Dinoflagellates

1.0What is a dinoflagellate?

Neither plant nor animal, dinoflagellates are unicellular protists. Functioning as a hybrid between plants and animals.

Dinoflagellates are single-celled aquatic organisms with two flagella, exhibit bioluminescence in oceans and can produce neurotoxic chemicals in large quantities. 

2.0History and Classification

• The first modern dinoflagellate was described by Baker in 1753, the first species was formally named by Muller in 1773. The first fossil forms were described by Ehrenberg in the 1830's from the flint of Cretaceous age.
• In 1993 Fensome and Taylor linked dinoflagellates to their cysts emphasizing the tabulation/paratabulation in their classification. Dinoflagellates are classified as Protists within the division Dinoflagellata, most of the members of this division are characterized by having, during at least one part of their life cycle, a motile stage with two dissimilar flagella. 
• Two subdivisions are recognised, of which the Dinokaryota possess a dinokaryon (the typical dinoflagellate nucleus) during at least part of their life cycle. 

3.0Characteristics of Dinoflagellates

• 90% of all dinoflagellates are marine plankton. Others are benthic, symbiotic, or parasitic. Although many dinoflagellates are microscopic and range from 15 to 40 microns in size, the largest, Noctiluca, may be as large as 2 mm in diameter! 
• Dinoflagellates swim by means of two flagella, movable protein and microtubule strands that propel the cell through the water. The longitudinal flagellum extends out from the sulcal groove of the hypotheca (posterior part of the cell); when it whips back and forth it propels the cell forward. The flattened transverse flagellum lies in the cingulum, the groove that extends around the equator of the cell. Its motion provides maneuvering and forward movement. As a result of the action of the two flagella the cell spirals as it moves.
• Many are thecate, having an internal skeleton of cellulose-like plates. The peripheral part of the cell has a series of membranes called the amphiesma. In “armored” species polysaccharide deposited between the membranes form rigid plates called thecae. “Naked” cells lack thecae.
• Chromosomes are always condensed. Dinoflagellate DNA always exists in a crystalline form in the nucleus, unlike other eukaryotes. In addition, lack proteins called histones that in other eukaryotic cells help organize the chromosomes. 
• Dinoflagellates contain a lot of DNA, which explains the large size of the nucleus. The metabolic requirements of supporting the large amount of DNA may explain the low growth rates of dinoflagellates compared to other unicellular protists.

4.0Nutrition in Dinoflagellates

Many dinoflagellates are photosynthetic, manufacturing their own food using the energy from sunlight, and providing a food source for other organisms. The photosynthetic dinoflagellates are important primary producers in coastal waters.

Some photosynthetic dinoflagellates are symbiotic, living in the cells of their hosts, such as corals. Called zooxanthellae, they are found in many marine invertebrates, including sponges, corals, jellyfish, and flatworms, as well as within protists, such as ciliates, foraminiferans, and colonial radiolarians.

Approximately half of all species are heterotrophic, eating other plankton, and sometimes each other, by snaring or stinging their prey. Non-photosynthetic species of dinoflagellates feed on diatoms or other protists (including other dinoflagellates); Noctiluca is large enough to eat zooplankton and fish eggs. Some species are parasites on algae, zooplankton, fish or other organisms.

Dinoflagellates employ a diverse set of nutritional strategies, encompassing photosynthesis and phagocytosis. As primary producers, many dinoflagellates conduct photosynthesis using chlorophyll-a and other pigments, capturing sunlight to convert carbon dioxide and water into essential carbohydrates. This process not only sustains their growth but also contributes significantly to oxygen production in aquatic environments. Complementing chlorophyll-a, dinoflagellates often harbour accessory pigments like peridinin, fucoxanthin, and chlorophyll-c, enhancing their light absorption efficiency across various aquatic conditions.

Some dinoflagellates showcase mixotrophy, adeptly toggling between photosynthesis and phagocytosis. This nutritional flexibility enables them to thrive in diverse ecological conditions, seamlessly transitioning between autotrophic (photosynthetic) and heterotrophic (feeding on other organisms) modes based on nutrient availability.

5.0Reproduction In Dinoflagellates

Dinoflagellates usually reproduce asexually. The most form of dinoflagellates reproduction is asexual, where daughter cells form by simple mitosis and division of the cell. The daughter cells will be genetically identical to that of the original cell. The thecal plates may either be divided, or completely shed and then reformed.

The life cycle of Dinoflagellates employs both asexual and sexual reproduction strategies, Here's an overview of their reproductive processes:

1. Asexual Reproduction: Dinoflagellates predominantly reproduce asexually through binary fission, wherein a single cell undergoes division to produce two identical daughter cells. This swift and effective process enables dinoflagellates to rapidly multiply in favorable conditions. The resulting cells from binary fission are haploid, possessing a single set of chromosomes each.

2. Sexual Reproduction:

Sexual reproduction in dinoflagellates involves the fusion of two cells to form a zygote. The zygote can take two different paths : 

It may develop into a resting stage known as a dinocyst, enabling the dinoflagellate to endure unfavorable conditions.

Alternatively, the zygote may remain motile, exhibiting typical dinoflagellate behavior.

• Planozygote and Hypnozygote Stages: Under unfavorable conditions, vegetative cells of dinoflagellates may fuse to form a Planozygote. The Planozygote undergoes changes, accumulating excess fat and oil, developing a harder shell, and increasing in size. This stage is known as the Hypnozygote, resembling a hibernating state. Occasionally, spikes may form.

• Transition to Planomeiocyte: In favorable conditions, dinoflagellates emerge from the Hypnozygote's shell. This transition phase is known as the Planomeiocyte, where dinoflagellates temporarily present a different structure before quickly reorganizing to their typical dinoflagellate shape.

6.0Bioluminescence by Dinoflagellate

Dinoflagellates exhibit bioluminescence, with over 18 genera known for this remarkable feature. These organisms possess scintillons, individual cytoplasmic bodies housing dinoflagellate luciferase, the key enzyme and luciferin, a tetrapyrrole ring derived from chlorophyll, acting as the substrate for light production. 

Bioluminescence is triggered by mechanical disturbance, resulting in a light-emitting reaction. The number of scintillons peaks at night, coinciding with maximal bioluminescence, and decreases by the end of the night. The luciferin-luciferase reaction is pH-sensitive; a drop in pH prompts a change in luciferase shape, allowing tetrapyrrole binding and initiating the bioluminescent response.

Red Tides by Dinoflagellates:

Red tides are conditions when a dinoflagellate population increases to such huge numbers that it discolors the water. This “bloom” may be caused by nutrient and hydrographic conditions, although the environmental conditions which result in red tides are not completely understood. For dinoflagellate red tides, the water is discolored red or brown due to as high as 20 million cells per liter. 

These red tides are composed primarily of one species of dinoflagellate that has been rapidly growing and accumulating.

Some red tides are luminescent; most in southern California create dramatic nighttime displays of bioluminescence in the wakes breaking on the beach.

Some but not all red tides are toxic. In toxic red tides, the dinoflagellates produce a chemical that acts as a neurotoxin in other animals. When the dinoflagellates are ingested by shellfish, for example, the chemicals accumulate in the shellfish tissue in high enough levels to cause serious neurological affects in birds, animals, or people which ingest the shellfish.

There are several types of neurotoxins produced by dinoflagellates. These chemicals may affect nerve action by interfering with the movement of ions across cell membranes, thus affecting muscle activity. 

The toxin saxitoxin, produced by Gonyaulax off the west coast of North America, and Alexandrium off the northeast coast, accumulates in shellfish. Eating contaminated shellfish causes paralytic shellfish poisoning (PSP). The worst cases of PSP result in respiratory failure and death within 12 hours. 

Another toxin that accumulates in shellfish is brevetoxin, produced by the dinoflagellate Karenia brevis. Brevetoxin is unique in that it becomes aerosolized when the dinoflagellates end up in the surf zone and then blow onto the beach causing respiratory irritation in humans. If you are on a beach on the Gulf Coast of Florida and notice asthma-like breathing symptoms, chances are you are experiencing toxicity from a Karenia bloom. 

A toxin produced by the dinoflagellate Dinophysis causes diarrhetic shellfish poisoning (DSP), which results in digestive upset but which is not fatal. 

Ciguatera is another form of dinoflagellate toxicity in tropical areas caused by eating fish contaminated by toxins of the dinoflagellate Gambierdiscus toxicus.

Red tides, triggered by specific environmental conditions conducive to the rapid growth of dinoflagellates, are a natural phenomenon observed in marine or coastal waters. Warm water temperatures, abundant sunlight, and nutrient-rich conditions, often resulting from agricultural runoff, contribute to the initiation of these events. 

Various dinoflagellate species, such as Karenia, Alexandrium, and Gymnodinium, can be responsible for red tide occurrences, with some producing harmful algal toxins (HABs) detrimental to marine life and potentially impacting human health through contaminated seafood. 

The characteristic reddish or brownish tint of water during red tides is attributed to pigments like chlorophyll-a and carotenoids present in dinoflagellates. The ecological impact of red tides is profound, leading to fish kills, shellfish closures, and disruptions in the marine food web. Additionally, the decomposition of vast quantities of dead algae after a red tide can deplete oxygen levels, creating "dead zones" in the affected waters.

7.0Examples of Dinoflagellates