Plastic waste as a novel substrate for industrial biotechnology.

Abstract

Two hundred and seventy five million tons of plastic waste were produced in 2010 alone (Jambeck et al., 2015), with Europe accounting for about 55 million tons per year. The environmental impact of these, primarily fossil-based, plastics has been broadly discussed. While the vast majority of these polymers are not biodegradable, their strength and light weight provide comparative advantages. Poly(ethylene terephthalate) (PET), for instance, has contributed significantly to reducing energy expenditure during transport, especially in the beverage industry. Due to its thermoplastic nature PET is also easy to recycle. However, recycled PET products struggle to compete with virgin PET on price and quality, leading to an overall European recycling rate of less than 30% (PlasticsEurope, 2015). Polyurethanes (PU) are used extensively in a wide range of applications including construction, transportation, furniture and medicine. Since many PU types have a thermoset nature with covalent cross links, one of the main concerns for this plastic is the notable lack of end-of-life recycling (< 5%). Finally, polyethylene (the most used plastic, ca. 140 million tons per year), is considered to be practically inert and its recycling (other than its downcycling into lumber) is economically unfavourable (Sivan, 2011), thereby creating a phenomenal environmental impact, especially in marine environments (Cozar et al., 2014; 2015). While a few countries manage major fractions of plastic waste through incineration in controlled industrial facilities, release of recalcitrant post-consumer plastic into nature remains a major problem globally. Significant amounts of plastic waste contribute to the large-scale pollution of the oceans (Katsnelson, 2015), with terms such as ‘the Great Pacific Garbage Patch’ and ‘the Trash Vortex’ mobilizing public opinion. The widespread distribution of microplastics in the food chain, with as yet unknown effects on biodiversity and human health is also appearing more in the scientific and general literature (Allsopp et al., 2006; Setala et al., 2014; Avio et al., 2015). The momentum for change away from current practices and towards a sustainable model of exploitation of waste and renewable resources is growing. In order to counteract the pollutant/recycling problems, the revised European Union (EU) Waste Framework Directive has set a minimum plastic recycling target of 50% for household waste and 70% for building and construction waste, which must be reached by all EU member states by 2020. However, without a clear technology roadmap – not to mention an attractive market strategy, the increase in recycling rates will in our opinion not be achievable. Given this background, we propose to use plastic wastes as substrates for the synthesis of added value products, which will empower the recycling industry to a qualitatively new dimension. How can microbial biotechnology contribute now to this enormous challenge? The advances in our understanding of microbial functions from enzyme, to pathways, and entire metabolic networks now allow the engineering of complex metabolic functions in microbes (Blank and Ebert, 2013). With the ever-advancing tools from synthetic biology e.g. for genome editing (Nikel et al., 2014), we can overcome the major challenges for a biotechnological plastic waste-based value chain. Some of the challenges and recent developments are highlighted below.