Penn StateNASA

Harmful Algal Bloom 101


Dinoflagellates are a group of microscopic single-celled organisms or protists that are dominantly autotrophic (ie primary producers). Interestingly, many species are also mixotrophic, having the ability to ingest their prey as a source of energy. Some species are entirely heterotrophic, lacking chloroplasts or plastids, and have been termed carnivorous. In fact, there have been reports in the scientific literature that some species have the ability to consume fish after having paralyzed them with neurotoxins. These claims are controversial but have given dinoflagellates a near-mythical reputation among the oceanic plankton.

Diagram illustrating the simplified life cycle of a dinoflagellate
Simplified life cycle of a dinoflagellate.

Diatoms and dinoflagellates are most common in the coastal oceans but also have the ability to live in freshwater environments and in intermediate salinity environments where fresh and marine waters mix in estuaries. They are the most prolific group of primary producers in the ocean. Dinoflagellates have a highly complex life cycle that consists of an alternation between a motile stage and a resting or cyst stage. In short, dinoflagellates enter the resting stage via sexual reproduction when conditions in the surface ocean are not suitable for them to thrive. They can remain dormant for weeks, months or years before they “excyst,” when surface conditions improve, reproduce vegetatively, and populate the surface ocean. Excystment and repopulation are triggered by changes in temperature, light, or oxygen levels or even resuspension of cysts by storms. Dinoflagellates generally thrive when nutrient levels are elevated, and, under conditions of extremely high nutrient levels, cell division can be so rapid that extremely high cell counts (millions of cells per milliliter of seawater) are reached, resulting in red tides. The cyst stage acts as a very effective mechanism for seeding blooms.


The following video summarizes the life cycle of the dinoflagellates.

Video: Life Cycle of the Dinoflagellates (1:27)

Dinoflagellates have a broad range of different ecologies. As we saw earlier in the module, they can be endosymbionts of corals, facilitating calcification in the host colony. They have this same role in foraminifera and radiolarian, a group of siliceous zooplankton.


Diatoms are autotrophic protists that produce a delicate, microscopic test of opaline silica.

Pseudo-nitzschia, a toxin-producing diatom.
Pseudo-nitzschia, a toxin-producing diatom.
Credit: Raphael Kudela, USCS Ocean Data Center

They are non-motile, and, for most of their life cycle, they reproduce asexually. Many nearshore diatom species also have a resting stage akin to the dinoflagellates, allowing them to exit the surface zone when conditions are unfavorable for their growth. This may occur in winter when surface waters are cold or at times when nutrients are depleted. The resting spore stage actually may resemble the vegetative stage of dinoflagellates. Like the dinoflagellates, diatoms are able to reproduce extremely rapidly by simple cell division, and this allows them to rapidly dominate the surface ocean when nutrients are readily available.

Video: Diatom Life Cycle (1:38)

Species that are harmful belong to the pennate diatoms that are long and thread-like and have the ability to attach to a host, although the relationship is not symbiotic. Both dinoflagellates and diatoms cysts can move around the oceans by currents, storms, dredging of the ocean bottom, and when cysts act as ballast on ships or even higher-level organisms. Toxins are not known in the cyst stage of either group.