Bioplastics from natural polymers could replace petroleum-based plastics, helping us keep nonbiodegradable plastics out of landfills and waterways. But there’s a problem: The most promising bioplastics—polyhydroxyalkanoates, or PHAs—are supplied via low-yield, high-cost production processes.
To overcome this problem, scientists at Washington University in St. Louis (WashU) decided to take a fresh look at PHA-producing bacteria. The scientists found that relatively obscure species of purple bacteria have untapped potential. Moreover, the scientists showed that for at least one species of purple bacteria, this potential could be realized with a little help from genetic engineering.
The scientists recently presented their work in two articles. One appeared in Microbial Biotechnology, in an article titled, “The phototrophic purple non-sulfur bacteria Rhodomicrobium spp. are novel chassis for bioplastic production.” The other appeared in Applied and Environmental Microbiology, in an article titled, “Overexpression of RuBisCO form I and II genes in Rhodopseudomonas palustris TIE-1 augments polyhydroxyalkanoate production heterotrophically and autotrophically.”
The scientists also described how PHA production by the purple bacteria could be increased through the control of growth conditions.
In the second article, the scientists focused on a purple bacterium called Rhodopseudomonas palustris TIE-1, and they explained how genetic engineering could be applied to increase the bacterium’s PHA production: “[Genes] encoding the putative regulatory protein PhaR and the depolymerase PhaZ of the polyhydroxyalkanoate (PHA) biosynthesis pathway were deleted. Genes associated with pathways that might compete with PHA production, specifically those linked to glycogen production and nitrogen fixation, were deleted. Additionally, RuBisCO form I and II genes were integrated into TIE-1’s genome by a phage integration system.”
The scientists observed that PHA production was greatest when RuBisCO form I and form I and II genes were overexpressed.
Both studies came from the lab of Arpita Bose, PhD, an associate professor of biology. Members of her laboratory, graduate student Eric Conners and research lab supervisor Tahina Ranaivoarisoa, were the lead authors of the first and second studies, respectively.
“There’s 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 show the importance of taking multiple approaches to finding new ways to produce this valuable material.”
Purple bacteria are a special group of aquatic microbes renowned 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.
The bacteria naturally produce PHAs and other building blocks of bioplastics to store extra carbon. Under the right conditions, they can keep producing those polymers indefinitely.
Rhodomicrobium bacteria have unusual properties that make them intriguing contenders as natural bioplastic factories. “These are unique bacteria that look very different from other purple bacteria,” Conners said. While some species float around cultures as individual cells, this particular genus forms interconnected networks that seem especially well-equipped to produce PHA.
Other types of bacteria can also produce bioplastic polymers with some help. “TIE-1 is a great organism to study, but it’s historically not been the best for producing PHA,” Ranaivoarisoa said.
Several genetic tweaks helped boost the output of PHA, but one approach was especially successful. Researchers saw impressive 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. With the help of the supercharged enzyme, the usually sluggish bacteria turned into relative PHA powerhouses. The researchers are optimistic a similar approach could be possible with other bacteria that might be able to produce even higher levels of bioplastics.
In the near future, Bose plans to take a closer look at the quality and possible uses of the polymers produced in her lab: “We hope these bioplastics will produce real solutions down the road.”
