“Instead of cursing the darkness, light a candle”–Benjamin Franklin
Some organisms don’t need a candle, a star (like the sun), or even electricity for light. A few bacteria, insects, fungi and marine creatures produce light on their own, a phenomenon known as bioluminescence (“bio-“=life, “-luminescence”=light). Bioluminescence, the end product of a chemical reaction occurring inside cells, has evolved multiple independent times in nature. A general chemical reaction is shown below depicting how light is produced by the enzyme luciferase, which cleaves the molecule luciferin to produce oxyluciferin and light. In some organisms, the chemical reaction is this simple; however, other organisms, such as bioluminescent bacteria, have more complex systems to produce light that is an expansion of this simple framework.
Luciferin + O2 ———————————-> oxyluciferin + light
What is the utility or evolutionary advantage of bioluminescence? Why expend so much energy? Light is an excellent mode of communication for organisms. You may recall the scene in the movie “Finding Nemo” in which Dory and Marlin encounter a bright spot that mesmerizes them, luring them in, only to discover that they nearly become ceviche for an anglerfish. The anglerfish hypnotized Dory and Marlin with his light lure because he thought they looked tasty. Other organisms produce light, or house light-producing organisms, for protection, mating, navigation and dispersal. Bioluminescent jellyfish produce sparks of light to ward off predators, fireflies are looking for a date, deep sea fish need flashlights to get around, and fungi attract predators with their light to aid their reproduction by spreading their spores.
Let’s reconsider the anglerfish. How does it produce light? The anglerfish cannot produce light itself, but it does provide a nice, nutrient dwelling for organisms that can. Bioluminescent bacteria live and grow inside the anglerfish’s lure. It’s a win-win situation: the anglerfish gets to find and eat more frequent meals, and the bacteria get a roof over their heads which is chock full of nutrients to help them grow. It’s a symbiotic relationship, meaning that they both benefit from this interaction.
The most fascinating symbiotic relationship involving bioluminescence, in my opinion, is between the bobtail squid and its resident bioluminescent bacterium Vibrio fischeri. Bobtail squid house Vibrio fischeri in a specialized organ called the “light organ”. Bobtail squid are nocturnal, so they burrow and hide during the day and forage at night. When looking for food at night, the squid are susceptible to predators below that can detect their shadows from moonlight. Vibrio fischeri produces light within the squid which eliminates their shadows, providing camouflage. In order to prevent light production while the squid hide during the day, bacteria are ejected from the squid’s light organ leaving only a few behind just before daylight arrives. How does this eliminate light if some of the bacteria are still present? Vibrio fischeri, and some other species of bacteria, can turn their lights on and off. This molecular light switch is controlled by chemicals emitted from the bacteria called “autoinducers”. Autoinducers are sensed by bacterial neighbors but only have an effect if there are lots of them sensed at the same time. If copious amounts of autoinducers are detected, then a relay of signals are sent throughout the bacteria telling it turn on the genes necessary to make light. Overall, a large amount of Vibrio fischeri must be present in a small space in order to produce light, so light is not made with only a few bacteria.
Much like the bobtail squid living in unison with Vibrio fischeri, many organisms (hosts) are colonized with bacteria, including us. Normally, bacteria are considered harmful by our immune systems. How is it that we live so “peacefully” with these tiny invaders? This question is the basis of an entire field of biology that scientists across the world are trying to answer. You harbor just as many bacterial cells in/on your body as the number of cells that compose you1. Scientists have made significant progress over the last few decades in understanding these host-bacterial relationships. Many factors including the immune system, physical and physiological barriers, and bacterial molecules contribute to the maintenance of bacteria in close proximity to host cells2. We scientists often refer to these close interactions in war-like terms because host and bacterial cells are constantly at battle keeping each other in check. However, we NEED each other. Hosts provide housing and nutrients for bacteria, and in turn, bacteria help break down our food to provide nutrients for us as well as influence our physiology (see previous blog post: https://speacatutsw.wordpress.com/2015/10/22/the-microbiome-our-gut-feeling/ – more-513).
SPEaC recently took part in the “Glow” Social Science event at the Perot Museum of Nature and Science in January, which was focused on light production by living organisms. We presented the bioluminescence produced by a cousin of Vibrio fischeri called Vibrio harveyi. Museum patrons were able to visualize bacterial bioluminescence in action. Dense cultures of Vibrio harveyi were grown with a magnetic stir bar in the culture. Then, we placed the cultures on a magnetic plate which caused the stir bar to spin and distribute oxygen throughout. (Oxygen is a requirement for the bioluminescent reaction.) Voilà! Vibrio harveyi produced a brilliant blue-green light for all to see.
Next time you see fireflies lighting up the night or visit Bioluminescent Bay in Puerto Rico, remember that the light you see is biologically-produced as a means to simply send a message.
References and Resources:
 Sender, R., Fuchs, S., and Milo, R. (2016). Revised estimates for the number of human and bacterial cells in the body. bioRxiv. Published online January 6, 2016. http://dx.doi.org/10.1101/036103
 Hooper LV, Littman DR, Macpherson AJ. (2012). Interactions between the microbiota and the immune system. Science. Jun 8;336(6086). http://www.ncbi.nlm.nih.gov/pubmed/22674334
SPEaC sincerely thanks Dr. Bonnie Bassler (Princeton University) for providing the Vibrio harveyi strain used in our demonstration. Thank you to the Perot Museum of Nature and Science for hosting SPEaC. We are also indebted to our volunteers: Wendy Tsai, Dr. Alejandro D’Brot, Dr. Lindsay Case, Dr. Breck Duerkop, Rashmi Voleti, Bishakha Mona, and Dr. Sharon Kuss.
Editor: Chris Hensley