The Effect of Acidification on Bioluminescent Marine Organisms

Photo by David Clode on Unsplash

It is the soft glow of stars in the night sky that spurs the imagination of artists and dreamers, but what most don’t realize is that if you look in the right places, you can find a similar dreamy glow in the ocean. Better yet, the stars of the ocean are alive.

It isn’t rare to be bioluminescent. In fact, 76% of all marine organisms are capable of it. Rather than using words as social cues, these organisms use light, and in the depths of the ocean, the ability to display color against a limitless darkness has impacts on courtship, defense, and camouflage. Usually, in the deep ocean and on the coastline, the most common colors are blue and green respectively, whereas freshwater and land animals, such as fireflies, are more partial to yellow. From the nightmarish image of an anglerfish to the strips of light running down the sides of a comb jelly, the deep sea is a light show more spectacular than any you could ever witness on even the most explosive of New Year’s Eves.

The glow emitted by these organisms occurs due to a highly efficient reaction between oxygen and organic compounds called luciferins, releasing 98% of all energy as light, which, for comparison, is quite a bit more than the 2% efficient traditional filament light bulb. 

However, the pH of the ocean is decreasing, leading to more acidic temperatures which will change the conditions in which these reactions are taking place. Enzymes require an optimum pH and temperature, meaning the effects of ocean acidification on luciferase enzymes will be critical for understanding the future of bioluminescent organisms as a whole. 

The reason for this acidification is the excessive amount of CO2 in the atmosphere, 25-30% of which ends up in aquatic ecosystems. When CO dissolves in water, carbonic acid(H2CO3) is produced, which then dissociates into bicarbonate(HCO3) and hydrogen ions(H+). Now is when you remember…hydrogen ions increase acidity. Therefore, the increasing levels of CO2 in the ocean lead to the lowering of the ocean’s pH overall, which is meant to be approximately 8.1(roughly the same as an egg white). Furthermore, with an increased temperature, the ocean is able to hold less oxygen, leading to hypoxic conditions which are only exacerbated by the increase in nutrient pollution and consequent algal blooms (those annoying microorganisms that hog up everyone else’s oxygen). 

So that leaves those 76% of marine organisms capable of bioluminescence in unknown terrain. While you might assume they are doomed, studies have shown that for some organisms, a lower pH actually increases bioluminescence. For example, sea fireflies, 3mm long bioluminescent shrimp which can be found scattered along the coast of Okayama Beach, Japan, have been found to increase their light production by 20% in acidic conditions, and sea pansies, fleshy leaf-shaped colonies of organisms typically found in warmer areas of Pacific & Atlantic coasts, have increased their light production twofold. On the other hand lie those organisms whose light production has decreased. One such example is the firefly squid, whose sparkling display of glowing blue decorates the darkness of the deep sea much like a firefly decorates the darkness of a summer sky. 

Therefore, while there is clear evidence that supports a correlation between bioluminescence and ocean acidity, it has not yet been clarified in exactly what way the response varies between species, nor what it will mean for the future of these organisms.

Nevertheless, acidification is not to be mistaken for the ideal, for an improvement on what existed before CO2 began dissolving into the ocean at a higher rate. Rather, it is another anthropogenic adjustment which only accelerates change, change being neither good nor bad, but only something vastly unknown and terrifying. 

Hence, with the concentration of CO2 in the ocean only increasing, the future holds almost as many uncertainties as there are stars in the sky, and with a big question mark hanging over the stars that light up the deepest corners of our oceans, we are left to hold our breath and wait, while doing our best to keep the terrifying uncertainty of change to a minimum. 

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