelp forest restoration, and hopes by writing pieces like this and investigating innovative solutions, he can inspire more people to make actionable changes in their environment.

Author’s Note

I wrote this piece as the final assignment for the class UWP102B: Writing in Biology. I would expect that this undergraduate community is already quite familiar with the problem of microplastics in our marine environment, or has heard of it at the very least. In writing this paper, I wanted to take a look at a relatively niche aspect of this problem that people may be less familiar with, as well as try to elicit future research questions. I currently work in a lab studying eelgrass microbiomes which prompted me to think about the effects microplastics may have on this vital coastal plant which comprises some of the most productive ecosystems in the world. Through analysis of cutting-edge primary research, I plan to showcase our current understanding of this issue and look to the future about the utilization of this plant. I want readers to come away with a new perspective on this problem and prompt them to think deeper about this plastic problem that pervades and affects every ecosystem in the world. Special thanks to Dr. Amy Goodman-Bide for her invaluable advice and support.

References

1. Barrett J, Chase Z, Zhang J, Holl MMB, Willis K, Williams A, Hardesty BD, Wilcox C. Microplastic Pollution in Deep-Sea Sediments From the Great Australian Bight. Frontiers in Marine Science. 2020;7:576170. doi:10.3389/fmars.2020.576170 
2. Alfaro-Núñez A, Astorga D, Cáceres-Farías L, Bastidas L, Soto Villegas C, Macay K, Christensen JH. Microplastic pollution in seawater and marine organisms across the Tropical Eastern Pacific and Galápagos. Scientific Reports. 2021;11(1):6424. doi:10.1038/s41598-021-85939-3 
3. Moyo S. An enigma: A meta-analysis reveals the effect of ubiquitous microplastics on different taxa in aquatic systems. Frontiers in Environmental Science. 2022;10:999349. doi:10.3389/fenvs.2022.999349 
4. Sutton R, Franz A, Gilbreath A, Lin D, Miller L, Sedlak M, Wong A. 2019 Aug. Understanding Microplastic Levels, Pathways, and Transport in the San Francisco Bay Region. Richmond, California: San Francisco Estuary Institute. SFEI Contribution No. 950.
5. Werbowski LM, Gilbreath AN, Munno K, Zhu X, Grbic J, Wu T, Sutton R, Sedlak MD, Deshpande AD, Rochman CM. Urban stormwater runoff: A major pathway for anthropogenic particles, black rubbery fragments, and other types of microplastics to urban receiving waters. ACS ES&T Water. 2021;1(6):1420–1428. doi:10.1021/acsestwater.1c00017
6. Teng J, Zhao J, Zhang C, Cheng B, Koelmans AA, Wu D, Gao M, Sun X, Liu Y, Wang Q. A systems analysis of microplastic pollution in Laizhou Bay, China. The Science of the Total Environment. 2020;745:140815. doi:10.1016/j.scitotenv.2020.140815
7. Li W, Lo H-S, Wong H-M, Zhou M, Wong C-Y, Tam NF-Y, Cheung S-G. Heavy metals contamination of sedimentary microplastics in Hong Kong. Marine Pollution Bulletin. 2020;153:110977. doi:10.1016/j.marpolbul.2020.110977 
8. Walter RK, O’Leary JK, Vitousek S, Taherkhani M, Geraghty C, Kitajima A. Large-scale erosion driven by intertidal eelgrass loss in an estuarine environment. Estuarine, Coastal, and Shelf Science. 2020;243:106910. doi:10.1016/j.ecss.2020.106910 
9. Unsworth RKF, Cullen-Unsworth LC, Jones BLH, Lilley RJ. The planetary role of seagrass conservation. Science. 2022;377(6606):609–613. doi:10.1126/science.abq6923 
10. Zhao L, Ru S, He J, Zhang Z, Song X, Wang D, Li X, Wang J. Eelgrass (Zostera marina) and its epiphytic bacteria facilitate the sinking of microplastics in the seawater. Environmental Pollution. 2022;292(Pt A):118337. doi:10.1016/j.envpol.2021.118337 
11. Gerstenbacher CM, Finzi AC, Rotjan RD, Novak AB. A review of microplastic impacts on seagrasses, epiphytes, and associated sediment communities. Environmental Pollution. 2022;303:119108. doi:10.1016/j.envpol.2022.119108 
12. de Los Santos CB, Krång A-S, Infantes E. Microplastic retention by marine vegetated canopies: Simulations with seagrass meadows in a hydraulic flume. Environmental Pollution. 2021;269:116050. doi:10.1016/j.envpol.2020.116050 
13. de Smit JC, Anton A, Martin C, Rossbach S, Bouma TJ, Duarte CM. Habitat-forming species trap microplastics into coastal sediment sinks. The Science of the Total Environment. 2021;772:145520. doi:10.1016/j.scitotenv.2021.145520 
14. Sanchez-Vidal A, Canals M, de Haan WP, Romero J, Veny M. Seagrasses provide a novel ecosystem service by trapping marine plastics. Scientific Reports. 2021;11(1):254. doi:10.1038/s41598-020-79370-3 
15. Horn D, Miller M, Anderson S, Steele C. Microplastics are ubiquitous on California beaches and enter the coastal food web through consumption by Pacific mole crabs. Marine Pollution Bulletin. 2019;139:231–237. doi:10.1016/j.marpolbul.2018.12.039 
16. Nielsen KJ, Morgan SG, Dugan JE. Baseline characterization of sandy beach ecosystems in California’s North-Central Coast region. 2013. California Ocean Protection Council Data Repository.
17. Abidli S, Lahbib Y, Trigui El Menif N. Microplastics in commercial molluscs from the lagoon of Bizerte (Northern Tunisia). Marine Pollution Bulletin. 2019;142:243–252. doi:10.1016/j.marpolbul.2019.03.048 
18. Sussarellu R, Suquet M, Thomas Y, Lambert C, Fabioux C, Pernet MEJ, Le Goïc N, Quillien V, Mingant C, Epelboin Y, et al. Oyster reproduction is affected by exposure to polystyrene microplastics. Proceedings of the National Academy of Sciences of the United States of America. 2016;113(9):2430–2435. doi:10.1073/pnas.1519019113 
19. Goss H, Jaskiel J, Rotjan R. Thalassia testudinum as a potential vector for incorporating microplastics into benthic marine food webs. Marine Pollution Bulletin. 2018;135:1085–1089. doi:10.1016/j.marpolbul.2018.08.024 
20. Molin JM, Groth-Andersen WE, Hansen PJ, Kühl M, Brodersen KE. Microplastic pollution associated with reduced respiration in seagrass (Zostera marina L.) and associated epiphytes. Frontiers in Marine Science. 2023;10:1216299. doi:10.3389/fmars.2023.1216299
21. Gerstenbacher CM, Finzi AC, Rotjan RD, Novak AB. A review of microplastic impacts on seagrasses, epiphytes, and associated sediment communities. Environmental Pollution. 2022;303:119108. doi:10.1016/j.envpol.2022.119108
22. Viel T, Manfra L, Zupo V, Libralato G, Cocca M, Costantini M. Biodegradation of plastics induced by marine organisms: future perspectives for bioremediation approaches. Polymers. 2023;15(12):2673. doi:10.3390/polym15122673 
23. Masiá P, Sol D, Ardura A, Laca A, Borrell YJ, Dopico E, Laca A, Machado-Schiaffino G, Díaz M, Garcia-Vazquez E. Bioremediation as a promising strategy for microplastics removal in wastewater treatment plants. Marine Pollution Bulletin. 2020;156:111252. doi:10.1016/j.marpolbul.2020.111252