For each species below, observe the illustration and microscope image, then identify the phytoplankton group and key morphological features.

Cyanobacteria

Dolichospermum spp.
Filamentous chains; cylindrical/barrel cells; heterocysts for N₂ fixation; freshwater cyanotoxins

Coccolithophore

Emiliania huxleyi
Spherical; calcium carbonate coccolith plates; no HAB; abundant in CCE

Diatom

Pseudo-nitzschia spp.
Elongate fusiform; silica frustule; chains; ~½ species produce domoic acid (ASP)

Diatom

Chaetoceros spp.
Rectangular cells; long corner setae; coiled/curved chains; no HAB; abundant in CCE

Dinoflagellate

Dinophysis spp.
Armored; laterally flattened; large hypotheca (~¾); girdle lists; okadaic acid (DSP)

Dinoflagellate

Karenia brevis
Unarmored; apical groove; dorsoventrally flattened; brevetoxins (NSP); Florida red tide

Dinoflagellate

Margalefidinium spp.
Unarmored; cingulum rotates >1.5×; reactive oxygen species; fish kills / brown tide

Dinoflagellate

Tripos spp.
Heavily thecate; 1–4 apical/antapical horns; no HAB; common in CCE

Dinoflagellate

Gambierdiscus spp.
Lens-shaped; anterio-posteriorly compressed; thecate; ciguatoxins (CFP); tropical reefs

Dinoflagellate

Alexandrium catenella
Armored; spherical; chains; saxitoxins (PSP); abundant in CCE

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