Despite the practice of reducing, reusing, and recycling, plastics waste still poses a major problem due to its increasing diversity of properties and quantity. In 2013, plastics' global packaging production was 78 million tonnes, of which, 98% were produced using new non-recycled material (Geyer et al. 2017). Although 14% of these plastics were collected for recycling, only 2% were eventually recycled (Zhu, Yao, and Liu 2006). About 32% of the produced plastics end up in oceans or on land. In 2018, plastics' global production had reached 8.3 billion metric tons, and 76% of them became plastic waste. Only 9% of plastics produced was recycled, and 12% was incinerated. Approximately 79% of the plastics waste produced end up accumulating in a landfill or leaked into the environment (Geyer et al. 2017). By comparing the data from 2013 and 2018, it is evident that although the recycling rate has increased, the total volume of plastic waste produced has also been increased more than a hundred times.
The low recovery of plastics waste happened due to some reasons. Recycling facilities are expensive to built, and segregation requires extensive workforce. Regardless of the damaging effect towards the environment, these facilities are unable to recycle all type of plastics waste. For example, China's recycling facilities were the main cause of its enormous environmental pollution (Katx, 2019). On that note, China declared that they would stop importing plastics waste from any developed countries from July 2017. Due to this policy, other countries like Malaysia, Vietnam, Thailand, Indonesia, Taiwan, South Korea, Turkey, India and Poland were the new dumping sited for the developed counties (Asia 2019; BBC Reality Check Team 2019). This is one of the evidences of a global plastic waste issue. Neither all plastics can be recycled, nor their quality is the same after recycling (Achilias et al. 2007; Hopewell, Dvorak, and Kosior 2009; Schyns and Shaver 2020). Camacho and Karlsson (2000) reported that recycled plastics (food packaging plastics) were five times more hazardous contaminants (ethylbenzene and xylenes) than the non-recycled ones. Under these circumstances, bioplastics and biodegradable plastics have been promoted as an alternative to the non-degradable fossil-based
Bioplastics are renewable and sustainable resources, known primarily by aliphatic bio-polyester such as polyhydroxyalkanoates (PHAs) or poly-3-hydroxybutyrate (PHB), polyhydroxy valerate (PHV), polyhydroxyalkanoate (PHH) and polylactic acid (PLA) (Altaf et al., 2007; Carus, 2012). There has been increasing demand on this type of plastics particularly the PLA (polylactic acid) and PHAs (polyhydroxyalkanoates). For example, the PLA production reached 140,000 metric tons of PLA per year in the USA by Blair facility alone. It was estimated that the global bioplastics production would reach 2.62 tonnes in 2023 (Rosevelt et al. 2013). This shows that PLA maybe one of the leading bioplastics for the future (Jamshidian et al., 2010). Due to the increase in the production of bioplastics, it is highly expected to contribute to plastic waste problem. Since most biodegradation testing of bioplastic are done in a controlled conditions for a start, it is highly impossible to ensure a complete degradation of these materials. Their accumulation and fragmentation into smaller particles in the environment is likely. These fine particles have been showing various impacts over ingested organisms (González-Pleiter et al. 2019; Shruti and Kutralam-Muniasamy 2019; Zuo et al. 2019).
In general, plastics are complex long-lasting materials. However, with prolonged exposure to weathering condition, plastics tend to shatter into smaller pieces, both in the open ocean or buried inside the soil. These fragmentations are known as microplastics with a size range starting from 5 mm to nanometres. These fine size particles possess high possibilities for interfering with the food chain and can cause damage to both flora and fauna (de Souza Machado et al., 2018a). The land is the primary source of plastics and microplastics, production and disposal, with estimated annual release of 4 to 23 times more than in the oceans (Horton et al. 2017; de Souza Machado et al. 2018). Despite the widespread of microplastics presence inland, long-term or large-scale monitoring data are limited. Sources of terrestrial of microplastics and its effect on terrestrial microorganism are overlocked until recent years (Rillig et al., 2017; de Souza Machado et al., 2018a).
Soil ecosystem facilitates a variety of services such as carbon sequestration, biogeochemical cycling, and promotion of biodiversity. There has been an increase in the documentation of microplastics polluting soil, with a high potential effect on soil biodiversity and function. There is also a lack of evidence on this pollutant destructive behaviour in the soil (de Souza Machado et al., 2018b). The earthworms positively affect the soil structure and the decomposition and mineralization of litter by breaking down organic matter and increasing soil fertility. Due to their impact on soil properties and their influence on the availability of resources for other species, including microorganisms and plants, earthworms are known as soil engineers. Nevertheless, less attention has been paid to their impacts on the soil ecosystem (Kooch and Jalilvand, 2008). Thus, the potential of earthworm for plastics degradation and toxicity is underexplored.
Microplastics can enter the soil ecosystems in a few different ways such as by mulching materials used in the agriculture, during the production of microplastics, secondary sources from the breaking down of plastics materials and sludge produced by wastewater treatment. A few researchers reported the possibilities of surface microplastics transported by the earthworms into the more in-depth soil profile (Huerta Lwanga et al. 2016; Matthias C. Rillig, Ziersch, and Hempel 2017; Yu et al. 2019; Zhang et al. 2018). Worms move particles in two ways; by the adhesion on the their body when it comes to contact with the pollutant, and by their cast as a result of ingestion. A higher concentration of microplastics has been detected in the L. Terrestris burrow compared to the surface area (Huerta Lwanga et al., 2016). Therefore, there is a potential effect of leaching microplastics to the groundwater throughout the earthworm burrows.
The mortality of earthworms caused by petroleum and bio-based microplastics were observed in a few studies. For example, according to Cao et al. (2017), 2% of polystyrene (PS) which is petroleum-based plastics mixed with the earthworms feed cause a significant inhibition to the growth of the earthworms. In the case of low-density Polyethylene (LDPE), the mortality rate of the earthworms was between 8–25% when the LDPE concentration was 28 % and 60%, as observed by Huerta Lwanga et al. (2017). Nevertheless, a contradictory result was observed by Zhang et al. (2018). In his study, exposure of the earthworms to LDPE films resulted in zero per cent mortality. This result might be caused by selective behavior of the earthworms toward their feed. Polylactic acid (PLA) is known as bio-based plastics. Even though the mortality rate was zero, the weight loss of worms subjected to PLA was evident (Alauzet et al. 2002). The earthworms were also unable to digest and consume the PLA as carbon source (Qi et al. 2018). Even though PLA is plant-based plastics, it shows resistance to degradation without exposure to hydrolysis degradation first. However, the two studies were conducted using PLA 50 and PLA 96, and any research on the effect of commercial PLA towards the fitness of Eudrilus eugeniae is yet to be conducted. Eudrilus eugeniae has a natural ability to colonize organic waste, with high endurance and handling resistance, possess tolerance to a wide range of environmental factors and capable of digestion and assimilation of organic matter (Kooch and Jalilvand, 2008).
The purpose of this research is to observe the possibility of Eudrilus eugeniae earthworms to degrade PLA microplastics, as well as the effect of different concentrations of PLA on their weight changes, growth rate and cast concentration factor (CF) when the specific concentration of PLA microplastics was added into their feed.