Newsletter #2 topic - Introduction on PHAs

Polyhydroxyalkanoates (PHAs) are polymers produced naturally by bacteria as a carbon and energy store.  They are both biobased and biodegradable.  This means that they are produced from renewable resources such as sugars and plant oils and they can be broken down by microorganisms to carbon dioxide and water. They can also be produced from waste resources such as used cooking oil or agricultural byproducts. This solves two of the major problems associated with traditional plastics.  Traditional plastics are produced from non-renewable fossil-based resources, and they persist in the environment for many years.

Polyhydroxyalkanoates can be thought of as some microorganisms’ equivalent of fat stores.  When humans eat more food than they need for energy they store the excess as fat.  In a similar way, some bacteria can store excess carbon (which they usually use for energy to grow) inside the cells in storage granules, when they have a shortage of other vital nutrients such as phosphorous and nitrogen.  The carbon is stored in these granules as polyhydroxyalkanoates. When carbon becomes limited, the bacteria can break down stored PHAs for growth and energy.

Figure 1 Bacterial cell containing PHA granules

We can grow the bacteria that produce PHAs under conditions that induce PHA accumulation.  The bacteria are harvested and the PHA can be extracted for use. Depending on the type of microorganism used and the carbon source provided, PHAs with different properties can be produced. Some of these properties are similar to traditional petrochemical plastics which means the PHAs could potentially be used to replace fossil fuel-based plastics in many applications. 

Figure 2 Growth of bacterial cells for PHA production in a bioreactor.

PHAs can be classified into two main types based on the number of carbon atoms in each monomer (the building blocks which make up the polymer).  Short-chain-length PHAs have 3-5 carbons in each monomer. They are rigid and brittle polymers.  Medium-chain-length PHAs have 6-14 carbons in each monomer.  They are more elastomeric and rubber-like.  In the BIOMAPS project we are using medium-chain-length PHAs to develop biobased and biodegradable materials for use in manufacturing.

Figure 3 medium-chain-length PHA

Use of PHAs in place of petrochemical based plastics in manufacturing will change the process from a linear one to a circular process where renewable resources derived from plants are used to produce products which return to the earth as nutrients at the end of their life.