19.6: Dinoflagellates

Learning Objectives

• Explore some of the ecological roles of dinoflagellates.
• Describe the symbiosis between corals and zooxanthellae.
• Explain what happens in a red tide.

There are currently around 2,000 species of dinoflagellates. They are unicellular, though dinoflagellates exhibit extensive morphological diversity and can be photosynthetic, heterotrophic, or mixotrophic. In photosynthetic dinoflagellates, most use the pigments chlorophylls a and c. Many dinoflagellates are encased in interlocking plates of cellulose. Two perpendicular flagella fit into the grooves between the cellulose plates, with one flagellum extending longitudinally and a second encircling the dinoflagellate (Figure \(\PageIndex{1}\)). Together, the flagella contribute to the characteristic spinning motion of dinoflagellates. Interestingly, dinoflagellates have a unique nucleus structure, where chromosomes are attached to the nuclear membrane. This is not found in other eukaryotes and so has received its own name: a dinokaryon.

Figure \(\PageIndex{1}\): The dinoflagellates exhibit great diversity in shape. Many are encased in cellulose armor and have two flagella that fit in grooves between the plates. Movement of these two perpendicular flagella causes a spinning motion. Descriptive text: Two dinoflagellates. The first is walnut-shaped, with a groove around the middle and another perpendicular groove that starts at the middle and extends back. Flagella fit in each groove. The second dinoflagellate is horseshoe-shaped, with the body extending from the wide part of the horseshoe toward the narrow end. Like the first dinoflagellate, this one has two perpendicular grooves, each containing a flagellum.

These protists exist in freshwater and marine habitats, and are a component of plankton, the typically microscopic organisms that drift through the water and serve as a crucial food source for larger aquatic organisms. Some dinoflagellates generate light, called bioluminescence, when they are jarred or stressed. Large numbers of marine dinoflagellates (billions or trillions of cells per wave) can emit light and cause an entire breaking wave to twinkle or take on a brilliant blue color (Figure \(\PageIndex{2}\)). For approximately 20 species of marine dinoflagellates, population explosions (also called blooms) during the summer months can tint the ocean with a muddy red color. This phenomenon is called a red tide due to the abundant red pigments present in dinoflagellate plastids. In some cases, such as the 2018/19 event on the Gulf Coast, these dinoflagellate species can secrete an asphyxiating toxin that can kill fish, birds, and marine mammals. Red tides can be massively detrimental to commercial fisheries, and humans who consume these protists may become poisoned.

Figure \(\PageIndex{2}\): Bioluminescence is emitted from dinoflagellates in a breaking wave, as seen from the New Jersey coast. (credit: “catalano82”/Flickr)

As plankton, dinoflagellates are essential sources of nutrition for many other organisms. In some cases, they are consumed directly. Others serve as producers of nutrition in a more indirect way. For instance, photosynthetic dinoflagellates called zooxanthellae use sunlight to fix inorganic carbon. In this symbiotic relationship, these protists provide nutrients for coral polyps (Figure \(\PageIndex{3}\)) that house them, giving corals a boost of energy to secrete a calcium carbonate skeleton. In turn, the corals provide the protist with a protected environment and the compounds needed for photosynthesis. This type of symbiotic relationship is important in nutrient-poor environments. Without dinoflagellate symbionts, corals lose algal pigments in a process called coral bleaching, and they eventually die. This explains why reef-building corals do not reside in waters deeper than 20 meters: insufficient light reaches those depths for dinoflagellates to photosynthesize.

Figure \(\PageIndex{3}\): Coral polyps obtain nutrition through a symbiotic relationship with dinoflagellates. The dinoflagellate partners, called zooxanthellae, are what give corals their color.

Summary

Dinoflagellates are a group of morphologically and nutritionally diverse acquatic organisms, from the zooxanthellae that live inside coral polyps to the toxin-releasing microbes that cause red-tides. They have essential roles in marine food webs. They are typically unicellular, with cellulose plates and two flagella.

Attributions

Curated and authored by Maria Morrow, CC BY-NC, using the following sources:

• 19.1.2 Protists from Biology by John. W. Kimball (licensed CC-BY)
• 23.3 Groups of Protists from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.