Do biodegradable plastics really degrade in nature?
Plastic pollution is a threat to ecosystems around the world and the animals living in these ecosystems. Additionally, plastic pollution potentially causes food safety issues for humans. Nano-sized particles called microplastics act as a raft for microorganisms, contributing to the spread of pathogens and antibiotic resistance. While most research focuses on ocean plastic pollution, information on the fate and potential hazards of land-based plastic waste is scarce and its impact is likely to be significantly underestimated.
Some scientists have studied the amount of plastic in terrestrial ecosystems and revealed the harmful effects of plastics in the soil on earthworms and microorganisms, as well as on agricultural productivity. It is estimated that the amount of plastic released into terrestrial environments is 4–23 times higher than in marine environments. Plastics are released into the environment not only by the transport of directly disposed of garbage through rivers and oceans but spread also by air transport. The most striking example of this is plastic pollution, which is reported even in remote areas of the Swiss Alps and the Arctic.
Biodegradable plastics
The best way to deal with plastic pollution in the environment is to develop biodegradable plastics. However, they only make up 1% of the total plastic currently produced. Also, little is known about the microbial colonization and degradability of biodegradable plastics in cold environments. Because the degradation tests of these materials are usually done at temperatures above 20°C.
In one study, no signs of biodegradation were observed for both polyethylene (PE) and biodegradable carrier bags after a 3-month incubation at 10°C. In a second study, a 37% weight loss was observed for biodegradable plastic bags incubated in agricultural soil at 25°C for 3 months in the laboratory. However, in the same study, the biodegradation observed in samples incubated in the field under natural conditions was much less.
What does effect the biodegradation of biodegradable plastics?
The biodegradation rate of plastics largely depends on the material used, as well as external factors such as temperature, humidity and UV radiation. Considering this situation, it is estimated that biodegradation in soils in cold regions will be quite slow. The existing microorganisms in soils may be another critical parameter that determines the biodegradability of plastics. The composition of the soil microorganisms is highly dependent on a variety of climate and soil variables.
Alpine and Arctic soils are low in carbon and nitrogen content, and information on their microbial communities is limited. Plastic deposition can introduce a source of carbon and energy into oligotrophic soils, possibly leading to major changes in the soil microbiome and its functions in alpine and Arctic ecosystems. However, it has not been investigated whether enzymatic degradation of plastic is possible in all soils or whether there are microorganisms capable of degrading plastic, especially in distant soils that have just been affected by pollution.
What should be done?
If we can learn more about the plastisphere, we can better understand the role bacteria play in the biodegradation of plastics. As emphasized here, the biodegradability of plastics in the natural environment is very low. The biodegradation of plastic depends on many factors related to its composition, as well as the various ecosystems and environmental conditions encountered during its very long lifespan. For these reasons, plastic polymers continue to accumulate in the sea and on land, and biodegradation rates reported in laboratories are never achieved in the natural environment.
Therefore, studies on the biodegradation of plastic polymers in natural ecosystems should be validated by in situ experiments, especially in the field. Given the complexity and magnitude of the plastic problem, scientists, policymakers and the public need to come together under broad consortia to confront this environmental challenge.
Source
Rüthi J, Bölsterli D, Pardi-Comensoli L, Brunner I and Frey B (2020) The “Plastisphere” of Biodegradable Plastics Is Characterized by Specific Microbial Taxa of Alpine and Arctic Soils. Front. Environ. Sci. 8:562263. doi: 10.3389/fenvs.2020.562263