Alexandrium species

Alexandrium is an armored dinoflagellate that can occur as single cells or in chains of two or more cells. The photo below shows a chain of eight cells of Alexandrium. This dinoflagellate species is described as "armored" because the cell is made up of many cellulose plates. Alexandrium produces the paralytic shellfish poisoning (PSP) toxins, a series of related compounds that have over 20 different versions identied so far. A given clonal population of Alexandrium will not produce every single possible form of the PSP toxins, but is thought to produce the same subset of these toxins over and over again with each succeeding generation. PSP toxins are among the most potent natural toxins known.

The individual cells of Alexandrium are not round, but rather a bit compressed. The central groove around the cell, which contains one of the two flagella, is clearly visible. This photograph also contains a chain of three Alexandrium cells as well as several chains of Pseudo-nitzschia, the domoic acid producer, for comparison.