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Anabaena circinalis is a species of Gram-negative, photosynthetic cyanobacteria common to freshwater environments throughout the world. Much of the scientific interest in A. circinalis owes to its production of several potentially harmful cyanotoxins, ranging in potency from mildly irritating to lethal.

Morphology
The cyanobacteria  Anabaena circinalis  exhibits a filamentous morphology, each filament consisting of several task-specific cells. This presence of differentiated cell types was a great evolutionary leap for cyanobacteria; an ancient prokaryote and one of the first multicellular organisms on Earth. The most numerous cells are photosynthetic vegetative cells, responsible for the production of high-energy sugars from environmental carbon, water, and sunlight. During periods when combined nitrogen (e.g. ammonia or nitrate) is unavailable,  A. circinalis  form heterocysts, differentiated cells with the ability to fix environmental nitrogen (N2) into useable compounds, such as ammonia or nitrate. This process requires nitrogenase, an oxygen-sensitive enzyme produced within the heterocysts. The cell envelopes of heterocysts tend to be very thick, providing the anoxic environment essential to nitrogenase activity and subsequent nitrogen fixation Although functioning independent of each other, vegetative cells and heterocysts are both essential to the survival of the organism; vegetative cells providing energy-rich sugars to heterocysts, which in turn use the energy to fix nitrogen for biosynthesis. Also found within the filaments are gas vacuoles, specialized compartments that inflate or deflate with air to provide upward or downward movement. This adaptation positions  A. circinalis  at a favorable depth, determined by available sunlight, water temperature, or O2 concentration. When conditions for growth are favorable,  Anabaena circinalis  form large blooms that can appear as slime at the surface of the water. In harsh conditions (i.e. winter),  A. circinalis  form spore-like cells called akinetes. Akinetes are thick, durable structures, resistant to low temperature, desiccation, and darkness. Often, akinetes will hibernate in sediment until environmental conditions allow germination and re-growth.

Neurotoxins
North American and European strains of  Anabaena circinalis  produce anatoxin-a, one of the first cyanobacterial neurotoxins identified. In the late 1950’s, research began in earnest following several cattle deaths attributed to contaminated drinking water. Due in part to the relatively short time from ingestion to death, the toxin was ominously coined Very Fast Death Factor (VFDF). Anatoxin-a functions as a postsynaptic nicotinic agonist, ultimately disrupting the neuromuscular blockade. Disruption to this particular cascade can result in a loss of coordination, paralysis, muscle twitching, shortness of breath, and possibly death. Besides its biological toxicity,  Anabaena circinalis  blooms also disrupt water-treatment facilities and recreational waterways, causing substantial economic damage as well. In some freshwater environments of Australia,  A. circinalis  are known to produce paralytic shellfish toxins (PSTs), a potent neurotoxin found mostly in marine dinoflagellates. PST intoxication can result in a potentially fatal illness known as paralytic shellfish poisoning (PSP). PSTs are a potent saxotoxin that function as an acetylcholine (ACh) analog, binding to Ach receptors in the neuromuscular junction. However, unlike acetylcholine, PSTs are not readily degraded by cholinesterase. Toxicity can therefore result in persistent muscle contraction, respiratory paralysis, and eventually death.

Phylogeny
Regardless of geographic location, A. circinalis are monophyletic, having evolved from a cyanobacterial ancestor approximately 1-2 billion years ago. Some progress is currently being made to subdivide groups of Anabaena circinalis further. Specifically designed PCR primers have been employed to isolate and analyze 16S rRNA, a highly conserved region of the bacterial ribosomal subunit. This analysis has revealed two distinct gene clusters based on the presence or absence of PSTs.

Applications
The toxins produced by  Anabaena circinalis  (and similar species) occur in many different analogs, each varying in toxicity. Biotransformation experiments have shown promise in the chemical conversion of highly toxic PSTs into less toxic forms. This process may provide a way to detoxify dangerous blooms before they cause irreparable damage. Additionally, some forms of PSTs have shown promise as long-lasting anesthetics for various chronic medical conditions The Chemical Weapons Convention (CWC) has categorized saxitoxin as a Schedule 1 substance, meaning: The United States Military has researched saxitoxin independently, naming the toxin Agent TZ. Further, researchers have found the aerosol dispersion of Agent TZ to have a much higher toxicity than sarin nerve gas or phytotoxin ricin
 * Saxitoxins can be used as a weapon itself, or as a precursor to another weapon.
 * Saxitoxins have no other practical use outside of weapons manufacture.