Heterosigma Akashiwo
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''Heterosigma akashiwo'' is a species of microscopic
alga Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, suc ...
e of the class
Raphidophyceae The raphidophytes, formally known as Raphidophycidae or Raphidophyceae (formerly referred to as Chloromonadophyceae and Chloromonadineae), are a small group of eukaryotic algae that includes both marine and freshwater species. All raphidophyte ...
. It is a swimming marine alga that episodically forms toxic surface aggregations known as
harmful algal bloom A harmful algal bloom (HAB), or excessive algae growth, sometimes called a red tide in marine environments, is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, water deoxygenation, ...
. The species name ''akashiwo'' is from the Japanese for "
red tide A harmful algal bloom (HAB), or excessive algae growth, sometimes called a red tide in marine environments, is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, water deoxygenation, ...
". Synonyms 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 μm 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 flat or round individual cells. Molecular techniques for identification (including
quantitative PCR A real-time polymerase chain reaction (real-time PCR, or qPCR when used quantitatively) is a laboratory technique of molecular biology based on the polymerase chain reaction (PCR). It monitors the amplification of a targeted DNA molecule duri ...
) are preferred over traditional microscope fixing, which may
lyse Lyse may refer to: People * Lyse Doucet (born 1939), Canadian journalist, presenter and correspondent for BBC World Service radio and BBC World television * Lyse Richer (born 1958), Canadian administrator and music teacher * Carl L. Lyse (1899– ...
the cells.


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, New Zealand and South Africa. 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.


Physiology

''Heterosigma akashiwo'' is a
mixotrophic A mixotroph is an organism that uses a mix of different sources of energy and carbon, instead of having a single trophic mode, on the continuum from complete autotrophy to complete heterotrophy. It is estimated that mixotrophs comprise more than ...
alga, supplementing nutrient uptake and
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
with ingestion of bacteria. Each cell may contain 18-27 chloroplasts. These cells have been observed to glide and twirl under microscopic examination, but nonmotile 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 Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two values (0 and 1) for each digit * Binary function, a function that takes two arguments * Binary operation, a mathematical o ...
. ''Heterosigma akashiwo'' produces
cysts A cyst is a closed Wikt:sac, sac, having a distinct Cell envelope, envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac ...
as a resting stage. The
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, ...
of these cysts leads to large-scale blooms, which can be laterally transferred by tides and currents. A 2007 review of these blooms in Puget Sound suggested that salmon farming was probably not a strong driver of their incidence or severity. 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. Blooms are known to be lethal once concentrations of cells reach 3x105 to 7 x 105 cells/L.
Viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almo ...
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
brevetoxin Brevetoxin (PbTx), or brevetoxins, are a suite of cyclic polyether compounds produced naturally by a species of dinoflagellate known as ''Karenia brevis''. Brevetoxins are neurotoxins that bind to voltage-gated sodium channels in nerve cells, lea ...
s, but others suggest the concentrations of these toxins are too low to account for such a large effect on fish populations. Some have argued the production of
reactive oxygen species In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...
like
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
may be responsible for gill damage. However, research suggests 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. The effective toxin possibly is chemically unstable, and therefore difficult to detect.
Sablefish The sablefish (''Anoplopoma fimbria'') is one of two members of the fish family Anoplopomatidae and the only species in the genus ''Anoplopoma''. In English, common names for it include sable (US), butterfish (US), black cod (US, UK, Canada), ...
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
RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
can be found.


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 Fishery can mean either the Big business, enterprise of Animal husbandry#Aquaculture, raising or Fishing, harvesting fish and other aquatic life or, more commonly, the site where such enterprise takes place (wikt:AKA, a.k.a., fishing grounds). ...
, compromise fish and
sea urchin Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...
egg development, and impact
copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...
s, as well as
oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but no ...
survival. Further ecological impacts to
plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
,
invertebrates Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum ...
, and wild fish are likely, but unknown. The 1997 ''H. akashiwo'' bloom in
British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...
, for example, coincided with a dramatic increase in mortality of captive
salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...
. ''H. akashiwo'' contributed to the loss of over 1,000 tons of
Atlantic salmon The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Hucho taimen, Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlan ...
in 2001. A bloom in
Puget Sound Puget Sound ( ; ) is a complex estuary, estuarine system of interconnected Marine habitat, marine waterways and basins located on the northwest coast of the U.S. state of Washington (state), Washington. As a part of the Salish Sea, the sound ...
in 2006 led to the loss of $2 million of farmed salmon. In 2014, a bloom near Port Hardy, British Columbia, killed nearly 280,000 Atlantic salmon. In 2018, a bloom near British Columbia killed near 250,000 Atlantic salmon at two seafood farms. In March 2022, a ''H. akashiwo'' bloom, triggered by sewage pollution, in the Swartkops River estuary in
Port Elizabeth Gqeberha ( , ), formerly named Port Elizabeth, and colloquially referred to as P.E., is a major seaport and the most populous city in the Eastern Cape province of South Africa. It is the seat of the Nelson Mandela Bay Metropolitan Municipal ...
, South Africa, killed thousands of fish and crustaceans, compromising the ecosystem value of the estuary as a "fish nursery" that benefits ecosystem diversity and the fishing industry. 1995 article noted that the global distribution of ''H. akashiwo'' is increasing, as is the frequency of ''H. akashiwo'' HAB formation.


References


External links


The harmful phytoplankton project

Heterosigma Research @ University of Washington

Heterosigma Research @ Western Washington University
{{Authority control Ochrophyta Ochrophyte species Protists described in 1987