Light from the Living

 

Firefly season! No, not the show, that would be great too, but the bugs! The light show that is occurring starting…now, or maybe, …now, or, well, soon anyway.

Side note: Okay, not true bugs either, true bugs, order Hemiptera include stink bugs and the like. Fireflies are insects in the order Coleoptera, along with the rest of the beetles. The insect order with the most species.

The impetus for this blog post is a book on darkness. I’m reading this book as part of my science geek book club. And the author, though they marveled at the light show, did not dive into the science of light creation by fireflies; I’m aghast. 

When organisms produce light, it’s called bioluminescence.

The reaction requires two key chemicals: Luciferin, the substrate that actually produces light, and either luciferase (an enzyme) or a photoprotein. The color of the glow depends on the arrangement of luciferin molecules. 

There is no single luciferin molecule. Marine organisms like dinoflagellates, fireflies, and fungi each use completely distinct chemical systems that evolved independently, convergent evolution at work again; the same solution through different chemical routes.

Though firefly lightings are well known, terrestrial bioluminescence is actually rare; it’s more frequently found in marine wildlife.

Luciferin (the substrate) is first "charged" using ATP, producing an activated intermediate called luciferyl-AMP. Pyrophosphate (PPi) is released as a byproduct. This step is carried out by the enzyme luciferase.

Molecular oxygen then attacks the luciferyl-AMP, and luciferase catalyzes the oxidation. The product, oxyluciferin, is left in an electronically excited state,  meaning one of its electrons has absorbed energy and jumped to a higher energy level. When that electron drops back to the ground state, it releases the energy difference as a photon of visible light, typically around 560 nm (yellow-green). The other byproducts are CO₂ and AMP. AMP is just ATP with fewer phosphates and less energy.

Summary of Lucifern to Oxyluciferin in fireflies

What makes this reaction remarkable is its efficiency: Roughly 95% of the chemical energy is converted to light rather than heat, which is why the light feels cold to the touch. The color of the flash can vary somewhat by species and temperature, because small changes in the shape of the luciferase active site shift the emission wavelength.

LED bulbs efficiency note:

Incandescent light bulbs convert more than 90% of their energy to heat and less than 10% to light, the opposite of the remarkable firefly. LEDs are better; about 45% of the energy goes to the visible light, and the rest is lost as heat. LED research continues, however, and some are pushing the envelope and reaching 60% or more energy going to the light.

Other critters have bioluminescence, particularly Marine organisms.

Dinoflagellates are among the most abundant bioluminescent organisms. When they bloom in dense surface layers, they produce a sparkly sheen as they move in waves at night. A common genus, Noctiluca, is found throughout the world's oceans. Noctiluca means night light, or light at night. 

The firefly squid produces light from its underside to blend in with light from the ocean surface, a form of counterillumination camouflage that makes it nearly invisible to predators below. This adaptation is common among the fishes as well, about 1,500 known species luminesce, not all for counter shading, however.

The Lanternfish does appear to use its rows of photophores along its belly to adjust its glow to match faint downwelling light from above, erasing its silhouettes from predators looking upward. Other fish, such as the barbeled dragonfishes, emit a red glow, which allows them to see red-pigmented prey that are normally invisible to other organisms in deep water where red light otherwise does not exist.  Beyond the fishes, the brittle star, when threatened, can detach a bioluminescent limb, which acts as a distraction for a potential predator while the rest of its body escapes.

Back to the land

Some fungi emit a glow. The honey fungus Armillaria ostoyae, which breaks down trees, glows faintly both at the gills, under the cap, and underground from the network of branching hyphal threads. The emitted light sometimes causes the decaying tree to appear to glow. This might be an adaptation to attract insects that would aid spore dispersal. Called foxfire or fairy fire, the glow is one likely source of many traditional folklore tales of things glowing in the woods.

Some terrestrial animals produce light. Quantula striata, a land snail native to tropical Southeast Asia, is the only known land snail to bioluminesce. The railroad worm (a beetle larva) is notable for glowing in two colors: red from its head and green from its body. This appears to be a toxic warning saying to potential predators, if you eat me, you will regret it. 

Bioluminescence serves a variety of functions, including camouflage, assisting in courtship, and playing roles in predator-prey stuff, that is, finding or avoiding.

Do all bioluminescent organisms produce light in the same way?

Bioluminescence works on the same general principle: a substrate molecule (called luciferin) is oxidized by an enzyme (called luciferase) in the presence of oxygen, releasing energy as light. That part is observed in all light-producing organisms.

 Basic pathway: Oxygen, luciferin, and its enzyme, luciferase.

Where fireflies and marine organisms diverge

Fireflies use a specific luciferin called D-luciferin, paired with their own luciferase enzyme, and the reaction also requires ATP (adenosine triphosphate) as an energy source. This ATP requirement is actually fairly unusual. Most marine bioluminescent organisms use a completely different luciferin called coelenterazine, which is widespread in the ocean, observed in dinoflagellates, jellyfish, squid, copepods, and many deep-sea fish. The coelenterazine-based reaction does not require ATP. Instead, it often works through a slightly different mechanism involving a photoprotein (like aequorin in jellyfish) that is triggered by calcium ions. 

Different, but similar, methods to produce light.

There is more variety, well, of course, that is nature’s thing, variety. Dinoflagellates use yet another distinct luciferin, unique to that group, though still oxygen-dependent. Ostracods (tiny crustaceans) use a luciferin called vargulin, which is chemically different again.

The light in many organisms is not produced by the organisms, but by bacteria; the organisms just house them. This is the case in some deep-sea fish, anglerfish, and the Hawaiian bobtail squid. The squid has an area colonized by bioluminescent bacteria directly after birth, which in turn becomes its light organ. Light-emitting bacteria can be used to measure unwanted chemicals in water. The bacterium Aliivibrio fischeri produces dimmer light when pollutants are present in the water. The pollutants affect the bacteria's growth and reproduction. 

Bioluminescence has evolved at least 10s of times, causing different chemical systems. Fireflies and marine organisms arrived at the same solution (light from chemistry) through separate paths. However, the processes and molecules are similar, showing the power of convergent evolution at the biochemical level, but also showing how similar the chemical makeup of cells is, whether a dinoflagellate or a firefly, to use a similar, but not precisely the same, chemical reaction. Evolution finds a way to the light, but there is no need to reinvent the wheel in the process.

Sources and Further Readings:

Leitão, João & Silva, Joaquim. 2010. Firefly luciferase inhibition. Journal of photochemistry and photobiology. B, Biology. 101. 1-8. 10.1016/j.jphotobiol.2010.06.015. 

Morciano, G., Sarti, A., Marchi, S. et al. 2017. Use of luciferase probes to measure ATP in living cells and animals. Nat Protoc 12: 1542–15622017. https://doi.org/10.1038/nprot.2017.052

https://oceanexplorer.noaa.gov/wp-content/uploads/2025/04/bioluminescence-fact-sheet.pdf

Viviani, Vadim. 2002. The origin, diversity, and structure function relationships of insect luciferases. Cellular and molecular life sciences: CMLS. 59. 1833-50. 10.1007/PL00012509. 

Images:

https://www.glpbio.com/blog.html

https://www.bas.ac.uk/blogpost/shedding-light-on-lanternfish/

https://oceanexplorer.noaa.gov/wp-content/uploads/2025/04/bioluminescence-fact-sheet.pdf