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Green Matter: Taming the PHB bug

There's an old joke that goes: What did the male bacteria say to the female bacteria? Who needs biology when we have chemistry...Today, it looks as if the microbes may actually need both, at least when it comes to producing the bioplastic known as poly-3-hydroxybutyrate, or PHB.

There's an old joke that goes: What did the male bacteria say to the female bacteria? Who needs biology when we have chemistry...Today, it looks as if the microbes may actually need both, at least when it comes to producing the bioplastic known as poly-3-hydroxybutyrate, or PHB. Researchers in Germany recently reported in the BioMed Central open access journal Microbial Cell Factories that they have found a method for rapidly producing this material in microalgae. No petroleum feedstock needed, just some fancy molecular biology and some simple chemistry.

PHB is a renewable material produced by bacteria as a storage compound, in response to stress from environment conditions. It is an aliphatic polyester with thermoplastic properties and is 100 % biodegradable into carbon dioxide and water.

The news about PHB is mostly good. It has a host of properties rendering it extremely suitable for a range of applications in which other bioplastics cannot even be considered. Unlike most bioplastics that are either water-soluble or sensitive to moisture,  PHB is insoluble in water and resistant to hydrolytic degradation. It also has good permeability for oxygen molecules. It has a tensile strength of 40 Mpa, comparable to PP. The material is UV resistant, meaning it will not degrade when exposed to UV rays. It has a good thermal resistance, compared to, for example, PLA, which, unblended, starts to wilt at 60°C. As a result, PHB is also suitable for applications using higher temperatures.

The problem hampering the commercial development of PHB up until now has been the high costs of producing this material. Bacterial fermentation has been the route chosen for commercial PHB production by, for example, the US bioscience company Metabolix, although the company is now looking at the commercial feasibility of producing PHB in the leaves of genetically modified switchgrass. Although ingenious, it not only takes time to grow such photosynthesis fueled low-cost production plant systems, these systems take up agricultural land that could otherwise be used to produce food crops. PHB produced via plant-based systems can therefore still not compete with the costs of petroleum-based plastics.

Microalgae share all the advantages of plant systems while lacking the mentioned disadvantages: they possess high growth rates, are easy to handle and do not need much more than light and water for cultivation. The method developed by the German researchers involved taking the genes coding for the proteins in the bacteria responsible for the synthesis of PHB and inserting these into diatoms, a type of phytoplankton, known as Phaeodactylum tricornutum. After only seven days, about 10% of the dried weight of the diatoms was PHB. A triumph, according to the researchers:

"Millions of tons of petroleum-based plastics are consumed every year worldwide causing immense amounts of waste that can take thousands of years to biodegrade - if at all. Bacterial fermentation is expensive and while people have introduced a similar system into plants, plants are relatively slow growing and biofuel agriculture uses up valuable land. P.tricornutum needs little more than light and water to grow and can produce similar amounts of PHB to the plant systems in weeks rather than months."

Altogether, they conclude that microalgae have a great potential for functioning not only as 'biosynthetic factories for recombinant proteins but also as photosynthetically fueled bioreactors for synthesizing biotechnologically relevant polymers like PHB.' And we hope that this means that ultimately, microalgae may offer a low-cost and environmentally friendly route for the commercial production of PHB.

TAGS: Materials
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