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CALIFORNIA – Researchers in the US have developed a new production platform for producing blue pigment which may provide a sustainable alternative to synthetic indigo dye. A research team was investigating microbe-based biomanufacturing when it came across the production platform for developing a blue pigment. With a similar, vividly saturated hue as synthetic indigo, a dye used around the world to colour denim, the team’s fungi-produced indigoidine offers a potential alternative to conventional indigo, most of which is synthesised at present. The researchers claim they have developed a method to efficiently produce a blue pigment that uses inexpensive, sustainable carbon sources instead of harsh precursors. They also believe it “ticks most of the boxes” in terms of cost structure for scaling.

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The work was carried out by the Department of Energy Joint BioEnergy Institute (JBEI) which initially set out to test how well a hardy fungi species called Rhodosporidium toruloides could express nonribosomal peptide synthetases (NRPSs) – large enzymes that bacteria and fungi use to assemble important compounds. The scientists examined this fungi’s NRPS expression capability by inserting a bacterial NRPS into its genome. They chose an NRPS that converts two amino acid molecules into indigoidine – a blue pigment – to make it easy to tell if the strain engineering had worked. Quite simply, when it did, the culture would turn blue.

Going into this experiment, indigoidine itself was not the main interest for the team. Instead, they were focused exploring how the assembly line functionality of these enzymes could be harnessed to create biosynthetic manufacturing pathways for valuable organic compounds, such as biofuels, and assessing whether or not the fungi represented a good host species for the production of these compounds. But when they cultivated their engineered strain, and saw just how blue the culture was, they realised the significance of the finding.

With an average titer of 86 grams of indigoidine per litre of bioreactor culture, the yield of the strain – which they named Bluebelle – is by far the highest that has ever been reported. In addition, the yield was obtained from a culture process that uses nutrient and precursor inputs sourced from sustainable plant material. Previous pathways required considerably more expensive inputs yet made about one-tenth the amount of indigoidine.

The team’s next steps will be to characterize how indigoidine could be used as a dye and to dig deeper into the capabilities of R. toruloides.

Importantly, commercial markets already have considerable demand for what the scientists hope to supply. After meeting with many key stakeholders in the textile industry, the team found that many companies are eager for more sustainably sourced pigments because customers are increasingly aware of the impacts of conventional dyes. “There seems to be a shift in society toward wanting better processes for creating everyday products,” said Maren Wehrs, a graduate student at JBEI and first author of the paper describing the discovery, now published in Green Chemistry. “That’s exactly what JBEI is trying to do, using tools derived from biological systems – it just so happens that our engineered biological platform worked very well.”

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