Photosynthetic Protists
Category : 11th Class
General characters
(1) This is well defined group of unicellular, golden-brown photosynthetic organisms. Majority of them are motile and flagellated but a few are non-motile and non-flagellated. Flagellated forms exhibit peculiar spinning movement. Hence, they are called whorling whips.
(2) The cell wall of dinoflagellates, if present, is composed of a number of plates made up of cellulose. It is called theca or lorica. The theca contains two grooves-longitudinal sulcus and transverse girdle or annulus.
(3) Usually the cells possess two flagella which are of different types (heterokont).
(4) Trichocysts are rod like or spindle shaped ejective structures which occur just below the cell membrane.
(5) Cells possess a relatively large and prominent nucleus known as mesokaryon. DNA is without association with histone.
(6) There are numerous discoid chloroplasts without pyrenoids. They are yellow-brown to dark-brown in colour due to presence of characteristic pigments – Chlorophyll a, c, \[\alpha \]- carotene and xanthophylls (including dinoxanthin and peridinin).
(7) The reserve food material is starch in fresh water forms and oil in marine forms.
(8) The cells possess an osmoregulatory organelle called pusule which superficially looks like contractile vacuole.
The cells posses mitochondria, ribosomes and golgi bodies. They also possess mucilage bodies or vesicles below the cell membrane.
(9) In dinoflagellates it is mainly holophytic or photosynthetic. However, some forms are saprobic, parasitic, symbiotic or holozoic. For example, an colourless Blastodinium is parasite on animals.
(10) Dinoflagellates reproduce asexually through cell division or by the formation of zoospores and cysts. The cell division starts from posterior end. During cell division, centromeres and spindle are not seen. The spindle is replaced by cytoplasmic microtubules. During mitosis, the chromosomes break up into pairs of chromatids. The nuclear envelops and nucleolus persists during division.
(11) If sexual reproduction occurs, is isogamous or anisogamous. Two cells conjugate by a conjugation canal where the two amoeboid gametes fuse to form a diploid zygote. Life cycle involves zygotic meiosis (e.g., Ceratium, Gymnodinium etc.) or gametic meiosis (e.g., Noctiluca).
Examples : Glenodinium, Peridinium, Gymnodinium, Gonyaulax, Ceratium, Noctiluca.
General character
(1) Most of the diatoms occur as phytoplanktons both in fresh and marine waters. A few forms occur as benthos the bottom of water reservoirs. Diatoms constitute a major part of phytoplankton of the oceans.
(2) The cells of diatoms are called frustules or shell. They are microscopic, unicellular, photosynthetic organisms of various colours and diverse forms. They may be circular, rectangular, triangular, elongated, spindle-shaped, half-moon shaped, boat-shaped or filamentous. Incipient filament occur in Melosira.
(3) They exhibit mainly two types of symmetry-radial symmetry as in centrales (e.g., Cyclotella, Biddulphia, Triceratium, Melosira) and isobilateral symmetry as in Pennales (e.g., Pinnularia, Synedra, Actinella, Navicula).
(4) The cells of diatoms are called frustules. The cell wall is chiefly composed of cellulose impregnated with glass-like silica. It shows sculpturings and ornamentations. It is composed of two overlapping halves (or theca) that fit together like two parts of a soap box. The upper half (lid) is called epitheca and the lower half (case) is called hypotheca.
(5) Diatoms do not possess flagella except in the reproductive stage. They show gliding type of movement with the help of mucilage secretion. They float freely on the water surface due to presence of light weight lipids.
(6) Raphe is a cleft in the valve which occur in diatoms performing gliding movement. Raphe is often longitudinal, sigmoid with central and polar nodules.
(7) Each cell has a large central vacuole in which a prominent nucleus is suspended by means of cytoplasmic strands. The cells are diploid (2N). In case of centrales, the nucleus lies in the peripheral region.
(8) The cells possess plate-like or discoid chromatophores (or chloroplasts). They contain chlorophyll a, and c, carotenes, diatoxanthin, diadinoxanthin and fucoxanthin (chl. b is absent).
(9) Some species of diatoms are devoid of chromatophores, e.g., Nitzschia alba. The are saprotrophic in nutrition.
(10) The reserve food material is oil and a polysaccharide – chrysolaminarin (or leucosin).
(11) Asexual reproduction is most common method of multiplication is binary fission (cell division) that occurs at night.
(12) Sexual reproduction takes place by the fusion of gametes. Meiosis is gametic i.e., takes place during the formation of gametes.
Examples : Triceratium, Pleurosigma, Navicula, Cymbella, Amphipleura, Nitzschia, Melosira, Pinnularia.
General characters
(1) It includes Euglena like flagellate protists of fresh water and damp soils which were formerly placed in phytomastigophora by zoologists. Euglenoids include about 36 genera and 800 species.
(2) Euglenoids and their non-motile relatives are unicellular flagellate protists.
(3) These protists are devoid of cellulose cell wall. The body is covered by thin and flexible pellicle. The pellicle has oblique but parallel stripes called myonemes. The pellicle is composed of fibrous elastic protein and small amount of lipid or carbohydrates.
(4) The euglenoids have two flagella, usually one long and one short. Each flagellum arises from a basal granule (blepharoplast). The flagella bear hair.
(5) They can undergo creeping through expansion and contraction of body called metaboly.
(6) The apical end bears an invagination having three parts–cytostome (mouth), cytopharynx (gullet or canal) and reservoir. The cytostome is generally eccentric.
(7) The two flagella join with each other at a swelling called paraflagellar body. An orange red coloured eye-spot or stigma is located at the base of flagellum attached to the membrane of reservoir at the level of paraflagellar body. They contain red pigment astaxanthin. Both paraflagellar body and eye spot act as photoreceptors and direct the organism towards the optimum light.
(8) An osmoregulatory contractile vacuole occurs in the anterior part of the cell below the reservoir.
(9) A single large nucleus lies roughly in the middle. The nuclear envelope persists during division. The nucleolus also persists upto metaphase.
(10) Nutrition is holophytic (photoautotrophic), saprobic (e.g., Rhabdomonas) or holozoic (e.g., Peranema). Even holophytic forms can pick up organic compounds from the outside medium. Such a mode of nutrition is called mixotrophic.
(11) Product of photosynthesis is paramylon which is stored in the form of paramylum granules in the paramylum bodies in cytoplasm.
(12) Sexual reproduction has not yet been definitely proved. Under favourable conditions, euglenoids multiply by longitudinal binary fission.
(13) These protists perennate during unfavourable periods as cysts.
Example : Euglena, Phacus, Eutreptia, Trachelomonas, Peranema.
Euglena
General characters
(1) Euglena is a connecting link between animals and plants.
(2) Euglena resembles the ancestral form which the plants and animals evolved.
(3) Euglena is a free swimming fresh water flagellate.
(4) Body is covered by pellicle, a small cytostome (cell mouth) and cytopharynx is present at the anterior end of the body.
(5) Euglena contains chlorophyll, yet it resembles animals, because it feeds like animals in the absence of sunlight.
(6) Nutrition in Euglena is mixotrophic, when light is available it is photosynthetic, in darkness it is saprophytic absorbing food from surrounding water.
(7) Reserve food is stored in the form of paramylum or paramylon.
(8) Movements are called euglenoid movements (wriggling movements), also moves with help of flagella.
(9) Asexual reproduction occurs by longitudinal binary fission, no sexual reproduction.
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