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

Two new studies from biologists at Washington University in St. Louis highlight a potential source of breakthrough materials: purple bacteria that, with a little encouragement, could act as microscopic factories for bioplastics.

A study led by doctoral student Eric Conners found that two relatively unknown species of purple bacteria can produce polyhydroxyalkanoates (PHAs), natural polymers that can be purified to make plastic.

Another study, led by research lab supervisor Tahina Ranaivoarisoa, found that genetic engineering could trick a well-studied but notoriously stubborn strain of purple bacteria into dramatically increasing its production of PHAs.

Conners and Ranaivoarisoa work in the lab of Arpita Bose, an 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 to the atmosphere and are completely biodegradable. These two studies demonstrate the importance of using 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 convert carbon dioxide into food using energy from the sun. But instead of green chlorophyll, they use other pigments to capture sunlight.

The 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 from the genus Rhodomicrobium showed a remarkable willingness to produce polymers, especially when supplied with small amounts of electricity and fed nitrogen.

“It’s worth looking at bacteria that we haven’t looked at before,” Conners said. “We’re nowhere near realizing their potential.”

Rhodomicrobium bacteria have unusual properties that make them interesting candidates as natural bioplastic factories. “It’s a unique bacterium that looks very different from other purple bacteria,” Conners said. While some species float around in cultures as individual cells, this particular genus forms interconnected networks that seem particularly well-suited to producing PHA.

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

Several genetic tweaks helped increase PHA output, but one approach was particularly successful. Researchers saw dramatic results when they inserted a gene that increased the natural enzyme RuBisCO, the catalyst that helps plants and bacteria capture carbon from air and water.

Using the supercharged enzyme, the normally sluggish bacteria turned into relative PHA powerhouses. The researchers are optimistic that a similar approach could be possible with other bacteria that could potentially produce even higher levels of bioplastics.

In the near future, Bose plans to further investigate the quality and potential applications of the polymers produced in her lab. “We hope that these bioplastics will provide real solutions in the future,” Bose said.