Heterosigma akashiwo

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Heterosigma akashiwo
CCMP452.jpg
Scientific classification
(unranked):
SAR
Superphylum:
Phylum:
Class:
Order:
Family:
Genus:
Heterosigma
Species:
H. akashiwo
Binomial name
Heterosigma akashiwo
(Y. Hada) Y. Hada ex Y. Hara & M. Chihara

Heterosigma akashiwo is a species of microscopic algae of the class Raphidophyceae. [1] [2] It is a swimming marine alga that episodically forms toxic surface aggregations known as harmful algal bloom. The species name akashiwo is from the Japanese for "red tide". [1]

Contents

Synonyms include Olisthodiscus luteus (Hulburt 1965), and Entomosigma akashiwo (Hada 1967). [3] H. akashiwo and H. inlandica have been recognized as two species of Heterosigma. However, Hara and Chihara (1987) described both specimens as one species, validly describing them as H. akashiwo. [4]

Description

H. akashiwo cells are relatively small, ranging in size from 18 to 34 μm in diameter. [5] They appear golden brown, and appear in clusters. Morphology is highly variable, but does not appear to vary significantly between locations. One culture may contain flat or round individual cells. [5] Molecular techniques for identification (including quantitative PCR) are preferred over traditional microscope fixing, which may lyse the cells. [5]

Heterosigma akashiwo anatomy from Hara and Chihara 1987 Hara and Chihara Fig 21.jpg
Heterosigma akashiwo anatomy from Hara and Chihara 1987

Distribution

Heterosigma akashiwo has been identified off the coasts of the United States, Canada, Chile, the Netherlands, Scotland, Ireland, Sweden, Norway, Japan, S.Korea, Hong Kong, Australia, and New Zealand. [1] [6] [7] Most of the literature suggests H. akashiwo is associated with shallow water within 10 m of the surface, but this is not a universal rule. [7]

Physiology

Heterosigma akashiwo is a mixotrophic alga, supplementing nutrient uptake and photosynthesis with ingestion of bacteria. [8] Each cell may contain 18-27 chloroplasts. [5] These cells have been observed to glide and twirl under microscopic examination, but nonmotile cells have been associated with toxic blooms. [5] [7] Blooms are clearly visible by air, appearing as a red area in otherwise blue water. [7] Optimal growth occurs at 25 °C and 100 μE m−2s−1, conditions which are associated with very low toxicity. [9] Maximum toxicity occurs (and relatively slow growth) occurs at 20 °C and 200 μE m−2s−1. [9] H. akashiwo reproduces asexually by binary fission. [4]

Heterosigma akashiwo produces cysts as a resting stage. [2] [10] The germination of these cysts leads to large-scale blooms, which can be laterally transferred by tides and currents. [7] These blooms are neither caused nor exasperated by fish farming. [7] Bottom water temperature must reach at least 15 °C for germination to occur. Blooms are most often associated with summer months, and some areas may see two blooms within one year. [7] Blooms are known to be lethal once concentrations of cells reach 3x105 to 7 x 105 cells/L. [7] Viruses may act as a natural control on bloom populations, as H. akashiwo viruses (HaV) have been shown to only leave resistant alga alive. [11] Similarly, certain bacteria may also reduce H. akashiwo populations. [12]

The exact mode of bloom toxicity is currently unknown, but gill damage leading to hypoxia is the proposed cause for fish death. [5] [7] H. akashiwo may produce brevetoxins, but others suggest the concentrations of these toxins are too low to account for such a large effect on fish populations. [7] [13] Some have argued the production of reactive oxygen species like hydrogen peroxide may be responsible for gill damage. However, research suggests hydrogen peroxide concentrations are far too low to have significant effects on fish. [14] Mucus production is another proposed, but poorly supported, mechanism for fish mortality. [7] The effective toxin possibly is chemically unstable, and therefore difficult to detect. [7] Sablefish appear to be unaffected by H. akashiwo blooms, while many other marine fish are decimated. [7]

Genetics

Genetic sequences are highly conserved between Pacific and Atlantic populations. Relevant probe sequences for small subunit RNA can be found. [5]

Economic impact

Heterosigma forms massive golden tides that impact the survival of organisms at every trophic level. This alga has been shown to kill finfish, compromise fish and sea urchin egg development, and impact copepods, as well as oyster survival. [15] Further ecological impacts to plankton, invertebrates, and wild fish are likely, but unknown. [7] The 1997 H. akashiwo bloom in British Columbia, for example, coincided with a dramatic increase in mortality of captive salmon. [16] H. akashiwo contributed to the loss of over 1,000 tons of Atlantic salmon in 2001. [6] A bloom in Puget Sound in 2006 led to the loss of $2 million of farmed salmon. [7] In 2014, a bloom near Port Hardy, British Columbia, killed nearly 280,000 Atlantic salmon. [17] In 2018, a bloom near British Columbia killed near 250,000 Atlantic salmon at two seafood farms. [18] A 1995 article noted that the global distribution of H. akashiwo is increasing, as is the frequency of H. akashiwo HAB formation. [19]

Related Research Articles

Red tide A common name for a worldwide phenomenon known as an algal bloom

Red tide is a common name for algal blooms, which are large concentrations of aquatic microorganisms, such as protozoans and unicellular algae. The upwelling of nutrients from the sea floor, often following massive storms, provides for the algae and triggers bloom events. Harmful algal blooms can occur worldwide, and natural cycles can vary regionally.

Algal bloom Rapid increase or accumulation in the population of planktonic algae

An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems, and is often recognized by the discoloration in the water from their 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 rapid growth of microscopic, unicellular algae, not macroscopic algae. An example of a macroscopic algal bloom is a kelp forest.

Paralytic shellfish poisoning Syndrome of shellfish poisoning

Paralytic shellfish poisoning (PSP) is one of the four recognized syndromes of shellfish poisoning, which share some common features and are primarily associated with bivalve mollusks. These shellfish are filter feeders and accumulate neurotoxins, chiefly saxitoxin, produced by microscopic algae, such as dinoflagellates, diatoms, and cyanobacteria. Dinoflagellates of the genus Alexandrium are the most numerous and widespread saxitoxin producers and are responsible for PSP blooms in subarctic, temperate, and tropical locations. The majority of toxic blooms have been caused by the morphospecies Alexandrium catenella, Alexandrium tamarense, Gonyaulax catenella and Alexandrium fundyense, which together comprise the A. tamarense species complex. In Asia, PSP is mostly associated with the occurrence of the species Pyrodinium bahamense.

Phycodnaviridae is a family of large (100–560 kb) double-stranded DNA viruses that infect marine or freshwater eukaryotic algae. Viruses within this family have a similar morphology, with an icosahedral capsid. As of 2014, there were 33 species in this family, divided among 6 genera. This family belongs to a super-group of large viruses known as nucleocytoplasmic large DNA viruses. Evidence was published in 2014 suggesting that specific strains of Phycodnaviridae might infect humans rather than just algal species, as was previously believed. Most genera under this family enter the host cell by cell receptor endocytosis and replicate in the nucleus. Phycodnaviridae play important ecological roles by regulating the growth and productivity of their algal hosts. Algal species such Heterosigma akashiwo and the genus Chrysochromulina can form dense blooms which can be damaging to fisheries, resulting in losses in the aquaculture industry. Heterosigma akashiwo virus (HaV) has been suggested for use as a microbial agent to prevent the recurrence of toxic red tides produced by this algal species. Phycodnaviridae cause death and lysis of freshwater and marine algal species, liberating organic carbon, nitrogen and phosphorus into the water, providing nutrients for the microbial loop.

<i>Karenia brevis</i> species of alga

Karenia brevis is a microscopic, single-celled, photosynthetic organism that is part of the Karenia (dinoflagellate) genus, a marine dinoflagellate commonly found in the waters of the Gulf of Mexico. It is the organism responsible for the "Florida Red Tides", commonly referred to as red tides that affect the Gulf coasts of Florida and Texas in the U.S., and nearby coasts of Mexico. K. brevis has been known to travel great lengths around the Florida peninsula and as far north as the Carolinas.

<i>Marnaviridae</i> family of viruses

Marnaviridae is a family of positive-stranded RNA viruses in the order Picornavirales. The first marnavirus that was isolated, and which is the type species for the family, infects a Microphyte: the toxic bloom-forming Raphidophyte, Heterosigma akashiwo. Algae therefore seem to serve as natural hosts. There is only one genus (Marnavirus) and one species in this family, the type species Heterosigma akashiwo RNA virus (HaRNAV).

Predatory dinoflagellate

Predatory dinoflagellates are predatory heterotrophic or mixotrophic alveolates that derive some or most of their nutrients from digesting other organisms. About one half of dinoflagellates lack photosynthetic pigments and specialize in consuming other eukaryotic cells, and even photosynthetic forms are often predatory.

Alexandrium fundyense is a species of dinoflagellates. It produces toxins that induce paralytic shellfish poisoning (PSP), and is a common cause of red tide. A. fundyense regularly forms massive blooms along the northeastern coasts of the United States and Canada, resulting in enormous economic losses and public health concerns.

Harmful algal bloom Population explosion of organisms that can severely lower oxygen levels in natural waters, killing marine life

A harmful algal bloom (HAB) contains organisms that can severely lower oxygen levels in natural waters, killing marine life. Some HABs are associated with algae-produced toxins. Blooms can last from a few days to many months. After the bloom dies, the microbes which decompose the dead algae use up even more of the oxygen, which can create fish die-offs. When these zones of depleted oxygen cover a large area for an extended period of time, they are referred to as dead zones, where neither fish nor plants are able to survive.

<i>Karenia</i> (dinoflagellate) genus of algae

Karenia is a genus that consists of unicellular, photosynthetic, planktonic organisms found in marine environments. The genus currently consists of 12 described species. They are best known for their dense toxic algal blooms and red tides that cause considerable ecological and economical damage; some Karenia species cause severe animal mortality. One species, Karenia brevis, is known to cause respiratory distress and neurotoxic shellfish poisoning (NSP) in humans.

<i>Gonyaulax</i> genus of protists (fossil)

Gonyaulax is a genus of dinoflagellates with the type species Gonyaulax spinifera Diesing. Gonyaulax belongs to red dinoflagellates and commonly causes red tides.

Akashiwo sanguinea is a species of marine dinoflagellates well known for forming blooms that result in red tides. The organism is unarmored (naked). Therefore, it lacks a thick cellulose wall, the theca, common in other genera of dinoflagellates. Reproduction of the phytoplankton species is primarily asexual.

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.

Chattonella is a genus of marine raphidophytes associated with red tides. A technique using monoclonal antibodies can be used to identify the genus, while the RAPD reaction can be used to distinguish between different species within the genus. It includes the species Chattonella antiqua, a bloom forming alga responsible for large scale fish deaths due to the synthesis of toxic compounds related to brevetoxin.

All living cells produce reactive oxygen species (ROS) as a byproduct of metabolism. ROS are reduced oxygen intermediates that include the superoxide radical (O2) and the hydroxyl radical (OH•), as well as the non-radical species hydrogen peroxide (H2O2). These ROS are important in the normal functioning of cells, playing a role in signal transduction and the expression of transcription factors. However, when present in excess, ROS can cause damage to proteins, lipids and DNA by reacting with these biomolecules to modify or destroy their intended function. As an example, the occurrence of ROS have been linked to the aging process in humans, as well as several other diseases including Alzheimer's, rheumatoid arthritis, Parkinson's, and some cancers. Their potential for damage also makes reactive oxygen species useful in direct protection from invading pathogens, as a defense response to physical injury, and as a mechanism for stopping the spread of bacteria and viruses by inducing programmed cell death.

<i>Gonyostomum semen</i>

Gonyostomum semen is a species of freshwater algae in the genus Gonyostomum, with worldwide distribution. They cause nuisance algal blooms and are known to cause allergic reactions to people swimming in lakes.

<i>Cochlodinium polykrikoides</i> species of protist

Cochlodinium polykrikoides is a species of red tide producing marine dinoflagellates known for causing fish kills around the world, and well known for fish kills in marine waters of Southeast Asia.

Prymnesin-2 is an organic compound that is secreted by the haptophyte Prymnesium parvum. It belongs to the prymnesin family and has potent hemolytic and ichthyotoxic properties. In a purified form it appears as a pale yellow solid. P. parvum is responsible for red harmful algal blooms worldwide, causing massive fish killings. When these algal blooms occur, this compound poses a threat to the local fishing industry. This is especially true for brackish water, as the compound can reach critical concentrations more easily.

Aureoumbra lagunensis is a unicellular planktonic marine microalga that belongs in the genus Aureoumbra under the class Pelagophyceae. It is similar in morphology and pigments to Aureococcus anophagefferens and Pelagococcus subviridis. The cell shape is spherical to subspherical and is 2.5 to 5.0 μm in diameter. It is golden-coloured and is encapsulated with extracellular polysaccharide layers and has a single chloroplast structure with pigments.

References

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