Research Highlight: Blue Light Special

In the ocean, many bioluminescent organisms illuminate their surroundings in flashes lasting only a tenth of a second.  One tubeworm, however, is the compact lightbulb of the seas, possessing a unique ability to keep its blue light glowing for hours, and sometimes days on end.

The parchment tubeworm Chaetopterus secretes thick glowing mucus when disturbed by predators, but why and how the mucus glows is a mystery. The worm usually lives in a tube that it builds in the mud, sometimes in sunlit shallow areas of the ocean where light production is only necessary at nighttime, when it could have powerful defensive functions.

While understanding why the light is produced is an important ongoing question, learning how the light is produced and maintained may be equally important: this could potentially provide the key to a biological source of “everlasting” light.

Researchers at Scripps Institution of Oceanography at the University of California San Diego are working to discover the secret behind the worm’s glow. The group’s overall goal: to learn about natural processes and mimic them to improve biotechnology. Uncovering the unique mechanism used by the worm to produce long-lasting light in a natural material could have great potential in a range of applications including biomedical diagnostics among others.

In a study led by former Scripps postdoctoral researcher Renu Rawat and published in the journal Scientific Reports Nov. 10, the Scripps scientists discovered that the worm’s mucus contains large amounts of a common iron-binding protein called ferritin. This enables the worm’s sustained light production, propose the authors.

This is the first time that ferritin, a ubiquitous iron-binding protein was shown to be involved in producing bioluminescence.

Some unique features of the worm’s bioluminescence provided clues that led the researchers to the discovery. The light is produced in the worm’s secreted mucus, which is unusual. In other organisms it is most often produced inside of cells, supported by metabolic processes and linked to oxygen consumption. This is not the case for the mucus that, oddly, has high iron content.

“Light production in organisms usually relies on oxygen. So if you remove the oxygen, there is no light. In this worm, you remove oxygen and there is light,” said the paper’s senior author, Scripps marine biologist Dimitri Deheyn. Furthermore “most systems use calcium as a co-factor for light production. In this worm, it’s not calcium, it’s iron. If you remove iron, there is no light.”

This led the researchers to surmise that a new reaction involving iron-binding proteins, functional in the absence of oxygen, was responsible for light production.

After devising methods for working with the thick and difficult-to-study mucus, the scientists discovered that three major proteins were present in the glowing mucus. One of them was ferritin.

Ferritin is found in nearly all living organisms. It regulates iron in cells by storing and releasing it in a controlled manner. However, finding it in extracellular mucus was unexpected

The researchers’ experiments continued to point towards ferritin as an essential protein for light production. When the mucus was analyzed, all parts that produced light appeared to contain ferritin. Furthermore, the addition of trace metals known to interfere with ferritin activity impaired light production in the mucus similarly.

The authors also found a unique secretion-related gene sequence through close analysis of the ferritin’s DNA sequence. The Scripps researchers were able to identify the sequence using very similar sequences from closely related tubeworms. The related worms have almost identical (99.7 percent) ferritin proteins, but do not produce light or secrete the protein, suggesting the Chaetopterus worm has uniquely repurposed the protein.

Although the ferritin protein appears to be secreted and involved in light production, it is probably not the only compound involved. The group previously isolated two compounds from the worm’s mucus that were shown to possibly produce light, also known as chromophores: riboflavin and FMN (flavin mono nucleotide), both of which are known to interact with ferritin in other systems.

Research is continuing on how this unique light-producing system works.

“The ferritin appears to be the source of energy for the long-lasting glow, providing electrons to a chromophore, the light-emitting entity,” said Deheyn. “Now we are going after the actual chromophore while also trying to identify the two other proteins we found in the mucus that are also part of the light production – but we don’t know what they do. Ferritin, iron, flavin – every single ingredient is there, but we still don’t know the exact reaction and molecule that produces the light.”

The Air Force Office of Scientific Research funded the study.

– Rachel Diner is a third-year Ph.D. student in the lab of Andrew Allen