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The diatom genus Pseudonitzschia (family Bacillariaceae) contains about 40 species, with a growing number of cryptic novel species defined using molecular methods.  In most species the elongate, 40-175 micrometer Pseudo-nitzschia cells form colonies, commonly of 20 or more cells, by overlapping ends to form a long thin chain-like morphology.

The genus is found in ocean and coastal waters around the globe.  In the Canadian arctic, these organisms have been found living under ice floes at sub-zero oC (<32oF) temperatures; species also flourish in tropical waters where temperatures exceed 30oC (>86oF).  When temperatures increase in high nutrient waters, “blooms” of one or more Pseudo-nitzschia species can make this genus the dominant member of the phytoplankton community, reaching numbers up to 107 cells/L.  As eutrophication and fertilization of waters increases, incidences of Pseudo-nitzschia blooms (often referred to as red tides, as these diatoms turn the water a rust color) have become more prevalent around the world.  Causes of blooms are complex and not well understood, involving changes in water conditions and nutrients sources.

During a large Pseudo-nitzschia bloom off Prince Edward Island in 1987, 100 people were sickened and 3 died after eating cultured mussels.  This incident first brought recognition of Psudo-nitzschia as a producer of Dolomic Acid (DA), a rare but naturally occurring amino acid that binds glutamate channels to act as a potent neurotoxin in vertebrates and cause permanent neurological damage.  Some animals bio-accumulate the toxin in concentrations dangerous to humans, so during blooms consumption of filter feeding shellfish, planktivorous fish such as sardines and anchovies, and internal organs of crabs should not be avoided.  Symptoms from exposure to DA are gastrointestinal and neurological in effect, and can include short-term memory loss, giving the syndrome the name Amnesic Shellfish Poisoning (ASP).  Severe cases result in death.  Sea birds and marine mammals have experienced large die-offs and strandings from eating contaminated fish prey.  After blooms the toxins may settle into sediments to further impact benthic food webs. 

Since 1987, at least 14 species of Pseudo-nitzschia have been recognized as producers of Dolomic Acid, and the genus receives a lot of attention as a major player in harmful algal blooms (HABs) around the world.  A bloom stretching along the Pacific Coast of North America from southern California to Alaska in spring of 2015 has grown to an unprecedented extent and concentration of DA in Pacific coast waters, causing high-cost closures of fishing industries.  Scientists are investigating possible connection between the bloom and higher than usual water temperatures in the area which creating conditions that may be more and more common with climate change, and developing robotic detection and modeling to assist in forecasts of future blooms.  Because species that produce DA do not do so all the time, ecology and physiology of the different species and conditions triggering toxin production are important research avenues.

For the first time ever recorded, US Pacific Coast shellfish have been found with coincident bioaccumulation of toxic Pseudo-nitzschia diatoms as well as dangerous dinoflagellates that produce Paralytic Shellfish Toxins (PSTs) in 2015.

(Anderson et al. 2010; Hayashi et al. 2007; Lelong et al 2012; Lim et al 2013; Milstein 2015; Schnetzer et al. 2007)

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