The tiny culprits behind our glowing seas


Written by Alesha Brewer

We seem to hear about it once or twice a year – strange glowing stuff washing up on some of SA’s best-loved beaches. News teams race out to film kids and adults alike excitedly splashing around in water filled with what looks like the inside of a particularly vivid glowstick. But what exactly is this unusual phenomenon?

Bioluminescent algae graced Port Lincoln in August of 2023 (Nine News SA)

It’s best to first understand how this glowing happens. It’s a result of bioluminescence, a chemical tool that evolved numerous times in marine life that is used in a wide range of applications. That giant, creepy, evil-looking anglerfish in Finding Nemo was probably most people’s first introduction to this concept – here we see bioluminescence used as a hunting tactic to lure in unsuspecting prey.

While a great educational moment, I have to question the necessity of making children (and adults) terrified of what might be hiding in the ocean (Walt Disney Studios)

Heaps of other open ocean animals can glow too, to signal to members of the same species, scare off potential threats, or create distractions to throw predators off their trail. Many species, such as the lanternfish, can create specialised chemical compounds that react and glow (just like the inside of a glowstick!). Other species, however, need to rely on a symbiotic relationship with special glowing bacteria.

A firefly squid in the Sea of Japan using bioluminescence to camouflage with light from the surface (National Geographic)

It's the enzyme called luciferase that is responsible for catalysing the reaction of glowing chemical compounds and ultimately creating some strikingly glowing bacteria. There are a number of bacterial groups that can glow, some of which are free-floating throughout the world’s oceans, and many of which form symbiotic relationships with larger organisms. Usually, the bacteria provide the larger species with the ability to glow, and in return, the bacteria have a safe environment to live in and a reliable source of energy.

But it’s not just bacteria that glows. In fact, what we see washed up on our beaches is usually bioluminescent algae. Unlike bacteria, true algae belong to the domain eukaryota, meaning their cells contain a nucleus (amongst other differences). The underlying mechanisms of their bioluminescence are largely the same, however, in algae, it’s almost exclusively used as a defence mechanism.

The tiny, individual cells of algae glow when there is a disturbance in their environment, such as the movement of the waves or of other, larger creatures. That’s why it becomes more visible along the shoreline, where the tide is constantly pushing it up against the sand. It’s also why people kicking and splashing in the water create impressive, glowing splashes.

An abundance of bioluminescent algae washed up on a beach in the Maldives in 2010 (Doug Perrine, Nature Picture Library)

However, the reason why we see such large and concentrated amounts of bioluminescence every now and then is the result of large algal blooms. When the conditions are just right, algae can rapidly multiply and dominate their environment, taking advantage of plentiful energy from waste in the water or stagnant currents. Bioluminescence in this case is believed to be a tool to ward off grazing from predators, but produced alongside this beautiful glow is several kinds of highly toxic compounds. These not only prevent predation but can actively harm marine life, poisoning fish as well as shellfish and important filter feeders. While algae and their defence mechanisms are a natural part of marine ecosystems, in exponentially increasing populations they can cause more harm than good.

That’s why it’s important to note that, despite its beautiful and alluring appearance, it’s best to avoid touching or swimming in glowing waters. Especially important is to keep pets away from the water, lest they drink and ingest harmful toxins. There’s plenty of beautiful photos and videos you can take without being in the water yourself – a well-thrown rock can create some dazzling splashes!

A  little creativity in your photography can help keep yourself and others safe (Hasan Jasim)