Sarah Bottorff
Technical Support Specialist, Live Materials

October 2017

Bioluminescence has perplexed humans since ancient times. Greek philosopher Aristotle described this biological process in De Anima (On the Soul) and in other writings. Pliny the Elder referenced various bioluminescent organisms he encountered in his travels. Charles Darwin observed and wrote of bioluminescence during his journey around the world, noting that among closely related organisms, the ability to emit light was restricted to a select few.

Bioluminescence occurs on several branches of the tree of life. It has evolved independently in at least 40 different organisms (Valiadi and Inglesias-Rodriguez, 2013). Some of the most frequently encountered bioluminescent organisms are several species of dinoflagellate algae living on the surface layers of temperate and tropical seawaters.
The actual mechanism used to produce the bright blue light emanated by marine dinoflagellates is one of the fastest cellular responses known to science. The time from stimulus to the emission of light is less than 20 milliseconds!

Bioluminescence as burglar alarm

One prevailing theory regarding the evolution of bioluminescence in marine dinoflagellates states that expressing bioluminescence in response to physical stimulation serves as a mechanism to deter predators. Known as the burglar alarm hypothesis, it was first proposed by Burkenroad in 1943.

According to the theory, animals will modify their behavior in response to the presence of bioluminescent dinoflagellates in their hunting environment. Primary consumers feeding on dinoflagellates induce light production. The light could potentially attract higher-level predators.

Fleisher and Case (1995) found evidence to support the burglar alarm theory in a series of experiments in which feeding rates of high-level predators on lower-level grazers correlated with the presence or absence of bioluminescent dinoflagellates. In these experiments, the high-level predators used the trail of bioluminescence provided by the dinoflagellates to track prey they would otherwise be unable to detect. The burglar alarm theory contends that a sudden flash of light draws the attention of higher predators to the bioluminescent organism’s predator, thus decreasing grazing behavior when bioluminescent organisms are present.

Additional experimental evidence indicates that bright flashes of bioluminescence startle would-be grazers into retreat, thus decreasing the rate at which they feed on the bioluminescent organisms. More research is needed to truly understand the evolutionary function of this phenomenon.

Scintillons produce the light

In a pathway unique to marine dinoflagellates, light production occurs in organelles called scintillons. Scintillonsare cellular vesicles found on the periphery of individual cells and contain the substrate luciferin and the enzyme luciferase. Light is produced when a dinoflagellate experiences mechanical disturbance caused by contact with another organism or a breaking wave.

The image on the right shows a simplified version of the reactions producing bioluminescence in scintillon.

1. The bioluminescent response begins when pressure is applied to the scintillon membrane.
2. The pressure disturbs the integrity of a G-protein receptor complex lining the membrane.
3. The signal is then transmitted to the inside of the vesicle by the activation of a G-protein. 
4. A second messenger molecule from the G-protein complex activates gated channels on the scintillon membrane specific to calcium ions (Ca²⁺).
5. With these channels open, calcium ions can cross the scintillon membrane in large quantities.
6. The influx of positively charged calcium ions depolarizes the membrane. This charge differential spreads over the surface of the membrane.
7. The charge differential causes conformational changes in voltage-gated channels that allow hydrogen ions (H+) to enter the scintillon.
8. The influx of H+ ions from the vacuole is required to induce a conformational change in the luciferase/luciferin enzyme/substrate complex.

The rapid change in the surrounding biochemistry generated by the influx of H+ ions allows luciferase to expose its binding site to luciferin. This change in protein conformation induces the bioluminescent chemistry.

Despite the amount of detail known about the activation of the luciferin/luciferase complex, the exact details of chemical processes and intermediates that produce light remain unknown. Recent advances in sequencing and gene cloning technologies may provide researchers with additional tools to unlock the secret of a natural phenomenon that has fascinated humans since ancient times.

Get tips on how to raise bioluminescent dinoflagellates in your classroom.

Resources

Bioluminescent Dinoflagellates Care Guide
Video: Bioluminescent Dinoflagellates
Carolina® Culturing Algae Manual

References

Burkenroad, M.D. (1943). “A Possible Function of Bioluminescence.” Journal of Marine Research 5: 161–164.

Fleisher, K.J., and Case, J. F. (1995). “Cephalopod Predation Facilitated by Dinoflagellate Luminescence.” The Biological Bulletin, 189(3), 263–271. doi: 10.2307/1542143

Valiadi, M., and Iglesias-Rodriguez, D. (2013). “Understanding Bioluminescence in Dinoflagellates—How Far Have We Come?” Microorganisms, 1(1), 3–25. doi:10.3390/microorganisms1010003