Alexandrium

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A Microbial Biorealm page on the genus Alexandrium

Alexandrium catenella colony. Photo courtesy of Jan Rines.

Alexandrium tamarense cysts. Photo by D. Wall.

Classification

Higher order taxa:

Eukaryota; Alveolata; Dinophyceae; Gonyaulacales; Gonyaulacaceae

Species:

Alexandrium andersoni, A. taylori, A. lusitanicum

NCBI: Taxonomy Genome

Description and Significance

While they share many morphological characterists with other Dinoflagelata, Alexandrium is a very unique genus. It is most famous (or infamous) for the effects some of its species can have on the environment. Many Alexandrium species are toxic, which can cause both medical and economic harm to humans. The non-toxic species can cause trouble as well.

Genome Structure

DNA sequencing is one of the most popular ways to identifiy different Alexandrium species.

Cell Structure and Metabolism

Diagram of life cycle. By Jack Cook, Woods Hole Oceanographic Institution.

Alexandrium species are considered armored dinoflagellates, because they are covered with thecal plates. Alexandrium have two flagella.

Alexandrum tamarense is an autotrophic organism, as is Alexandrium minutum, which obtains energy through photosynthesis. However, there are heterotrophic species as well.

Asexual reproduction through binary fission is most common (steps 1-3 on the life cycle). However, Alexandrium species can also undergo a sexual cycle. In sexual reproduction, motile mating types fuse, which produces a motile diploid zygote (called a planozygote). The planozygote will swim, then take the form of a cyst. It will wait until environmental conditions are ideal before germinating. There are several stages to the Alexandrium life cycle: motile vegetative cells, haploid gametes, diploid zygotes, resting cysts, and temporary cysts.

Ecology

Red tide in the St. Lawrence River (Alexandrium tamarense). Pêches et Océans Canada.

Alexandrium are toxic organsims. They live in marine environments, mainly in costal regions. They are also associated with high-nitrogen environments. Alexandrium tamarense is noted for its ability to adapt to changes in the amount of nitrogen in its environment. Some species of Alexandrium are colonial organisms. For example, Alexandrium catenella is a catenate (chain-forming) organism.

Alexandrium produce paralytic shellfish poisoning toxins (PST). PSTs are neurotoxins. They inhibit transmisison of nerve impulses by blocking sodium channels. PSTs are extremely dangerous humans if they consume contaminated fish. Paralytic shellfish poisoning (PSP) can be fatal. While not all strains and species are harmful, many can have serious consequences on animal health. In addition to PSP, Alexandrium cause the infamous red tides. The most severe red tide outbreak occured in 1972. However, the summer of 2005 yielded another extremely severe outbreak. Although algal blooms are natural, they have increased in recent years. This is due to activities such as aquacultures and agriculture increasing them amount of fertilizers entering waters where Alexandrium species live. Even non-toxic species can be harmful. Large algal blooms lead to an increase in biomass. This can cause water to become anoxic, killing fish and other organisms.

Alexandrium can also be affected by other organisms. For example, the parasitic flagellate Parvilucifera infectans can be lethal to Alexandrium ostenfeldii. Alexandrium ostenfeldii has a defense mechanism, however. It can avoid infection by entering a temporary cyst stage. These temporary cysts can break open within a few hours, once danger has passed.