Dinoflagellates
Superclass Dinoflagellata
Dinoflagellates are unicellular algae living in mostly marine environments, but freshwater representatives exist. Most are heterotrophic and parasitic, but others are photosynthetic. This latter group is known for forming a symbiotic relationship with non-motile marine animals, such as corals, anemones, and giant clams. These dinoflagellates (called zooxanthellae) live inside the tissues of these animals, providing sugars from photosynthesis. Coral bleaching is a phenomenon inwhich warming and acidifying oceans kill these colorful dinoflagellate symbionts, leaving the coral polyps looking gray or whitish. The loss of the symbiont can cause the coral host to die.
Some dinoflagellates are also known for having bioluminescence which can be seen at night in tropical waters. The agitation of water from waves and animals can make the water appear to glow.
Lastly, this group is known for creating red tides, which appear as a discoloration of coastal waters. These blooms of dinoflagellates are usually caused by an influx of nutrients, sometimes from pollution runoff. These blooms can produce harmful neurotoxins
Ecology & form
Unicellular, aquatic organisms
They possess 2 flagella in grooves: 1 equatorial (around mid-section) and 1 apical (at the tip)
These algae have external plates called theca
Dinoflagellates can form cysts, when conditions become unfavorable
Dinoflagellates have permanently condensed chromosomes, which is distinctive among eukaryotes
Most are heterotrophic; parasites on fish or protists
Some are photosynthetic, using chlorophyll a and c, and peridinin pigments
Some do not have their own chloroplasts, but host a phototrophic endosymbiont, and a few may use alien chloroplasts (cleptochloroplasts), obtained from photosynthetic organisms they consumed (kleptoplasty)
Above: the dinoflagellate, Ceratium
Bioluminescence
At least 18 genera of dinoflagellates are able to produce bioluminescence (Haddock et al. 2010)
The algae glow when agitated by waves or swimming animals
Dinoflagellates may use bioluminescence for defense against invertebrate predators, shining when they detect a predator, possibly making the predator more vulnerable by attracting the attention of predators from higher trophic levels (Haddock et al. 2010)
Above: bioluminescence of algae in the water
Left: Dolphins swimming through waters with bioluminescent algae
Zooxanthellae
A symbiotic relationship between dinoflagellates and marine animals (e.g. anemones, corals, clams, etc.)
Non-flagellated dinoflagellates living inside animal cells
Dinoflagellates produce sugars through photosynthesis and share with the host animal
They provide their host with the organic carbon products of photosynthesis, sometimes providing up to 90% of their host's energy needs
In return, they receive nutrients, carbon dioxide, and an elevated position with access to sunshine
Above: The zooxanthellae dinoflagellates inside the polyps of coral
Above: The zooxanthellae dinoflagellate, Symbiodinium, commonly found in corals and anemones
Red tides
Cause of discoloration of water along tropical oceans
Some species produce neurotoxins, and the genus Alexandrium is the most numerous and widespread toxin producers
Paralytic shellfish poisoning is caused by eating affected shellfish, such as clams, mussels, and oysters
Humans can be affected by eating fish or shellfish containing the toxins; affects muscle function
Above: red tide caused by dinoflagellates
Additional Resources
Things to know about FDA warning on paralytic shellfish poisoning in Pacific Northwest (Chicago Tribune 10June2024)
Changing climate fuels harmful algae blooms (University of Delaware 27Oct2021)
Sex and the Symbiont: Can algae hookups help corals survive? (Rice University 22Sep2021)
Paralytic shellfish poisoning - Univ. of Maryland
Zooxanthellae - NOAA
Dinoflagellate bloom (May 2012) - The Baltimore Sun
Algae blooms and Climate Change (EurekAlert 2015)
Ancient dinoflagellates survived glaciation in warm ocean pools - Ghent University
Giant clams harness the sun by growing dinoflagellates (Holt et al. 2014)
Some dinoflagellates harpoon their prey (NY TImes 2017)
Bio-engineering dinoflagellates to save coral (Science Daily 2017)