Bacteria can be adapted genetically to produce the pigment that grants octopuses their ability to camouflage, achieving yields that are up to a thousand times greater than conventional techniques.
There is optimism that this plug-and-play method might also be utilized to speed up the creation of various other beneficial chemicals and materials, facilitating a transition away from materials derived from fossil fuels.
The remarkable capacity of octopuses, squid, cuttlefish, and other cephalopods to alter their skin color to merge with their surroundings has intrigued scientists and the general public for a long time. The pigment in question is xanthommatin – a structurally intricate, color-changing ommochrome with potential applications in materials and cosmetics. Nevertheless, generating this pigment at a large scale for thorough investigation has been difficult and costly.
In pursuit of a more effective and eco-friendly option, a research group from the Scripps Institution of Oceanography in the U.S. turned to bacteria. “No one wants to extract a bunch of octopuses for this pigment… it’s not a feasible method,” says Bradley Moore, a marine chemist at Scripps. “Since it isn’t a microbial compound, we thought it would be a fascinating opportunity to direct a microbe to produce this material.”
The team implemented a “growth-coupled biosynthetic strategy” that involved a feedback loop, where the survival of a bacterium – Pseudomonas putida – was modified to depend (auxotrophic) on a byproduct of the pathway – formic acid.
P. putida, a soil bacterium, was selected due to its resistance to xanthommatin, which can be harmful to many microbial species, along with its capability to encode the tryptophan-to-kynurenine pathway necessary for xanthommatin production.
They engineered a 5,10-methylenetetrahydrofolate auxotroph in P. putida and further altered the bacterium to possess a crucial deficiency that could only be compensated by formate produced during xanthommatin synthesis. This involved performing five genomic modifications to the original strain, including the removal of four genes and the addition of a formate assimilation module.
“We’re asking the microbe to create a material for us while ensuring we provide something that makes the microbe want to produce this material,” Moore states. “Ultimately, it doesn’t care about xanthommatin; it cares about formic acid; formic acid is vital for this organism’s survival.”
As a result of this approach, for every pigment molecule produced, the cell concurrently generated one molecule of formic acid, which in turn fueled the cell’s growth. “The more formic acid it produces, the better it thrives because we made it an auxotroph for that compound. Most organisms prefer to have ATP. The more ATP you produce, the more xanthommatin you generate – it became beautifully interconnected.”
Bacterium as an expert in cephalopod pigment production
While conventional methods of synthesizing xanthommatin yield about 5mg per liter, this innovative technique was shown to yield over a thousand times more, achieving between 1–3g per liter. This method can also be extended to the creation of other valuable materials, even though the group has yet to publish these results. “We’ve primarily trained the organism to focus on formic acid currently, but as you can imagine, there are numerous other forms of C1 metabolism as byproducts, and we’re now branching out to all of those as well… We envision thousands of pathways being leveraged in this manner.”
The optoelectronic characteristics of xanthommatin suggest potential uses in areas such as photoelectronic devices, thermal management coatings, dyes, and UV protectants. Moore indicates that the material scientists on the team are eager to take xanthommatin and create devices capable of changing color in reaction to various stimuli.
Florent Figon, a specialist in pigment biochemistry and ecology from Grenoble Alpes University in France, describes the research as “very clever.” “There are indeed many challenges in synthesizing xanthommatin in sufficient quantities, especially using a method similar to in vivo conditions,” he observes. “Here, the authors present a robust method for scalable production. However, I’m uncertain how it might be adopted by other laboratories since it appears to necessitate specific tools and expertise.”
Figon notes that the xanthommatin biosynthesis pathway was first unraveled around the mid-20th century, yet the compound has only been synthesized in recent years. “Nevertheless, the method is quite labor-intensive, costly, and yields only a few milligrams.”