Dinoflagellates

Dinoflagellates are a diverse group of mostly unicellular flagellated protists in the phylum Dinophyta. They occur in marine and freshwater environments and include photosynthetic, heterotrophic, and mixotrophic species. Many contribute to primary production in the oceans, while others obtain energy by ingesting prey or by symbiosis with other organisms.

Morphology: typical cells are motile and often armored with cellulose plates forming a theca. They possess two flagella—one that encircles the cell in a groove (the cingulum) and a second that trails along the body—producing a distinctive spinning movement. Pigments such as chlorophylls a and c and peridinin give brownish colors in photosynthetic species; some lineages are nonphotosynthetic.

Ecology: dinoflagellates are important marine primary producers, especially in coastal ecosystems. Several form intracellular symbionts with corals (zooxanthellae), providing energy to hosts. Others form harmful algal blooms that produce toxins affecting shellfish, fish, and humans. Toxins include saxitoxins, brevetoxins, and others. Blooms can cause red tides and bioluminescence in some species.

Life cycle: reproduction is mainly asexual by binary fission, but many species undergo sexual reproduction and produce resting cysts that persist in sediments, enabling survival during unfavorable conditions and contributing to dispersion.

Fossil record: the cysts of dinoflagellates, known as dinocysts, are common fossils and are used in biostratigraphy and studies of past marine environments. This lineage forms an important component of both modern and fossil marine ecosystems.