Microscopic Manufacturers Produce Eco-friendly Plastics

"Organic waste from agriculture, industries and households forms a very large resource that is currently discarded or at best transformed into biogas. From a sustainability point of view it is desired to convert these organic resources in chemicals," says Mark van Loosdrecht of Delft University of Technology in the Netherlands, who has been working on using bacteria to transform this waste into bioplastics known as polyhydroxyalkanoates (PHAs).

PHAs are linear polyesters produced by bacterial fermentation of sugar or lipids (fats). They are produced by the bacteria to store carbon and energy. More than 150 different monomers can be combined within this family to give materials with extremely different properties. These plastics are biodegradeable and are used in the production of bioplastics. However, the high cost of PHA production compared to conventional plastics has limited their use in a wide range of applications.

Using technology derived from wastewater treatment systems, van Loosdrecht and his lab have developed a process using open microbial cultures to convert organic wastes to PHAs. This new process is able to produce just as much PHA as existing processes at specific rates that are up to three times faster.

Kevin O'Connor at the University College in Dublin, Ireland, has also developed a new process using bacteria to produce PHAs from waste, only the waste is not organic. O'Connor has found a way to transform traditional plastics into biodegradable plastics. Using a process called pyrolysis, the waste plastics are heated in the absence of air, causing a breakdown of the molecular bonds. What's left is an oil that is then fed to natural soil bacteria that use it to produce PHA.

The process was initially developed using polystyrene, one of the most widely used plastics, but O'Connor says it also works on other plastics including polyethylene terephthalate (PET), the plastic used to make water bottles.

Richard Gross from the Polytechnic University in Brooklyn, New York, is using bacteria that produce a building block from vegetable oils that can be used to make a plastic that is very much like polyethylene. However, unlike polyethylene, when it becomes waste it can be converted by mild enzymatic methods to biodiesel fuel.

"We were challenged by the Defense Advanced Research Projects Agency (DARPA) come up with a plastic that could be broken back down to liquid fuel. I thought about that and realized that we needed to make plastic from building blocks that could later serve as fuel elements," says Gross.

That basic starting material for this work are vegetable oils that consist of fatty acids. Gross in collaboration with colleagues at DNA 2.0 (Menlo Park, CA) engineered a yeast to specifically ferment a fatty acid into a compound that can be processed into a bioplastic.

"The plastic is very much like polyethylene. It will process like it, it will feel like it, people will be comfortable with it," says Gross.

When the plastic becomes waste, it can be broken down and processed into biodiesel using an enzyme. While the process for conversion of the plastic to biodiesel works in the lab, it is not efficient enough for commercial viability.

"We are now looking for a really efficient enzyme that can convert the plastic back to its building blocks. We have found microbes and enzymes that do break it down completely but we still need to improve their efficiencies," says Gross.