Bioremediation to Overcome Microplastic Contamination in The Water Environment

Microplastics are a global environmental issue that is receiving increasing attention. Microplastic particles, which are plastic fragments measuring less than 5 mm, are widely distributed worldwide and have negative impacts on the environment, living organisms, and human health. Bioremediation is one method to address this problem, using living organisms to remove microplastics from the environment. Bioremediation involves the use of living organisms, such as bacteria, algae, worms, and other organisms, to clean up or degrade microplastics that are widely dispersed in the environment. This article explains the concept and role of bioremediation in addressing microplastic contamination, the factors influencing its effectiveness, and the advantages and challenges of using bioremediation. This article is a systematic literature review. The data used in this research consist of 23 scientific articles, with 21 indexed in Scopus and 2 indexed in Google Scholar. Through this article, it is hoped to obtain comprehensive insights into the future potential of sustainable bioremediation technology in addressing the issue of microplastic contamination in aquatic environments.


Introduction
Plastic waste in landfill sites leads to soil and water pollution due to the presence of toxic and hazardous chemicals in plastic waste [1].Plastic waste forms particulate matter and undergoes degradation due to UV radiation, making it easily assimilated into the food chain, leading to the potential death of living organisms.The degradation process of plastic in the environment results in a new type of waste known as microplastic waste.The suspended plastic particles in the surface waters of the sea become small, reducing light transmission, photosynthetic efficiency [2] of microalgae, marine organism productivity, and causing sedimentation due to the accumulation of plastic waste on the ocean floor.Microplastics can act as a medium for enhancing the adsorption of hydrophobic chemicals that are not easily degraded, such as polychlorinated biphenyls (PCBs) [3], when they enter the food chain.
In aquatic environments, the most effective method for treating microplastics currently is through bioremediation technology using biofilms [4].Macroorganisms in water, such as seagrasses and macrophytes, are good candidates for microplastic bioremediation efforts.The production method of microplastics is divided into primary and secondary microplastics.Primary microplastics include microbeads in cosmetic and personal care products, abrasive materials used in industrial explosion 1317 (2024) 012027 IOP Publishing doi:10.1088/1755-1315/1317/1/012027 2 cleaning, microfibers released from synthetic textiles, and pure resin particles that end up in wastewater pipes due to artificial flushing, rainfall, or in natural water bodies.Secondary plastics are formed from the breakdown of larger plastics through natural weathering (environmental factors such as sunlight, temperature, and ocean waves) [4].
Plastic waste entering aquatic environments results from various human activities, such as the discharge of industrial waste, marine fisheries, and the deposition of anthropogenically released plastic fibers in the atmosphere.The migration and transformation pathways of microplastics in the environment are illustrated in Figure 1.As the particle size of microplastics decreases, its specific surface area increases, enhancing its adsorption capacity and facilitating interactions with other pollutants in aquatic environments [5] [6], including heavy metals, persistent organic compounds, and pathogens.Therefore, the removal of microplastics from aquatic environments is currently a research focus [4].[4] Plastics entering the marine environment can entangle or even be ingested by marine animals.The duration of plastic exposure in aquatic environments leads to its breakdown into microplastics [7] due to fragmentation, degradation, and weathering caused by UV radiation and microbial activities.Some microplastic particles settle at the seabed, serving as a substrate for microbial growth and biofilm formation, facilitating the degradation of microplastics by marine microorganisms [8].The biodegradation of microplastics into nanoplastics, along with the chemical compounds constituting the plastic, can be absorbed by marine plants [9].

Figure 1. Schematic model depicting the cycle of microplastics in aquatic environments
This article explains the concept and role of bioremediation in addressing microplastic contamination, the factors influencing its effectiveness, the advantages, and the challenges of using bioremediation.The objective of writing this article is to understand the role of bioremediation in addressing microplastic contamination in aquatic environments.The data used in this research are based on literature findings and bibliographic studies.

Materials and methods
The method used in this article involves conducting a literature review indexed in Scopus and Google Scholar.The number of articles used in writing this article is 23 articles, with 22 indexed in Scopus and 2 others indexed in Google Scholar.Data collection techniques involve summarizing important information with the aim of understanding and testing various theories related to bioremediation in addressing microplastic contamination in aquatic environments.This report evaluates the effectiveness of bioremediation methods in reducing microplastic contamination in aquatic environments, identifying its benefits, pinpointing its obstacles, and evaluating the factors that influence it.

Definition of Microplastics
Microplastics are defined as particles with a diameter of less than 5 mm and consist of polymers [10].The lower size limit of microplastics has not been established, but most studies investigate particles with a minimum size of 330 μm3 and categorize them into two groups: large-sized (1-5 mm) and small-sized (<1 mm).Based on their sources, microplastics are classified into two types: primary and secondary.Primary microplastics are the result of plastic production in micro forms, such as microbeads in skincare products.Secondary microplastics result from the fragmentation of larger plastic pieces [11].
Plastic waste that enters water can break down over time through chemical, physical, and biological processes, transforming the plastic waste into micro-sized particles, referred to as microplastics [12].Microplastics entering the environment accumulate in water and are not easily removed due to their persistent nature.The abundance of microplastics is strongly influenced by the activities and sources of pollution.Currently, microplastic pollution is ubiquitous in the environment, including in the sea, wastewater, freshwater, food, air, and water sources, even in drinking water such as refillable and bottled water.Some microplastics found in drinking water may originate from treatment and distribution systems for tap water or bottling processes for bottled water [11].

Sources and impacts of microplastic pollution in aquatic environments
Microplastics are not visible to the naked eye but can have negative impacts on both biota and water.They can have chemical, physical, and biological effects on organisms that directly ingest them or indirectly ingest them through the consumption of contaminated prey.The negative impacts can include digestive tract blockage, enzyme production inhibition, inhibited growth, decreased steroid hormone levels, and reproductive failure [13].
Microplastics are easily ingested by low trophic level organisms, leading to biomagnification in higher trophic level organisms that consume those at lower trophic levels.Human health issues may be influenced by the accumulation of microplastics in the food chain.Chemical additives used in plastic manufacturing [14], as well as pollutants and persistent organic metals adsorbed on the surface of microplastics, may also be ingested by marine organisms during microplastic consumption, increasing the potential for toxicity [9].The ingestion of microplastics and the potential increase in the concentration of harmful chemicals in species intended for human consumption raise concerns about human health as well.

Bioremediation
Bioremediation is the process of using living organisms, such as bacteria, fungi, or plants, to clean contaminants or pollutants from a polluted environment.The biological degradation process of organic waste under controlled conditions transforms it into non-hazardous substances or reduces its concentration below limits set by regulatory authorities.According to the United States Environmental Protection Agency, bioremediation is a natural process for cleaning hazardous chemicals.This method has become a primary focus in global efforts to address soil, water, and air pollution issues [15].When microbes degrade these hazardous substances, they produce harmless byproducts such as water and gases like CO2.
In the context of microplastics, bioremediation involves the use of organisms, such as bacteria, algae, worms, and other living beings, to degrade or collect microplastics from the environment.Some bacteria, like Ideonella sakaiensis, have been found to have the ability to break down polyethylene terephthalate (PET), a commonly used material in plastic bottle manufacturing [16].Other bacteria are also being researched to assess their ability to degrade various types of microplastics.
Certain species of algae and microalgae show potential in degrading microplastics through enzymatic mechanisms or by absorbing microplastic particles.Types of fungi, such as Aspergillus tubingensis and Penicillium spp.[9], have also been studied for their ability to degrade microplastics.They use specific enzymes to break the chemical bonds in microplastics.Some early studies suggest that earthworms may also play a role in recycling microplastics by altering their chemical structure through digestion and biochemical processes in their intestines.Living organisms have the potential to degrade microplastics through various mechanisms.Organisms have demonstrated the ability to break down microplastics, although this process is still in the evolving stages of ongoing research.

The Role of Bioremediation in Addressing Microplastic Pollution
Aquatic biota experience severe impacts such as reduced food consumption, fertility, and inflammation due to microplastic exposure.Additionally, microplastic consumption induces physical, chemical, and genetic changes in aquatic organisms.Therefore, it is crucial to prevent further entry (from the source) of microplastics and eliminate existing microplastics in the environment [17].Bioremediation mechanisms, such as the biodegradation process, are urgently needed.Microorganisms, such as certain bacteria, have the ability to degrade plastic, using it as a source of carbon and energy.However, the biodegradation process often takes a long time.
Biological filtration involves using microorganisms in the filtration process to collect microplastics from water.This method can be effective in removing microplastics from wastewater.Animal Feeding on Microplastics: Some marine creatures, such as small crustaceans, have been found capable of consuming microplastics [8].In some cases, this can help reduce the amount of microplastics in aquatic environments.Artificial Photosynthesis [2]: This technology combines bacteria capable of degrading plastic with an artificial photosynthesis system that produces oxygen and energy, contributing to the bioremediation process [17].

Factors influencing the effectiveness of bioremediation
Factors influencing bioremediation depend on the technique or method used in the remediation process.In this discussion, we will focus on the biodegradation process involving organisms (including microorganisms).Influential factors in bioremediation include internal factors (the microorganisms involved) and external factors (the supportive environment for microorganism performance).These factors include: pollutant concentration, salinity, pH, temperature, humidity, oxygen concentration, nutrient concentration (such as nitrate, ammonium, and phosphate), and microbial community/population [18].
Sun et al [4] reported that the degradation of microplastics by biofilm is significantly influenced by environmental factors.Conditions such as temperature, pH, and ultraviolet light must be within an optimal range for the degradation process.Biofilm degrades microplastic clusters, initially altering hydrophobicity and molecular weight as well as size through physical and chemical means.Meanwhile, bacterial strains affecting the properties of microplastics must be specifically cultivated [8].Optimal environmental conditions must be provided to encourage biofilm growth on the surface of microplastics.The use of fungi as bioremediation agents is influenced by several factors, such as nutrient content like nitrogen and phosphorus, which can be limiting factors.The biomass content of fungi, the duration of the remediation process, and the type of substrate and mobilization agent are also known to affect the efficiency of mycoremediation.

Case study on the use of bioremediation to address microplastic pollution
Bioremediation of microplastics involves the use of bacteria capable of degrading plastic.Bacteria such as Ideonella sakaiensis have been proven to possess the ability to break down PET plastic [8], commonly used in plastic bottles.They produce enzymes that can break the plastic bonds, transforming it into safer compounds.In addition to bacteria, some studies also explore the use of other organisms such as worms that can consume microplastics in the soil.Biological filtration systems are also employed to collect microplastics from water.
Ideonella sakaiensis bacteria have gained attention in microplastic bioremediation efforts, particularly for Polyethylene Terephthalate (PET) plastic [8].I. sakaiensis has demonstrated a unique ability to degrade PET plastic, which is widely used in plastic bottles, packaging, and fibers.The degradation process of PET plastic by I. sakaiensis occurs through two main enzymes produced by these bacteria.The first enzyme is PETase, which breaks the ester bonds within PET plastic.After these bonds are broken, two intermediate compounds are formed, which can be further broken down by the second enzyme, MHETase.The end result of this process is simpler compounds that can be metabolized by bacteria, such as terephthalic acid and ethylene glycol.
The ability of I. sakaiensis indicates significant potential in degrading existing PET plastic in the environment.Although these bacteria are found around landfill sites containing PET plastic, their industrial-scale use for cleaning PET plastic waste [8] is still in further research stages.The discovery of Ideonella sakaiensis provides hope that bioremediation technology can help address plastic waste issues, especially PET, and offer a more sustainable solution to reduce the environmental impact of plastic use.However, it is important to continue studying and developing this technology for efficient real-world applications.
In terms of the biodegradation ability of Ideonella sakaiensis on PET film, it was found that the bacteria could degrade 30 % of the total PET film measuring 2 cm x 1.5 cm within 18 hours, producing MHET BHET (Bis-hydroxyethyl Terephthalate) and TPA (Terephthalate Acid) compounds.The plastic film's surface area was found to be 3 cm 2 , and the degraded area was 0.9 cm 2 .With the same ratio, assuming the PET film is sized 10 cm x 7.5 cm in plastic processing in the field, the degraded plastic area would be 22.5 cm 2 .This degradation capability is deemed highly effective for PET film.The degradation speed is determined by the plastic's surface area, with a larger surface area leading to a longer degradation process [19].Therefore, compared to intact plastic, processed plastic in small pieces (PET film) would degrade more quickly.

Advantages and Challenges of Bioremediation
The degradation of microplastics [13] by microbes is an efficient way to reduce their accumulation and contamination in the environment.Microbes have the potential to degrade microplastics through biological mechanisms without producing toxic by products.However, the highest degradation achieved is only around 20% [4].The time required by microorganisms to alter their intrinsic properties is relatively long and depends on the characteristics of microplastics such as high molecular weight, stability, specific surface area, and hydrophobicity.
There are several advantages in efforts to address microplastics through bioremediation.One of them is a cleaner environment.Bioremediation helps reduce the levels of microplastics in the environment [20], potentially minimizing their negative impacts on ecosystems and living organisms.Bioremediation is a sustainable effort that utilizes local organisms, thus reducing the risk of introducing invasive species.

Challenges and constraints faced in the implementation of bioremediation
The degradation of microplastics [16] by microbes is an efficient way to reduce accumulation and contamination in the environment.Microbes have the ability to degrade microplastics without producing toxic substances.Another challenge in the degradation of microplastics requires further investigation into enhancing the effectiveness of biodegradation for field-scale applications.Therefore, the introduction of genetic engineering and DNA technology for modifying microorganisms may be prioritized to make these microbes more efficient in addressing issues related to the low rate of microplastic degradation.Further analysis of the genes and genetic products of microorganisms degrading MP can provide a clear picture, in-depth understanding, and knowledge of the molecular mechanisms of the degradation process [21].Further research is still highly needed to develop technology where microorganisms or microorganism-based products are commercially available for microplastic biodegradation [4].
The challenges in implementing bioremediation to address microplastic contamination lie in terms of efficiency.Some bioremediation methods are still not efficient enough in eliminating microplastics