Dinoflagellates

Introduction:

Dinoflagellates are a monophyletic group of single-celled eukaryotes.
They are motile and occur in many colour
They move with the help of two flagella, hence are called dinoflagellates.
They belong to the class Dinophyceae of division Pyrrophycophyta from the phylum Dinoflagellata.
While most of the dinoflagellates are photosynthetic, there are some heterotrophic species as well.
Example- Glenodinium, Peridinium, Gonyaulux

Habitat :

Mostly marine and photosynthetic.
Some of them could be found in freshwater also.
They inhabit different regions of the water bodies according to their habits.
Some species are called Zooxanthellae/golden dinoflagellates that are endosymbionts with other organisms and play an important role in the biology of corals.

Nutrition:

Dinoflagellates have a mixotrophic mode of nutrition.
About half of the species are phototrophic.
Many of the remaining are predatory (heterotrophs).
Some of the species exhibit parasitism as well as commensalism.

Pigments:

Most of the species of dinoflagellates contain chlorophyll c, the carotenoid - beta carotene, and xanthophylls that are unique to this group.
Different dinoflagellates possess different colours depending on the pigments present in their cells ranging from golden brown, yellow, green, blue, and red.
When some of these dinoflagellates multiply at an astonishing rate it makes the sea appear red. It is called Red tide.

Stored food:

As most of the dinoflagellates are photosynthetic, they store food in the form of starch, starch-like compounds, or oils.
They store food with the help of pigmented plastids, which are yellow or brown in colour.

Structure:

Dinoflagellates possess characteristic two flagellated single-celled bodies.
The cell wall has stiff cellulose plates on the outer surface.
The nucleus is large and contains condensed chromosomes even at the interphase stage of the cell cycle.
A noncontractile vacuole called Pusule enables cells to float and osmoregulate.
Many species consist of trichosysts (A rod-like ejected body located in the cell covering and generally present in the motile phase of dinoflagellates).
Some species have cnidoblast that enables them to protect themselves from predators.
A carotenoid pigment called Eyespot/Stigma is also found on paraflagellar bodies which filters light. Some of them resemble oceli.

Flagella:

Dinoflagellates have two types of flagella-
1. Transverse flagellum:
The transverse flagellum is responsible for the propelling action of the cell body and is also responsible for the characteristic whirling motion.
In some species, the flagellum is embedded in grooves called a sulcus.
2. Longitudinal flagellum:
It acts as a steering wheel inside the water and also provides some propulsive force to the main body.

Reproduction:

Dinoflagellates show characteristic asexual reproduction by binary fission.
They also show sexual reproduction by fusion forming a motile zygote that undergoes meiosis to form haploid cells.
In unfavorable conditions, the vegetative cells fuse together to form planozygote which then hardens its shell by taking in excess fat and oil to reach the hypnozygote stage.
Under favorable conditions, they break out the shell and reorganize to their original shape as dinoflagellates.

Economic importance:

They are the major source of marine producers second only to diatoms and an important part of the marine food web.
Most dinoflagellates produce the most potent toxins known and are the cause of toxic red tides. It also causes fish and shellfish poisoning.
Species that cause toxic red tides are:
1. Karenia brevis - Produces a neurotoxin named Bravetoxin
2. Alexendrium fundyense - Produces a toxin named Saxitoxin
3. Pfiesteria - Colourless dinoflagellates associated with harmful algal blooms and fish kills.
Dinoflagellates also show bioluminescence of various colours.
They have pigment luciferin, which is acted upon by the enzyme luciferase causing production of light without any heat, thus the name fire algae.
The enzymatic reaction of pigment produces light; pigment itself does not impart the fire-like appearance hence are also known as fire algae.

FAQ

Q-1 Are dinoflagellates eukaryotic or prokaryotic?

- Dinoflagellates are unicellular eukaryotic organisms found mostly in aquatic (marine and freshwater) habitats.

Q-2 Name the dinoflagellates that make the water look red and cause the so-called ‘Red tide’.

- Gonyaulux, Karenia, Gymnodinium are some of the main genera that turn seawater red. Waves of seawater look orange, red when they reach the coast. Hence, the name ‘Red tide’.

Q-3 Why dinoflagellates are called fire algae?

- Dinoflagellates are bioluminescent algae. They contain luciferin pigment that produces light in presence of the luciferase enzyme. No heat is produced but the overall appearance is similar to fire when multiple individuals are together. Hence the name ‘Fire algae’.

Q-4 How much time is required by the dinoflagellates to complete its lifecycle?

- Based on the exposure to sunshine, surrounding temperature, and nutrient requirement dinoflagellates tend to complete their life cycle in 1 to 3 months.

Q-5 What do dinoflagellates require to survive?

- Dinoflagellates require sunlight or artificial light. Range of temperature that is not extremely cold or hot.

Q-6 How many flagella do dinoflagellates have?

- Dinoflagellates have two flagella. One is called a transverse flagellum and the other is called a longitudinal flagellum.