Question: I found an interesting news article published in The Guardian about biodegradable plastics and how they may not be the ideal solution to the ocean pollution problem. This is mainly due to the fact that they only breakdown at very high temperatures generally not found in nature (around 50 degrees C). My question is what makes plastics so resistant to bacteria and fungi which normally breakdown organic materials?

Answer: Even though many other organic polymers like peptides and carbohydrates are readily broken down by bacteria and fungi, plastics are not. This is mainly due to the fact that they contain many carbon-carbon bonds which, unlike nitrogen-carbon or oxygen-carbon bonds found in peptides and carbohydrates, are extremely stable. For example, the common plastic polyethylene is composed only of carbon-carbon bonds (ignoring hydrogen). Plastics also often contain components which are very uncommon in biological polymers. This makes it difficult for enzymes, which have evolved to recognize and act on biological compounds, to break down these polymers. For example, the chlorine found in polyvinyl chloride is absent from most or all biological polymers (Figure 1). It should also be noted that many biodegradable plastics currently exist like nitrocellulose, and can be produced from starch and cellulose. They are not useable for many applications, however, because they are easily broken down by microbes and have a short shelf life. For example, PVC piping may cause pollution but is preferable than having decomposing pipes.

Increasing rates of plastic recycling is a good way to limit the effects of plastic pollution. Current rates of plastic recycling are very low, far below rates of paper and metal recycling, but have been rising since 1990. New technologies, such as integrating waste plastics into building material, are helping fuel this increase.

Even as rates of recycling increase, research and development of practical biodegradable plastics should continue. Work by Tokiwa et. al. suggests that the development and widespread use of biodegradable plastics will have far reaching consequences beyond simply less waste in landfills. They may increase soil fertility, reduce greenhouse emissions, and decrease waste management costs. In addition, they could potentially be recycled into useful metabolites and biological substances.

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Figure 1. The presence of chlorine in PVC makes biological decomposition difficult (Wikipedia).

  • Debanjan D.

References:

  • “Biodegradable plastic ‘false solution’ for ocean waste problem” The Guardian, 2016. Web. June 15 2016.
  • Why Doesn’t Plastic Biodegrade?” Livescience, 2011. Web. 15 June 2016.
  • Tokiwa Y, Calabia BP, Ugwu CU, Aiba S. Biodegradability of Plastics. International Journal of Molecular Sciences. 2009;10(9):3722-3742. doi:10.3390/ijms10093722.

Hopewell J, Dvorak R, Kosior E. Plastics recycling: challenges and opportunities. The Royal Society Publishing.  14 June 2009.DOI: 10.1098/rstb.2008.0311.

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