No abstract

Recently, the contribution of earthworms to plastic degradation and their capability to swallow smaller plastic fragments, known as microplastics, has been emphasized. The worm physically changes the size of microplastics and enhances microbial activities to increase the possibility of degradation. However, no research has shown that earthworms can chemically degrade microplastics to an element form, CO2 or H2O. In this review, previous research has been thoroughly explored to analyse the role that earthworms could play in plastic degradation in the soil. Earthworms can significantly affect the physical characteristics of plastics. However, earthworms’ abilities to chemically degrade or change the chemical structure of plastics and microplastics have not been observed. Additionally, earthworms exhibit selective feeding behaviour, avoiding areas containing a high plastics concentration and rejecting plastics. Consequently, earthworms’ abilities to adapt to the microplastics in soil in the environment can cause a problem. Based on this review, the challenges faced in earthworm application for plastic degradation are mostly expected to be associated with the toxicity and complexity of the plastic material and environmental factors, such as the moisture content of the soil and its temperature, microbial population, and feeding method.

The uptake of polylactic acid (PLA) microplastics, the most commonly used biodegradable plastics by earthworms Eudrilus eugeniae was investigated by observing their weight changes, biomass and the microplastics concentration after feeding them with a mixture of PLA and cow dung for 16 days at the concentrations of 0%, 10%, 30%, 60%, and 80% w/w dry weight. The mortality rate of the earthworms for all the PLA concentrations during the 16 days of feeding period was 0%. However, the microplastic affected the earthworms' weight significantly (P-value 0.00027), especially at the concentration of 80% of PLA. The earthworms had the lowest weight gain at 80%, followed by 60%, 30%, and 10% of PLA, respectively. The earthworms subjected to 80% and 10% of PLA had a similar pattern with the control. This was perhaps due to the high organic matter in the worm’s feed for the 10% PLA and the digestion mechanisms, which led the earthworms to preserve their energy and become less active in the 80% PLA, compared to all the other treatments. The PLA microplastics concentration factor (CF) in the vermicast was the highest at the 10 % PLA compared to other treatments, because Eudrilus eugeniae did not degrade the PLA. This study concludes that even though PLA is a biopolymer-based, it cannot be bio- assimilated by the earthworm.