Dinoflagellates: Blooms, Bleaching, and Bioluminescence
Dinoflagellates are eukaryotic cells with whirling flagella
It’s the middle of the night. Angry waves of high tide are smacking against the sandy shore and frisking about slippery rocks, when all of a sudden, a blue glowing wave — growing higher by the second, appears just off the coast. The dinoflagellates are coming. And through remarkable genetics, chemistry, and cell machinery, they warn of their arrival in an artistic display of marine bioluminescence.
Dinoflagellates, literally “whirling flagella”, are single-celled protists: a group of eukaryotic organisms not quite like plants, animals, or fungi. They are called eukaryotic because their DNA is packaged into a clearly defined nucleus — much like our own cells. They primarily reproduce asexually: by budding of genetically identical offspring. But, in some cases, like when they are in symbiosis with coral reefs, they can combine their genes and reproduce sexually.
Around ~90% of species, including those responsible for glowing blue waves, are planktonic: floating freely in the water. Their whip-like flagella are not strong enough to overcome the current of water around them, and they move at the mercy of wind and density-driven ocean currents.
Not all species get their energy from the sun through photosynthesis (some are heterotrophic: they snare and consume prey like diatoms and other dinoflagellates), but those that do serve as important primary producers in warmer waters. Together with diatoms, they provide organic carbon to marine ecosystems and support the metabolism of the coastal biome.
Dinoflagellates are crucial for the health of coral reefs
The ecological significance of dinoflagellates does not stop at primary production. Certain species, known collectively as Symbiodinium, are also essential symbionts for the survival of coral reefs: perhaps the most diverse, living marine habitats on Earth.
Symbiodinium consists of 9 main genetic groups, called clades. These clades are further broken down into subclades that are separated by genetics and physiology/physical appearance. In these subclades, genetic differences of just a few nucleotides (letters in the genetic code) can translate into much more dramatic differences in size and pigment.
Coral reefs create productive environments in waters that are otherwise barren. These are oligotrophic waters — meaning there aren’t a lot of nutrients to go around. Perhaps the most notable of these reefs is the Great Barrier Reef off the coast of Australia, large sections of which have died in recent years due at least in part to climate change (click here for a SparkNotes version of what threatens coral reefs).
The symbiotic (living together) “microalgae” that live within coral reefs are known collectively as zooxanthellae, and includes dinoflagellates and other golden cells such as diatoms.
In the diverse habitats of coral reefs, the dinoflagellates have a very important job — one that was originally thought to be mutualistic — with both parties gaining something from the relationship. First, corals ‘attract’ dinoflagellates with chemical signals. These signals are poorly studied but are known to have varying degrees of specificity. What ultimately determines the species of Symbiodinium that attach to a coral is the coral's geography — what water it happens to form in.
Once the Symbiodinium attach, the coral — which has little resources of its own, takes advantage of molecules produced by the photosynthetic dinoflagellates. In return, the coral metabolizes photosynthetic products and releases more CO2 and inorganic nutrients back to the Symbiodinium.
This seemingly harmonious relationship, may be anything but. Close examination reveals that there is an unfair trade-off between corals and dinoflagellates. In fact, corals may behave more like parasites: luring dinoflagellates, stealing nutrients, and giving very little in return. This suspicion gained some weight in 2015, when researchers at the University of Connecticut sequenced the genome of S. kawagutti (a species of Symbiodinium).
The sequence data revealed an unusually large, hearty genome with genes associated with sexual reproduction (which isn’t common in dinoflagellates). Symbiotes (and even parasites like Malaria) typically have small genomes and rely on the cellular machinery of their hosts. What makes S. kawagutti so different is still yet to be discovered.
What is known is that S. kawagutti doesn’t seem to love living in coral reefs. In order to have adapted to the environment, it must have evolved closely with the corals, forever intertwining their biological histories. The UConn researchers also found an that S. kawagutti has extensive gene regulatory system that can act on the gene expression of the corals themselves. This means that the Symbiodinium may exhibit some level of control over the corals to make the environment more suitable.
Symbiodinium can exist without corals, but when dinoflagellates abandon their reefs something terrible happens. The corals become bleached.
Bleached (which refers to a lonesome coral’s appearance) corals are ultimately the result of increasing CO2 levels and sea surface temperatures. When CO2 dissolves in water, it forms carbonic acid and lowers the pH of the water. Increasing sea surface and air temperatures can melt glacial ice, releasing freshwater into the ocean and diluting its salts (decreasing salinity). Combinations of these factors create an environment that is unwelcoming for symbiotic dinoflagellates. They flee from their corals into the surrounding waters.
When dinoflagellates reach high concentrations, they can cause toxic “red tides”
When dinoflagellates flee into the ocean, they can become so concentrated that they cause a phenomenon known as ‘algal blooms’, or more specifically ‘red tides’. Not all algal blooms are red tides. Brown tides, and even algal blooms only detectable because of the destruction they cause, are also possible. Red tides specifically are caused by dinoflagellate Karenia brevis, which produces a red pigment.
Like coral bleaching, red tides and algal blooms are influenced by sea surface temperatures rising and decreases in salinity. Other influencing factors include pockets of high nutrient concentration (which can be caused by fertilizer runoff), periods of rain followed by intense sunlight, and calm seas that fail to spread out dinoflagellate colonies.
Red tides do more than change the color of the water. They change its composition. The species found in algal blooms can produce compounds that, in high amounts, are toxic to larger organisms — including humans who ingest shellfish from red tide environments. These toxic molecules exist in the water and can become airborne if they get close to the surface.
Karenia brevis produces a powerful neurotoxin known as brevetoxin, which prevents neurons from firing. Manatees, dolphins, birds and other organisms who ingest or inhale the toxin are found dead, washed onto shore.
It is important to note that the dinoflagellates here are not evil (neither, really, are parasitic corals). They do not intend to kill the manatees or poison our seafood, and they are not inherently toxic beings. They are controlled by changes in ocean condition and chemistry, and through no fault of their own, release more of certain compounds than large mammals in the area can handle.
In addition to releasing toxic compounds, red tides also disrupt the amount of oxygen gas dissolved in ocean water. When the dinoflagellates in red tides die, they are consumed by other microbial species. When these microbes reproduce and multiply, they consume oxygen in the water, just like marine animals. When there are tons dinoflagellates to dine on, dividing microbial populations can consume so much oxygen that fish and other marine animals are unable to survive the deprivation. The result is mass fish death.
Human casualties are both medical (through disease caused by ingesting toxins) and financial. Towns undergoing red tide events see massive losses in monetary gain from tourism and restaurant industries.
In 2007, chemists at MIT made great strides in understanding red tides. Their work was based on the “Nakinishi Hypothesis”, a series of chemical reactions proposed over 30 years ago to explain the chemical mechanism that produces red tide toxins. The 2007 study was the first piece of evidence that this cascade of chemical reactions is actually feasible.
This discovery was important not only because it elucidated the mechanisms of dangerous and expensive red tides, but also because dinoflagellates produce other important products, including a related compound that is being looked at as a potential treatment for cystic fibrosis.
The study of red tides expands the possibilities of our future and sheds light on the Earth’s past. Because dinoflagellates are so old, their remains can reveal how landscapes have changed over deep time (millions of years). Just last summer, fossilized dinoflagellate remains were found in inland Australia. These fossils date back 40 million years and suggest that during the Jurassic age of the dinosaurs, that very spot was covered in sea water.
High concentrations of free-floating dinoflagellates light up when disturbed
Large colonies of dinoflagellates aren’t always damaging. Sometimes, they cause beautiful, lighted displays. Bioluminescence is a term used to describe all organisms that light up, including fireflies and anglerfish. Anaximander, a prominent Greek philosopher, casually wrote of lighted up seas in 500 B.C. Now, some historians look to that document as the earliest recorded mention of dinoflagellates (or perhaps some other bioluminescent microbe) at work.
Despite being found more commonly in marine species, bioluminescence is thought to have evolved independently at least 40 times in life’s history, in diverse organisms with diverse biochemistries. Despite this diversity, tiny dinoflagellates are to blame for most bioluminescence observable at the surface of oceans.
Organisms produce light through biochemical reactions that take place in certain parts of their cells. In exchange for the energy they spend on colorful shows, they gain an evolutionary advantage. Many luminescent organisms live in the deep ocean, where light from the sun cannot penetrate. They use luminescence to find or attract prey and divert predators. Alternatively, as is the case for fireflies, lighting up can be a way for males and females to communicate.
In marine species, bioluminescence is thought to be mechanically induced — spurred by the jostling of waves, kicking of feet, or waving of fins. In dinoflagellates, it is a way to startle potential predators. The light prevents dinoflagellates from being consumed by disrupting the feeding habits of predators. The light that they use to deter some predators can attract others. Dinoflagellates are also thought to ‘signal for help’ by alerting secondary predators to the presence of their prey. In contrast to the thick, red tides shown above, marine bioluminescence is typically blue in color — favoring wavelengths that travel the farthest in water.
The organisms and chemical reactions involved in marine bioluminescence are incredibly diverse. But in dinoflagellates, the light is produced from a reaction of oxygen, a substrate called “luciferin”, and an enzyme called “luciferase” that speeds up the reaction without being consumed.
This reaction — and subsequent production of light occurs in a specific unit within the cell called the scintillon: the flashing unit. So far, dinoflagellates are the only bioluminescent organisms to possess such a structure. During nightfall, numerous scintillons can be seen gathered around the edges of cells, where the shear stress of surrounding movements triggers the reaction.
Scintillons are important in the initiation of bioluminescence. In order for the light producing luciferin/luciferase reaction to take place, the structures have to be acidified by being exposed to a cellular vacuole filled with acidic material.
The cellular mechanisms responsible for the bioluminescence of just a few, closely related species of dinoflagellates are remarkable, and it is impossible to predict what future, broader studies of bioluminescence will reveal.
“Faith” is a fine invention
For Gentlemen who see!
But Microscopes are prudent
In an Emergency!— Emily Dickinson
