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What are dinoflagellates?

The dinoflagellates are a group of eukaryotic protists flagellates very numerous and of great environmental impact. They constitute the Dinoflagellate Phylum , which has approximately 2400 species.

Many dinoflagellates are photosynthetic and are part of aquatic plankton , mainly marine, and have been studied habitually as a type of microscopic algae ( Dinophyta , Pyrrophyta ). But not all dinoflagellates are autotrophs, there are also parasites, symbionts and mixotrophs (combine photosynthesis with phagocytosis), and their study as protists is more accurate than as algae.

Dinoflagellate populations vary according to temperature, salinity, depth, latitude and other habitat characteristics. In terms of number of species, dinoflagellates are one of the most extensive groups of marine eukaryotes, although it is considerably smaller than diatoms .

The first dinoflagellate was described in 1753 by Henry Baker. The name derives from the Greek ????? (dinos), meaning “to turn” or “spin”, and the Latin flagellum, or “flagellum”, the name given to mobile cell appendages, which refers to one of the distinctive characteristics of dinoflagellates: presence of flagella (they have two) and a rotary movement .

Some dinoflagellates are bioluminescent  and emit blue or green light, depending on the species, and their light is visible if they come in large numbers in the current and marine tide.

Many dinoflagellates can enter highly resistant resting states known as dinocysts or dinocysts . The ability of many species to move from strategies with cyst formation to strategies without cysts makes the study of the history and evolution of these organisms very complicated.

Examples of dinoflagellates include Alexandrium , Gonyaulax , Gymnodinium or Lingulodinium polyedrum .

Morphology and general characteristics

Dinoflagellates are unicellular organisms with two flagella . The two flagella leave the cell through the ventral part, but one of them, the transverse flagellum , is disposed surrounding the cell, while the other, the longitudinal flagellum , protrudes from the cell like a typical flagellum.

Each species has a characteristic shape given by its cell wall or cover, called an amphysic or cortex . The amphosm of the dinoflagellates is a complex structure formed by flattened vesicles called cortical alveoli or ephemeral alveoli.

From a morphological point of view, two types of dinoflagellates are distinguished: those that have teak ( dinoflagellate tecados ) and dinoflagellates without teak or naked ( atheninated dinoflagellates ).

Teak is a kind of armor formed by cellulose plates whose disposition and shape is characteristic of each species and, sometimes, also of the state of the dinoflagellate within its life cycle.

In general, the teak has a transverse groove, called the cingulum , where the transverse flagellum lodges, and a longitudinal groove, called the sulcus , where the longitudinal flagellum lodges. Both flagella exit through the same hole located in the ventral part.

Among the internal structures, photosynthetic plastids of photosynthetic dinoflagellates stand out. These chloroplasts have three membranes, suggesting that they were acquired by endosymbiosis with unicellular algae .

As photosynthetic pigments, dinoflagellates can present chlorophylls A and C2, beta-carotenes and certain xanthophylls exclusive of dinoflagellates: peridinin, dinoxanthin and diadinoxanthin.

 

Another cellular organelle that stands out in dinoflagellates is the nucleus, whose organization is called dinocarion . The chromosomes appear anchored to the nuclear membrane and lack histones, instead they present other nucleoproteins that seem to have a viral origin.

 

Reproduction and life cycle

Dinoflagellates can be reproduced asexually as well as sexually . Typical dinoflagellates have a haploid nucleus (dinocarion) and reproduce mainly asexually. Sexual reproduction occurs by fusion of two haploid individuals to form a diploid zygote that is subsequently divided again by meiosis to form haploid individuals again.

Asexual reproduction, if conditions are favorable, can be very rapid and cause population explosions of great impact. They can concentrate more than 60 million individuals per liter of water and give color to the tides .

The zygote formed by sexual reproduction can remain a flagellated and mobile cell, or it can form a temporal cyst without flagella and without mobility ( dinocyst ). In addition, dinoflagellates can form other cysts adapted to survive unfavorable conditions.

When conditions become critical for survival, for example due to lack of food, two dinoflagellates can fuse to form a special zygote called plano zigoto , which remains mobile, until it loses the flagella and forms the hypnozigote , a form of resistance with a tougher teak that is deposited in the benthos and aquatic funds.

If the conditions are again adequate, the dinoflagellate is activated, breaks the teak and goes through a temporary phase ( plano meiocyte ) to then return to the form of haploid dinoflagellate and start again the life cycle.

 

Ecology and physiological features of dinoflagellates

Dinoflagellates are present in any aquatic environment, both marine and brackish waters and in fresh water, including ice and snow.

Among the metabolites typical of dinoflagellates, sterol-type dinosterol stands out , in comparison with animal cholesterol, vegetable phytosterol or fungal ergosterol.

Dinoflagellates present different feeding strategies . Although most are phototrophs, there are also mixotrophs and heterotrophs , and among heterotrophs there are free living and there are also ecto and endoparasites , sometimes as symbiotic life forms .

Among the endosymbiont dinoflagellates, the best known are zooxanthellae , a group of dinoflagellate parasites of other protists and marine invertebrates, such as corals, sponges or jellyfish.

Mixotrophic dinoflagellates are organisms with photosynthetic capacity but also ingest nutrients from outside simultaneously. The facultative or amphitropic mixotrophic dinoflagellates are those that can only perform heterotrophic nutrition or only autotrophic nutrition according to environmental conditions.

There are dinoflagellates without chloroplasts but they carry in their interior a photosynthetic endosymbiont that they have for them. Some dinoflagellates without their own chloroplasts can use “alien”chloroplasts, chloroplasts that they ingest from other organisms that they have ingested ( kleptoplasty ).

Among the heterotrophic dinoflagellates there are multiple capture and ingestion strategies. Some direct the prey towards the sulcal region with the help of the scourge or pseudopods and ingest it through the sulcus. For example, Ceratium hirundinella , Peridinium globulus or Oxyrrhis marina .

Other heterotrophic dinoflagellates, such as Protoperidinium species , have a large pseudopod called pallium that captures the prey and digests it extracellularly. There are also some that emit extensible peduncles to capture the prey.

 

However, the mechanisms of capture of the majority of marine dinoflagellates remain unknown.

Explosion of dinoflagellate populations

Dinoflagellate populations, as with other aquatic plankton organisms, may experience very rapid growth under certain circumstances. In these explosions the diversity of species is usually very low but can exceed one million individuals in only one milliliter of water .

Some species of dinoflagellates produce toxins ( ichthyotoxin, saxitoxins ) and during these population explosions the amount of toxins can be enough to kill fish and other marine life.

These toxins can also accumulate in other organisms, for example in molluscs , and then be consumed by humans and cause poisoning, although they tend to have mild effects. The majority of species producing these toxins are reddish or brown and produce tides of these colors.

However, there are also colorless dinoflagellates that produce toxins, and there are also harmless red tides, but these are usually produced by algae and not by dinoflagellates.

Also characteristic are bioluminescent tides caused by dinoflagellate species that emit flashes of bluish light when agitated.

 

Classification and evolution

The taxonomy of dinoflagellates has been very difficult to study. Evolutionary data are obtained mainly through fossil dinocysts and marked geochemicals.

The oldest fossil dinocysts date from the Middle Triassic, but the geochemical markers suggest that they were already present in the early Cambrian, and the proto dinoflagellates in the Paleozoic and Precambrian.

The molecular phylogenetic of the dinoflagellates groups them in the Dinoflagellate phylum , which together with the ciliophora ( Ciliophora ) and the apicomplexans ( Apicomplexa ) form the alveolate protists ( Alveolata ).

All dinoflagellates have remnant plastids of red algae or other cellular organelles originating in red algae.

Dinoflagellates have been classified by the Code of Nomenclature for Algae, Fungi, and Plants (ICN) , and by the International Code of Zoological Nomenclature (ICZN) . Approximately half of the existing dinoflagellates are photosynthetic and the other half are heterotrophs.

The most representative classes of dinoflagellates are:

  • Ello Biopsea : marine and freshwater dinoflagellates mainly ectoparasites of crustaceans.
  • Oxyrrhea : only includes the genus Oxyrrhis . They are dinoflagellates fagotrofos that exceptionally among dinoflagellates do not present cingulate or sulcus.
  • Syndiniophyceae : exclusively endoparasitic dinoflagellates of marine animals and intracellular parasites of protozoa. They do not have teak and its nucleus does not present the characteristics of a dinocarion.
  • Dinophyceae : called dinophyceae, corresponds to the typical photosynthetic dinoflagellates. Some dinophyceae form colonies and others are ectoparasites that can affect protozoa, algae, invertebrates and fish.
  • Noctis Lucis Phyceae : contains the largest dinoflagellates, some can reach 2 mm. Some are heterotrophic and feed on plankton and others include green algae inside them as photosynthetic symbionts.

 

References

  1. Strassert JF et al. (January 20018). Single cell genomics of uncultured marine alveolates shows paraphyly of basal dinoflagellates. The ISME Journal  12 (1): 304-308. doi: https://doi.org/10.1038/ismej.2017.167
  2. A proposed mechanism for bioluminescence in dinoflagellates . National Science Foundation.

 

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