Heterosigma akashiwo
Encyclopedia
Heterosigma akashiwo is a microscopic alga of the class Raphidophyceae. Heterosigma akashiwo is a swimming marine alga that episodically forms toxic surface aggregations known as harmful algal bloom (HAB).
the cells.
with ingestion of bacteria. Each cell may contain 18-27 chloroplasts. These cells have been observed to glide and twirl under microscopic examination, but non-motile cells have been associated with toxic blooms. Blooms are clearly visible by air, appearing as a red area in otherwise blue water. Optimal growth occurs at 25°C and 100 μE m-2s-1, conditions which are associated with very low toxicity. Maximum toxicity occurs (and relatively slow growth) occurs at 20°C and 200 μE m-2s-1. H. akashiwo reproduces asexually by binary fission.
Heterosigma akashiwo produces cysts as a resting stages. The germination
of these cysts leads to large-scale blooms, which can be laterally transferred by tides and currents. These blooms do not seem to be caused by fish farming. Bottom water temperature must reach at least 15°C for germination to occur. Blooms are associated with summer months, and some areas may see two blooms within one year. Blooms are known to be lethal once concentrations of cells reach 3x105 to 7 x 105 cells/L. Viruses may act as a natural control on bloom populations, as H. akashiwo viruses (HaV) have been shown to only leave resistant alga alive. Similarly, certain bacteria may also reduce H. akashiwo populations.
The exact mode of bloom toxicity is currently unknown, but gill damage leading to hypoxia
is the proposed cause for fish death. H. akashiwo may produce brevetoxins
, but others suggest that concentrations of these toxins are too low to account for such a large effect on fish populations. Some have argued that the production of reactive oxygen species
like hydrogen peroxide
may be responsible for gill damage. However, research suggests that hydrogen peroxide concentrations are far too low to have significant effects on fish. Mucus production is another proposed, but poorly supported, mechanism for fish mortality. It is possible that the effective toxin is chemically unstable, and therefore difficult to detect. Sablefish
appear to be unaffected by H. akashiwo blooms, while many other marine fish are decimated.
can be found in Bowers et al. 2006.
egg development and impact copepod
as well as oyster
survival. Further ecological impacts to plankton
, invertebrates, and wild fish are likely, but unknown. The 1997 H. akashiwo bloom in British Columbia
, for example, coincided with a dramatic increase in mortality of captive salmon
. H. akashiwo contributed to the loss of over 1,000 tons of Atlantic salmon
in 2001. A bloom in Puget Sound
in 2006 led to the loss of $2 million of farmed salmon. Moreover, the global distribution of H. akashiwo is increasing as is the frequency of H. akashiwo HAB formation.
Context and Content
Synonymns include Olisthodiscus luteus (Hulburt 1965), and Entomosigma akashiwo (Hada 1967). 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.Description
H. akashiwo cells are relatively small, ranging in size from 18 to 34 um in diameter. 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 individual cells that are flat or round. . Molecular techniques for identification (including real-time PCR) are preferred over traditional microscope fixing, which may lyseLyse
Lyse may refer to:* Lyse Abbey, a former Cistercian abbey in Pakistan* Lyse, an alternative name of Lysebotn, Norway* Lyse Energi, a Norwegian power company* Łyse, Masovian Voivodeship, a village in east-central Poland...
the cells.
Distribution
H. akashiwo has been identified off the coasts of The United States, Canada, Chile, the Netherlands, Scotland, Ireland, Sweden, Norway, Japan, Australia, and New Zealand. Most of the literature suggests that H. akashiwo is associated with shallow water within 10 m of the surface, but this is not a universal rule.Physiology
H. akashiwo is a mixotrophic alga, supplementing nutrient uptake and photosynthesisPhotosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
with ingestion of bacteria. Each cell may contain 18-27 chloroplasts. These cells have been observed to glide and twirl under microscopic examination, but non-motile cells have been associated with toxic blooms. Blooms are clearly visible by air, appearing as a red area in otherwise blue water. Optimal growth occurs at 25°C and 100 μE m-2s-1, conditions which are associated with very low toxicity. Maximum toxicity occurs (and relatively slow growth) occurs at 20°C and 200 μE m-2s-1. H. akashiwo reproduces asexually by binary fission.
Heterosigma akashiwo produces cysts as a resting stages. The germination
Germination
Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
of these cysts leads to large-scale blooms, which can be laterally transferred by tides and currents. These blooms do not seem to be caused by fish farming. Bottom water temperature must reach at least 15°C for germination to occur. Blooms are associated with summer months, and some areas may see two blooms within one year. Blooms are known to be lethal once concentrations of cells reach 3x105 to 7 x 105 cells/L. Viruses may act as a natural control on bloom populations, as H. akashiwo viruses (HaV) have been shown to only leave resistant alga alive. Similarly, certain bacteria may also reduce H. akashiwo populations.
The exact mode of bloom toxicity is currently unknown, but gill damage leading to hypoxia
Hypoxia
Hypoxia may refer to:* Hypoxia , reduced dissolved oxygen content of a body of water detrimental to aerobic organisms* Hypoxia , a pathological condition in which the body as a whole or region of the body is deprived of adequate oxygen supply-Popular culture:* "Hypoxia" , a 2008 ambient song*...
is the proposed cause for fish death. H. akashiwo may produce brevetoxins
Brevetoxin
Brevetoxin , or brevetoxins, are a suite of cyclic polyether compounds produced naturally by a species of dinoflagellate known as Karenia brevis...
, but others suggest that concentrations of these toxins are too low to account for such a large effect on fish populations. Some have argued that the production of reactive oxygen species
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
like hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
may be responsible for gill damage. However, research suggests that hydrogen peroxide concentrations are far too low to have significant effects on fish. Mucus production is another proposed, but poorly supported, mechanism for fish mortality. It is possible that the effective toxin is chemically unstable, and therefore difficult to detect. Sablefish
Sablefish
The sablefish, Anoplopoma fimbria, is one of two members of the fish family Anoplopomatidae and the only species in the Anoplopoma genus...
appear to be unaffected by H. akashiwo blooms, while many other marine fish are decimated.
Genetics
Genetic sequences are highly conserved between Pacific and Atlantic populations. Relevant probe sequences for small subunit RNARNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
can be found in Bowers et al. 2006.
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 urchinSea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
egg development and impact copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
as well as oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....
survival. Further ecological impacts to plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, invertebrates, and wild fish are likely, but unknown. The 1997 H. akashiwo bloom in British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, for example, coincided with a dramatic increase in mortality of captive salmon
Salmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...
. H. akashiwo contributed to the loss of over 1,000 tons of Atlantic salmon
Atlantic salmon
The Atlantic salmon is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the north Atlantic and the north Pacific....
in 2001. A bloom in Puget Sound
Puget Sound
Puget Sound is a sound in the U.S. state of Washington. It is a complex estuarine system of interconnected marine waterways and basins, with one major and one minor connection to the Strait of Juan de Fuca and the Pacific Ocean — Admiralty Inlet being the major connection and...
in 2006 led to the loss of $2 million of farmed salmon. Moreover, the global distribution of H. akashiwo is increasing as is the frequency of H. akashiwo HAB formation.