Purple bacteria transformed into bioplastic factories

In a world overrun with petroleum-based plastics, scientists are looking for alternatives that are more sustainable, more biodegradable and much less toxic to the environment.

Two new studies by biologists at Washington University in St. Louis highlight a potential source of revolutionary materials: purple bacteria that, with a little encouragement, can act as microscopic bioplastic factories.

A study led by graduate student Eric Conners has revealed that two relatively obscure species of purple bacteria have the ability to produce polyhydroxyalkanoates (PHAs), natural polymers that can be purified to make plastic.

Another study led by Tahina Ranaivoarisoa, director of the research lab, showed that genetic engineering could prompt a well-studied but notoriously stubborn species of purple bacteria to dramatically increase its production of PHA.

Conners and Ranaivoarisoa work in the lab of Arpita Bose, associate professor of biology in Arts & Sciences and corresponding author of the new studies. “There is a huge global demand for bioplastics,” Bose said. “They can be produced without adding CO₂ “Phosphates are released into the atmosphere and are completely biodegradable. These two studies show the importance of taking multiple approaches to find new ways to produce this valuable material.”

Purple bacteria are a special group of aquatic microbes known for their adaptability and ability to create useful compounds from simple ingredients. Like green plants and some other bacteria, they can turn carbon dioxide into food using energy from the sun. But instead of green chlorophyll, they use other pigments to capture sunlight.

Bacteria naturally produce PHAs and other building blocks of bioplastics to store extra carbon. Under the right conditions, they can continue to produce these polymers indefinitely.

As WashU biologists report this week in Microbial biotechnologyTwo little-known species of purple bacteria of the genus Rhodomicrobium showed a remarkable willingness to produce polymers, particularly when powered by small amounts of electricity and fed with nitrogen.

“It’s exciting to look at bacteria that we haven’t studied yet,” Conners said. “We’re still a long way from realizing their potential.”

Rhodomicrobium bacteria have unusual properties that make them attractive candidates for the role of natural bioplastic factories. “This is a unique bacterium that looks very different from other purple bacteria,” Conners said. While some species float in cultures as single cells, this particular genus forms interconnected networks that seem particularly well-equipped to produce PHA.

Other types of bacteria can also produce bioplastic polymers with a little help. As noted in Applied and environmental microbiologyWashU researchers used genetic engineering to extract impressive levels of PHA from Rhodopseudomonas palustris TIE-1, a well-studied species typically reluctant to produce polymers. “TIE-1 is a great organism to study, but it has historically not been the best at producing PHA,” Ranaivoarisoa said.

Several genetic modifications have been used to increase PHA production, but one approach has proven particularly effective. The researchers achieved impressive results when they inserted a gene that increased the natural enzyme RuBisCO, the catalyst that helps plants and bacteria capture carbon from the air and water.

With the supercharged enzyme, the normally slow bacteria were transformed into PHA powerhouses. The researchers are optimistic that a similar approach could be used with other bacteria that might be able to produce even higher levels of bioplastics.

In the near future, Bose plans to further study the quality and possible uses of the polymers produced in his lab. “We hope that these bioplastics will give rise to real solutions in the future,” Bose said.