Pseudo-nitzschia

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Pseudo-nitzschia
Pseudonitzschia2.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Clade: Diaphoretickes
Clade: SAR
Clade: Stramenopiles
Phylum: Gyrista
Subphylum: Ochrophytina
Class: Bacillariophyceae
Order: Bacillariales
Family: Bacillariaceae
Genus: Pseudo-nitzschia
H. Perag. in H. Perag. and Perag.
Not to be confused with Pseudonitzschia , a genus of flatworm in the family Capsalidae.

Pseudo-nitzschia is a marine planktonic diatom genus that accounts for 4.4% of pennate diatoms found worldwide. [1] Some species are capable of producing the neurotoxin domoic acid (DA), which is responsible for the neurological disorder in humans known as amnesic shellfish poisoning (ASP). Currently, 58 species are known, 28 of which have been shown to produce DA. It was originally hypothesized that only dinoflagellates could produce harmful algal toxins, but a deadly bloom of Pseudo-nitzschia occurred in 1987 in the bays of Prince Edward Island, Canada, and led to an outbreak of ASP. [2] Over 100 people were affected by this outbreak after consuming contaminated mussels; three people died. [3] Since this event, no additional deaths have been attributed to ASP, though the prevalence of toxic diatoms and DA has increased worldwide. This anomaly is likely due to increased awareness of harmful algal blooms (HABs) and their implications for human and ecosystem health. [4]

Contents

Blooms have since been characterized in coastal waters and the open-ocean worldwide and have been linked to increasing marine nutrient concentrations, warming ocean temperatures, and bacterial interactions. [5] [4]

Morphology and physiology

The diatom Pseudo-nitzschia granii is a common responder to iron enrichment in iron-limited regions of the ocean Pseudo-nitzschia granii.png
The diatom Pseudo-nitzschia granii is a common responder to iron enrichment in iron-limited regions of the ocean

Pseudo-nitzschia species are bilaterally symmetrical pennate diatoms. Cell walls are made up of elongated silica frustules. The silica wall is fairly dense which leads to negative buoyancy, providing a number of advantages. The wall allows the diatoms to sink to avoid light inhibition or nutrient limitations, as well as to protect against grazing zooplankton. The silica frustules also contribute vastly to the sediment layers of the earth and to the fossil record, which makes them exceptionally useful in increasing understanding of numerous processes such as gauging the degree of climate change. [6] Before sinking to the ocean floor, every atom of silicon that enters the ocean is integrated into the cell wall of a diatom about 40 times. [6]

Silica frustules contain a central raphe, which secretes mucilage that allows the cells to move by gliding. [7] Cells are often found in overlapped, stepped colonies, and exhibit collective motility. [7] Pseudo-nitzschia species synthesize their own food through the use of light and nutrients in photosynthesis. The diatoms have a central vacuole to store nutrients for later use and a light-harvesting system to protect themselves against high-intensity light. [6]

Taxonomy

The diatom lineage may go back 180 to 250 million years ago (Mya). About 65 Mya, diatoms survived a mass extinction in which roughly 85% of all species perished. [6] Until 1994, the genus was known as Nitzschia , but was changed to Pseudo-nitzschia because of the ability to form chains of overlapping cells, as well as other minor morphological differences. [8] While the genus can be readily recognized using light microscopy, identification of distinct species can require taxonomic expertise and be extremely time-consuming. Species identification in this genus is notoriously difficult due to the presence of cryptic species. Similar species are often differentiated by very small differences in the frustule, such as shape, period, and band stria. [3]

The direct impacts of species identification on public health make this a serious concern. Toxogenic and nontoxogenic species commonly co-occur; therefore, discrimination between various Pseudo-nitzschia species is imperative to determine the potential toxicity of an algal bloom. Optical microscopy identification techniques are inadequate when a large number of samples must be routinely examined, such as is required for a monitoring program for public health. Recently, a DNA-microarray was developed for simultaneous detection of multiple harmful algal bloom species with an emphasis on Pseudo-nitzschia. The total assay is believed to have the potential to identify hundreds of species and accurately differentiate between large quantities of related species. Additionally, this technology has been shown to accurately identify toxic phytoplankton even at extremely low concentrations. The lower limit for detection of Pseudo-nitzschia is as low as 500 cells. [9]

The nomenclatural history is given in Hasle (1995) [10] and Bates (2000). [11]

Lifecycle

Ornithine-urea cycle

The physiological distribution, fixation, and recycling center for inorganic carbon and nitrogen plays a key role in the metabolic response of diatoms to prolonged nutrient deprivation. The cycle enables diatoms to respond immediately to the availability of nutrients and recover by increasing their metabolic and growth rates. [6]

Resting stages

Diatoms have the ability to enter two distinct resting stages to overcome periods of stress. A resting spore has a great capacity to survive over extended periods of nutrient deprivation. To avoid low nutrient concentrations during stratification, the resting spores can settle to the bottom where the nutrient concentration is higher. A resting cell is better able to rapidly respond when nutrients become available again. This is more often observed in freshwater and pennate diatoms like Pseudo-nitzschia. [6] There is contradictory evidence regarding the presence or absence of a resting stage in Pseudo-nitzschia. [12]

Reproduction

Among diatoms, reproduction is primarily asexual by binary fission, with each daughter cell receiving one of the parent’s cell’s two frustules. However, this asexual division results in a size reduction. To restore the cell size of a diatom population, sexual reproduction must occur. Vegetative diploid cells undergo meiosis to produce active and passive gametes. These gametes then fuse to form a zygote, which then develops into an auxospore. [13] Sexual reproduction leads to both an increase in genotypic diversity and the formation of large initial cells through formation of the auxospore. Cells need to be below a species-specific size threshold for the sexual phase to be induced. Many external cues also regulate the initiation, such as day length, irradiance, and temperature. [14]

The basic mode of the sexual phase of reproduction appears to be conserved among Pseudo-nitzschia species. Upon mixing two strains of compatible mating type and of appropriate cell size for sexualization, cells align side by side and differentiate into gametangia. One active (+) and one passive (-) gamete are then produced within each gametangium. The active gamete migrates toward the passive partner and conjugates. The zygote is then becomes an auxospore, which has no rigid frustule. Inside the auxospore, a large initial cell is produced. [13]

Sexual reproduction appears to occur exclusively in the exponential growth phase and be linked to cell density. Sexualization can only be initiated when a species-specific threshold cell concentration is met. Decreasing the distance to facilitate contact and/or perception of chemical cues between cells triggers the sexual phase, indicating that high cell density is favorable for sexual reproduction. Additionally, the onset of sexualization is linked to a significant reduction in growth of the vegetative and parental cells, suggesting that vegetative division is inhibited when the two strains of opposite mating type come in contact. [14]

Genome and transcriptome

Pseudo-nitzschia multiseries has a genome consisting of 219 megabases (Mb) and a full genome project is underway. [15]

Transcriptomes of three species, P. arenysensis, P. delicatissima, and P. multistriata, have been sequenced. The transcriptomes encode between 17,500 and 20,200 proteins. P. multistriata was found to uniquely encode nitric oxide synthase. [16]

Recently, transcriptome analysis of P. multiseries was used to identify a four-gene cluster linked to DA biosynthesis. [17] The identification of these genes presents an opportunity to monitor toxic blooms of Pseudo-nitzschia genetically in order to better understand the toxicity and environmental conditions that cause them.

Habitat

In general, diatoms flourish in nutrient-rich waters with high light penetration. [6] Some species of Pseudo-nitzschia can grow in a broad temperature range (4–20 °C or 39–68 °F), making it possible for them to inhabit a diverse range of habitats. [18]

Pseudo-nitzschia species have been observed in all oceans of the world, including the Arctic and Antarctic. [4] In North America, they have been documented along the Pacific coast from Canada to California, along the Atlantic Northeast coast of Canada, North Carolina, and the Gulf of Mexico. [19] Various species have been detected in the open ocean as well as gulfs and bays, showing a presence in many diverse environments, including off the coasts of Canada, Portugal, France, Italy, Croatia, Greece, Ireland, Australia, Morocco, Japan, Spain, Tunisia, Namibia, Singapore, Angola, Philippines, Turkey, Ukraine, Argentina, and Uruguay. [20] [4]

Given the warming temperatures of ocean water, decreasing sea ice, and increasing light penetration brought on by climate change, it is likely that the season for favorable growth of toxigenic Pseudo-nitzschia species will expand. [4] It is important to continue monitoring Pseudo-nitzschia blooms and their toxicity, particularly in Arctic and Antarctic habitats that may begin to see higher prevalence of these species.

Harmful bloom dynamics

Harmful algal blooms (HABs) of Pseudo-nitzschia and the like can cause diseases and death in many marine organisms, as well as the humans who consume them. HABs can result in oxygen depletion caused by increased biomass production. However, blooms of Pseudo-nitzschia more commonly cause harm through the production of the toxin DA, which can be transferred to other trophic levels through bioaccumulation.

DA can often be detected in shellfish flesh during and immediately following a toxic bloom. Though shellfish harvest closures are typically based on cells counts of Pseudo-nitzschia present, these cell counts do not always correlate with DA levels. [4] Thus, it is important to understand the other environmental drivers that may lead to higher production of DA.

The largest recorded DA event caused by Pseudo-nitzschia took place along the North American west coast in 2015, causing prolonged closures of razor clam, rock crab, and Dungeness crab fisheries. Later in 2015, DA was detected in whales, dolphins, porpoises, seals, and sea lions. This bloom was dominated by P. australis and likely caused by anomalous warm water and nutrients brought to the surface by upwelling conditions. [18]

Prior to this 2015 bloom, the largest Pseudo-nitzschia bloom recorded occurred in September 2004 off the northwest coast of the United States. The maximum cell densities during this bloom reached 13 x 106 cells per liter, with domoic acid levels of 1.3 pg DA/cell. [21]

Sediment cores indicate a link between increasing coastal nutrient levels (eutrophication) and an increase in Pseudo-nitzschia blooms. [5]

The largest toxic Pseudo-nitzschia bloom was recorded in 2015 along the west coast of North America. NOAA July 2015 Average Chlorophyll Concentrations (milligrams per cubic meter of water).jpg
The largest toxic Pseudo-nitzschia bloom was recorded in 2015 along the west coast of North America.

Domoic acid

Pseudo-nitzchia Pseudo-nitzschia.jpg
Pseudo-nitzchia

Shellfish become contaminated after feeding on toxic Pseudo-nitzschia blooms and can act as a vector to transfer domoic acid to humans upon ingestion. DA acts as a potent glutamate agonist and is responsible for amnesic shellfish poisoning in humans. Effects can be as minor as vomiting, cramps, and a headache, or as severe as permanent anterograde memory loss, coma, and death. [22] So, monitoring systems and management practices for recreational and commercial fishing are important to ensure the health of marine animals and their predators.

Photosynthesis is essential for the production of domoic acid. Periods of darkness or chemical inhibition of photosynthesis has been shown to inhibit toxin production. Additionally, DA production peaks in the stationary phase of the growth cycle when cell division is slowed or absent. Production is minimal or nonexistent during the exponential phase, and ceases completely during the death phase of the growth cycle. [21]

Factors affecting production

Many factors have been linked to promotion of DA production, including sufficient light, elevated or decreased pH, and nutrition limitations. [12] [4] In one species, P. cuspidata, a link has been indicated between toxicity and photosynthesis photon flux density (PPFD). At a low PPFD, the exponential growth rate approximately halved and the cells were significantly more toxic. [21]

While the effect of availability of different nitrogen sources on toxicity has been studied many times, no general rule could be demonstrated for differences in growth and DA production, with the results varying greatly by species. However, toxin production increases when the nitrogen source could not sustain a high biomass, suggesting growth limitation seems to induce toxicity. [23]

The presence of zooplankton has also been shown to affect the toxicity of Pseudo-nitzschia. The presence of copepods was shown to enhance toxin production of P. seriata. This effect appears to be chemically mediated, as it could be induced without physical contact. [24]

Pseudo-nitzschia species also appear to respond dramatically to differences in trace metal concentrations, such as iron (Fe) and copper (Cu). In Fe-limited conditions, Pseudo-nitzschia increases DA production by six to 25 times as a result of stress. [2] This increase allows them to enhance Fe acquisition needed for metabolic activities, and can have devastating effects.

Known species

Over fifty species of Pseudo-nitzschia have been described (following WoRMS [25] unless specified):

Many species of Pseudo-nitzschia have been shown to produce domoic acid, although not all strains are toxigenic. [12] [4] [37]

  • Pseudo-nitzschia abrensis
  • Pseudo-nitzschia australis
  • Pseudo-nitzschia batesiana
  • Pseudo-nitzschia bipertita [36]
  • Pseudo-nitzschia brasiliana
  • Pseudo-nitzschia caciantha
  • Pseudo-nitzschia calliantha
  • Pseudo-nitzschia cuspidata
  • Pseudo-nitzschia delicatissima
  • Pseudo-nitzschia fraudulenta
  • Pseudo-nitzschia fukuyoi
  • Pseudo-nitzschia galaxiae
  • Pseudo-nitzschia granii
  • Pseudo-nitzschia hasleana
  • Pseudo-nitzschia kodamae
  • Pseudo-nitzschia lundholmiae
  • Pseudo-nitzschia multiseries
  • Pseudo-nitzschia multistriata
  • Pseudo-nitzschia obtusa
  • Pseudo-nitzschia plurisecta
  • Pseudo-nitzschia pseudodelicatissima
  • Pseudo-nitzschia pungens
  • Pseudo-nitzschia seriata
  • Pseudo-nitzschia simulans
  • Pseudo-nitzschia subcurvata [38]
  • Pseudo-nitzschia subfraudulenta
  • Pseudo-nitzschia subpacifica
  • Pseudo-nitzschia turgidula

Related Research Articles

<span class="mw-page-title-main">Diatom</span> Class of microalgae found in oceans, waterways, and soil

A diatom is any member of a large group comprising several genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of the Earth's biomass: they generate about 20 to 50 percent of the oxygen produced on the planet each year, take in over 6.7 billion tonnes of silicon each year from the waters in which they live, and constitute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half-mile deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by transatlantic winds from the African Sahara, much of it from the Bodélé Depression, which was once made up of a system of fresh-water lakes.

<span class="mw-page-title-main">Algal bloom</span> Spread of planktonic algae in water

An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term algae encompasses many types of aquatic photosynthetic organisms, both macroscopic multicellular organisms like seaweed and microscopic unicellular organisms like cyanobacteria. Algal bloom commonly refers to the rapid growth of microscopic unicellular algae, not macroscopic algae. An example of a macroscopic algal bloom is a kelp forest.

<span class="mw-page-title-main">Domoic acid</span> Chemical compound

Domoic acid (DA) is a kainic acid-type neurotoxin that causes amnesic shellfish poisoning (ASP). It is produced by algae and accumulates in shellfish, sardines, and anchovies. When sea lions, otters, cetaceans, humans, and other predators eat contaminated animals, poisoning may result. Exposure to this compound affects the brain, causing seizures, and possibly death.

<i>Phaeodactylum tricornutum</i> Species of single-celled organism

Phaeodactylum tricornutum is a diatom. Unlike other diatoms, P. tricornutum can exist in different morphotypes and changes in cell shape can be stimulated by environmental conditions. This feature can be used to explore the molecular basis of cell shape control and morphogenesis. Unlike most diatoms, P. tricornutum can grow in the absence of silicon and can survive without making silicified frustules. This provides opportunities for experimental exploration of silicon-based nanofabrication in diatoms.

Amnesic shellfish poisoning (ASP) is an illness caused by consumption of shellfish that contain the marine biotoxin called domoic acid. In mammals, including humans, domoic acid acts as a neurotoxin, causing permanent short-term memory loss, brain damage, and death in severe cases.

A resting spore is a resistant cell, used to survive adverse environmental conditions. Resting spore is a term commonly applied to both diatoms and fungi.

<i>Thalassiosira pseudonana</i> Species of single-celled organism

Thalassiosira pseudonana is a species of marine centric diatoms. It was chosen as the first eukaryotic marine phytoplankton for whole genome sequencing. T. pseudonana was selected for this study because it is a model for diatom physiology studies, belongs to a genus widely distributed throughout the world's oceans, and has a relatively small genome at 34 mega base pairs. Scientists are researching on diatom light absorption, using the marine diatom of Thalassiosira. The diatom requires a high enough concentration of CO2 in order to utilize C4 metabolism (Clement et al. 2015).

<i>Ceratium</i> Genus of single-celled organisms

The genus Ceratium is restricted to a small number of freshwater dinoflagellate species. Previously the genus contained also a large number of marine dinoflagellate species. However, these marine species have now been assigned to a new genus called Tripos. Ceratium dinoflagellates are characterized by their armored plates, two flagella, and horns. They are found worldwide and are of concern due to their blooms.

<i>Chaetoceros</i> Genus of single-celled organisms

Chaetoceros is a genus of diatoms in the family Chaetocerotaceae, first described by the German naturalist C. G. Ehrenberg in 1844. Species of this genus are mostly found in marine habitats, but a few species exist in freshwater. It is arguably the common and most diverse genus of marine planktonic diatoms, with over 200 accepted species. It is the type genus of its family.

<i>Nitzschia</i> Genus of single-celled organisms

Nitzschia is a common pennate marine diatom. In the scientific literature, this genus, named after Christian Ludwig Nitzsch, is sometimes referred to incorrectly as Nitzchia, and it has many species described, which all have a similar morphology. In its current circumscription, Nitzschia is paraphyletic.

Grethe Berit Rytter Hasle was a Norwegian planktologist. Among the first female professors of natural science at the University of Oslo, she specialized in the study of phytoplankton.

Phycotoxins are complex allelopathic chemicals produced by eukaryotic and prokaryotic algal secondary metabolic pathways. More simply, these are toxic chemicals synthesized by photosynthetic organisms. These metabolites are not harmful to the producer but may be toxic to either one or many members of the marine food web. This page focuses on phycotoxins produced by marine microalgae; however, freshwater algae and macroalgae are known phycotoxin producers and may exhibit analogous ecological dynamics. In the pelagic marine food web, phytoplankton are subjected to grazing by macro- and micro-zooplankton as well as competition for nutrients with other phytoplankton species. Marine bacteria try to obtain a share of organic carbon by maintaining symbiotic, parasitic, commensal, or predatory interactions with phytoplankton. Other bacteria will degrade dead phytoplankton or consume organic carbon released by viral lysis. The production of toxins is one strategy that phytoplankton use to deal with this broad range of predators, competitors, and parasites. Smetacek suggested that "planktonic evolution is ruled by protection and not competition. The many shapes of plankton reflect defense responses to specific attack systems". Indeed, phytoplankton retain an abundance of mechanical and chemical defense mechanisms including cell walls, spines, chain/colony formation, and toxic chemical production. These morphological and physiological features have been cited as evidence for strong predatory pressure in the marine environment. However, the importance of competition is also demonstrated by the production of phycotoxins that negatively impact other phytoplankton species. Flagellates are the principle producers of phycotoxins; however, there are known toxigenic diatoms, cyanobacteria, prymnesiophytes, and raphidophytes. Because many of these allelochemicals are large and energetically expensive to produce, they are synthesized in small quantities. However, phycotoxins are known to accumulate in other organisms and can reach high concentrations during algal blooms. Additionally, as biologically active metabolites, phycotoxins may produce ecological effects at low concentrations. These effects may be subtle, but have the potential to impact the biogeographic distributions of phytoplankton and bloom dynamics.

<i>Ditylum brightwellii</i> Species of diatom

Ditylum brightwellii is a species of cosmopolitan marine centric diatoms. It is a unicellular photosynthetic autotroph that has the ability to divide rapidly and contribute to spring phytoplankton blooms.

<i>Chaetoceros elegans</i> Species of single-celled organism

Chaetoceros elegans is a species of diatom in the family Chaetocerotaceae. According to Li, 2017, the type locality is Dapeng Bay, Guangdong Province, P. R. China.

<i>Eucampia zodiacus</i> Species of single-celled organism

Eucampia zodiacus is a common marine centric diatom species. It is known to be cosmopolitan except in polar regions. E. zodiacus is a harmful diatom that has become known as the predominant organisms causing the bleaching of aqua-cultured nori seaweed.

<i>Skeletonema</i> Genus of single-celled organisms

Skeletonema is a genus of diatoms in the family Skeletonemataceae. It is the type genus of its family. The genus Skeletonema was established by R. K. Greville in 1865 for a single species, S. barbadense, found in the Barbados deposit [Jung 2009]. These diatoms are photosynthetic organisms, meaning they obtain carbon dioxide from their surrounding environment and produce oxygen along with other byproducts. Reproduce sexually and asexually [Guiry 2011]. Skeletonema belong to the morphological category referred to as centric diatoms. These are classified by having valves with radial symmetry and the cells lack significant motility [Horner 2002]. Skeletonema are cylindrical shaped with a silica frustule. Cells are joined by long marginal processes to form a filament [Horner 2002]. Their length ranges from 2-61 micrometers, with a diameter ranging from 2-21 micrometers [Hasle 1997]. They are found typically in the neritic zone of the ocean and are highly populous in coastal systems [Jung 2009]. The genus is considered cosmopolitan, showing a wide range of tolerance for salinity and temperature [Hasle 1973]. For example, they have been found in various aquatic environments such as brackish or freshwater. Skeletonema are found worldwide excluding Antarctic waters [Hevia-Orube 2016]. Some harmful effects these diatoms may have on an ecosystem are attributed to large blooming events which may cause hypoxic events in coastal systems. Additionally, they are known to cause water discoloration [Kraberg 2010].

Pseudo-nitzschia australis is a pennate diatom found in temperate and sub-tropic marine waters, such as off the coast of California and Argentina. This diatom is a Harmful Micro Algae that produces toxic effects on a variety of organisms through its production of domoic acid, a neurotoxin. Toxic effects have been observed in a variety of predatory organisms such as pelicans, sea lions, and humans. If exposed to a high enough dose, these predators will die as a result, and there is no known antidote. The potential indirect mortality associated with P. australis is of great concern to humans as toxic algae blooms, including blooms of P. australis, continue to increase in frequency and severity over recent years. Blooms of P. australis are believed to result from high concentrations of nitrates and phosphates in stream and river runoff, as well as coastal upwelling, which are also sources of other harmful algae blooms.

<span class="mw-page-title-main">Marine protists</span> Protists that live in saltwater or brackish water

Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as marine single-celled prokaryotes and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics because they are paraphyletic.

Greta Albrecht Fryxell was a marine scientist known for her work on the biology and taxonomy of diatoms. In 1996, she was elected a fellow of the American Association for the Advancement of Science.

<i>Skeletonema costatum</i> Species of single-celled organism

Skeletonema costatum is a cosmopolitan centric diatom that belongs to the genus Skeletonema. It was first described by R. K. Greville, who originally named it Melosira costata, in 1866. It was later renamed by Cleve in 1873 and was more narrowly defined by Zingone et al. and Sarno et al. Skeletonemacostatum is the most well known species of the genus Skeletonema and is often one of the dominant species responsible for red tide events.

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