Dinoflagellates are single-celled eukaryotes that belong to the kingdom Protista. They are considered to be among the most primitive eukaryotes. These organisms are mostly marine and photosynthetic in nature. They have characteristics of both plants and animals. Dinoflagellates are also responsible for phenomena such as red tides and bioluminescence on the ocean surface.
Dinoflagellates
Dino is a Greek word for ‘whirling ‘ and flagellate is a Latin word for ‘whip’ For example, to save the tiger, we save the whole forest. it has a tail-like structure and the motion it produces when it swims. Dinoflagellates have two whip-like tails called flagella used for movement. One flagellum lies longitudinally, and the other transversely in a furrow between the wall plates. The cell wall consists of stiff cellulose on the outer surface. They are mostly marine but can also be found in freshwater all around the world. Their population varies with latitude, temperature, salinity, and depth. Depending on the main pigment present in the cell, they appear red, yellow, green, blue, and brown. Most species of dinoflagellates are phototrophic, and the other few are mixotrophic (combining photosynthesis and phagocytosis).
Dinoflagellates Classification
Dinoflagellates belong to the domain Eukaryota, classified under Kingdom Protista and phylum Dinoflagellata. Dinoflagellates are kept in the supergroup Chromalveolata and group Alveolata. They are characterized by ribosomal DNA sequences and flattened vesicles inside plasma membranes called alveoli.
- Domain: Eukaryota
- Kingdom: Protista
- Supergroup: Chromalveolata
- Group: Alveolata
- Phylum: Dinoflagellata
Characteristics of Dinoflagellates
- Dinoflagellates are unicellular, eukaryotic organisms.
- Most dinoflagellates are marine, but they are also found in freshwater.
- Their distribution depends on the pH level, temperature, salinity, and depth of the aquatic ecosystem.
- They show three modes of nutrition – phototrophic, heterotrophic, and mixotrophic.
- The flagella are responsible for locomotion and facilitate a spinning top-like motion.
- They possess an organelle called an eyespot that is light-sensitive. It provides them with a sense of direction.
- Dinoflagellates store food in the form of starch.
Structure of Dinoflagellates
- The size of dinoflagellates ranges between 15 to 40 microns.
- They have a complex outer covering called amphiesma.
- Flattened vesicles are present inside the plasma membrane called alveoli, which contain cellulose plates, which are permeated with silicates.
- The cytoplasm of dinoflagellates contains typical eukaryotic organelles, including; rough and smooth endoplasmic reticulum, Golgi apparatus, mitochondria, lipid and starch grains, food vacuoles, etc.
- Dinoflagellates have two grooves- the Sulcus groove, which is longitudinal, and the cingulum groove, which is transverse. These grooves possess two flagella that help in movement. One of the flagella wraps around the transverse groove like a belt, and the other flagellum projects behind the cell and is present in the longitudinal groove that is perpendicular to the transverse groove.
- The nucleus of the dinoflagellates is called dinokaryon, which has chromosomes attached to the nuclear membrane.
- They lack histones and have a fibrillar appearance.
- Floatation and osmoregulation are further facilitated by Pusule, a non-contractile vacuole present near the flagellar base.
Dinoflagellates Reproduction
- Reproduction in dinoflagellates is both asexual and sexual.
- Primarily they reproduce asexually through binary fusion. The cells are haploid in asexual reproduction.
- In sexual reproduction, a zygote is formed through fusion, which undergoes meiosis to form a haploid cell. The zygote may undergo a resting stage known as dinocyst or may remain motile.
- Under unfavorable conditions, vegetative cells of dinoflagellates fuse to form Planozygote. It consumes excess fat and oil and increases in size. They form a hard outer shell known as Hypnozygote (similar to hibernation).
- Under favorable conditions, the shell of the Hypnozygote breaks, and the dinoflagellates enter a temporary stage, Planomeiocyte. They regain their actual size and shape to dinoflagellates.
Dinoflagellates Nutrition
Dinoflagellates show three modes of nutrition – phototrophic, heterotrophic, and mixotrophic.
- Most dinoflagellates are phototrophs. They are important producers in marine ecosystems.
- While there are some heterotrophic dinoflagellates, they ingest other microorganisms and protozoa to get nutrition.
- Some dinoflagellates show endosymbiont relation with other marine invertebrates like coral, jellyfish, etc. They are called zooxanthellae and provide nutrients in the form of carbohydrates to their host.
- Some dinoflagellates are both heterotrophic and phototrophic, they are known as mixotrophic.
Bioluminescence in Dinoflagellates
Bioluminescence can be defined as the production of visible light by living organisms. Around 18 species of dinoflagellates are bioluminescent, and most of them emit blue-green light. These species contain scintillons, individual cytoplasmic bodies present in the cell vacuole. They contain dinoflagellate luciferase, the enzyme involved in dinoflagellate bioluminescence, and luciferin, a chlorophyll-derived tetrapyrrole ring that acts as the substrate to the light-producing reaction. The reaction is sensitive to the pH. When the pH drops, luciferase changes its shape, allowing luciferin to bind. When mechanically stimulated—by boat, swimming, or waves, a blue flash of light occur as luminescence on the ocean surface at night. Dinoflagellates use bioluminescence as a defense mechanism. They scare their predators by flashing lights. Examples of bioluminescence-producing species are Gonyaulax, Alexandrium, and Ceratium.
Dinoflagellates – Red Tide
Dinoflagellates undergo rapid multiplication as a result of the abundant nutrients in the water. The rapid multiplication results in population explosion or algae bloom. Bloom may contain more than a million cells per ml of water. Bloom can be due to natural reasons or due to human activity, like the addition of phosphate in the water. Although red tide gives an interesting visual experience, they contain toxins that are harmful to marine life and to individuals who consume them. For example, shellfish is a specific carrier. This phenomenon is also known as “Harmful algal bloom (HAB)”. Example: Gonyaulax: It secretes a poisonous toxin known as ‘saxitoxin’, which causes paralysis in humans. Karenia brevis produces a neurotoxin called ‘brevetoxin’.
FAQs of Dinoflagellates
Q1: What are Dinoflagellates?
Answer:
Dinoflagellates are single-celled eukaryotes that belong to the kingdom Protista. They are mostly marine and photosynthetic.
Q2: What are the benefits of Dinoflagellates?
Answer:
Dinoflagellates are an important group of phytoplankton that produce oxygen in marine and freshwater. Some species form symbiotic relationships with larger animals, including corals (zooxanthellae), jellyfish, sea anemones, nudibranchs and others.
Q3: Are dinoflagellates harmless?
Answer:
Dinoflagellates normally have a low toxin production rate. Therefore, in small concentrations, their toxins are not potent. However, their toxins are highly poisonous in large concentrations.
Q4: What are dinoflagellates also called?
Answer:
Dinoflagellates are also called ‘fire algae’, as they show bioluminescence.