BIOLUMINESCENCE AND CTENOPHORES

TAVISHI

Ctenophores are a phylum of invertebrates separate from cnidaria, which contains jellyfish, known as cone jellies. Several species of ctenophores are bioluminescent, but not all. Ctenophore bioluminescence uses a combination of luciferins and photoproteins. Luciferins are a class of light producing molecules typically activated by oxygen and a luciferase. Luciferins vary widely in structure across species. Firefly luciferin is one prominently known type of luciferin, but it varies in terms of structure and method of action from the luciferin present in ctenophores, coelenterazine. Firefly luciferin is unique in its usage of magnesium and ATP for production of light. Coelenterazine is unique in its usage of photoproteins in the place of luciferase.

Coelenterazine is a derivative of two of my favorite amino acids: tyrosine and phenylalanine. In ctenophores, obelin is the photoprotein that activates it. Obelin is activated specifically by the alkali earth metal calcium. Usually, when coelenterazine reacts with oxygen, carbon dioxide is produced, and the oxidized form of coelenterazine, coelenteramide, is produced.

Obelin is composed of an apoprotein, molecular oxygen, and a chromophore. Obelin itself is considered to contain coelenterazine, which is its chromophore. Calcium ions bind to a site in the apoprotein. Although I could go in further depth about the biochemistry, at this point, I'm going to switch over to the functionality of the bioluminescence itself.

In ctenophores, the cells that produce bioluminescence are called photocytes. Photocytes likely produce their own coelenterazine, rather than obtaining it from diet. One of the most prominent and well known comb jellies is the Arctic comb jelly, known for its rainbow cilia. These colors are frequently attributed to bioluminescence, but rather, this multicolored phenomenon is a direct result of refraction instead. The same process forming rainbows after a storm creates these undersea rainbows.

However, although they aren't rainbow, sea walnuts, or Mnemiopsis leidyi, are true bioluminescent organisms. Rather than relying off of physics (ew...) to create color, the process described above occurs. We aren't 100% sure what the role of bioluminescence is in comb jellies, but the sea walnut gave us a tiny peek into its role. In the sea walnut's genome, ten different photoproteins, like obelin, were discovered. All of which produce different colors. Furthermore, the genes coding for the sea walnut's photoproteins were very, very close to the opsin genes. Opsins are light receptor genes found in nearly all organisms, and the closeness to the genes coding for bioluminescence indicates a potential connection. Perhaps these opsins allow for bioluminescence to be moderated.