Algae Lecture 2 PDF

THE ALGAE General characteristics The study of algae is called Phycology  The algae are chlorophyll bearing organisms hence are able to make their own food.  They have a plant body with no differentiation into roots , stems and leaves – this is called a thallus(sing.) thalii (plural)-thalloi...

THE ALGAE General characteristics The study of algae is called Phycology  The algae are chlorophyll bearing organisms hence are able to make their own food.  They have a plant body with no differentiation into roots , stems and leaves – this is called a thallus(sing.) thalii (plural)-thalloid  They have unicellular or multicellular sex organs ALGAE CONT’D  The thallus is non-vascular and thus has no vascular tissue  They are aquatic, both marine and fresh water, on and within soil, on moist stones and wood as well as in association with fungi and certain animals.  Many algae contain secondary pigments such as carotenes which may be brown, yellow, xanthophylls and phycobillins ALGAE CONT’D  They have eukaryotic type of cell except in the blue green algae which has prokaryotic cell.  Have rigid cell walls composed of cellulose except in diatoms whose cell wall is composed of silica  They exhibit ranges of alternation of generation – sporophyte which is spore bearing and diploid and gametophyte which is haploid and produces gametes. ALGAE CONT’D  Algae range in forms such as unicellular, colonial, filamentous, siphonaceous and parenchymatous. Unicellular forms  May be motile –have flagella e.g Euglena has a single flagellum while Chlamydomonas has two flagella  Non motile – no flagella , no cytoplasmic projections e.g Chlorella  Amoeba-like – lack rigid cell wall, have cytoplasmic projections e.g Rhizochrysis  Unicellular Rhizopodial – Rhizochrysis Non-motile-Single cell-Chlorella  Unicellular Chlamydomonas Motile-unicellular, Euglena Colonial forms Flagellate colonies. Motile flagellate cells aggregate to form simple colonies in some species. In most cases the cells are embedded in mucilage with only flagella projecting. Volvox sp. A vegetative colony Colonial forms cont’d Non-flagellate colonies  Non-motile cells aggregate to form a colony  It forms multinucleate. In some cases the non-motile cells are embedded in mucilage. Filamentous forms  Filamentous (Unbranched)  Formed by repeated cell division in one plane without separation of the daughter cells.  They may be free-living, attached initially,or aggregated into colonies. Single filamentous unbranched colony Filamentous forms cont’d  Filamentous (Branched)  There are two main types of thalli.  (i) a simple branching system attached to the substratum by a basal disc secreted by the lowest cell  (ii) the heterotrichous type with a basal attachment system of filaments giving rise to many upright branches.  Heterotrichous type Simple branching type Siphonaceous forms Siphonaceous thalli  The simplest organization include a small unbranched vesicle containing a central vacuole and peripheral cytoplasm in which the chloroplasts are located.  An irregular branching system with rhizoids and occasional cell walls cutting off old siphons is also found. Siphonaceous type Parenchymatous forms Parenchymatous thalli  This forms a paper-like thalli or tubular-like thalli.  A dichotomously branched parenchymatous thallus is forms of larger inner cells sandwiched between outer cells. REPRODUCTION  REPRODUCTION IN ALGAE  There are two common methods; (a) vegetative/ asexual, (b) sexual. Vegetative/Asexual reproduction  By cell division. The mother cell divide and the daughter cells are produced, which become new plants. REPRODUCTION CONT’D Fragmentation.  The plant body breaks into several parts or fragments and each such fragment develop into an individual. Hormogone formation.  When the trichomes break in small pieces of two or more cells such piece is called hormogone.  Each hormogone develops into a new plant. REPRODUCTION CONT’D  Adventitious thalli.  Certain special structures of thalli are formed which help in vegetative propagation. They are known as propagule.  Tubers.  Usually these bodies are rounded and filled with abundance of starch.  Bulbils. Small bud-like structures usually develop on the rhizoids.  Akinetes. Thick-walled, dormant, often elongated spores. Formed during dry season. REPRODUCTION CONT’D  Zoospores.  They are formed from certain older cells of the filaments.  zoospores escape from the mother plant and is able to develop into a new plant. They are normally formed in favourable conditions. The zoospores are motile and may be biflagellate or tetraflagellate. REPRODUCTION CONT’D Aplanospores  These are zoospores that have lost means of motility – no flagella.  Normally develop in unfavorable conditions. Each spore is surrounded by a wall. REPRODUCTION (b) Sexual reproduction CONT’D  This is an advanced method of reproduction shown in all groups of algae except in the blue-green algae.  There are three types;  Isogamy; It is fusion of similar motile gametes. These gametes are morphologically similar but genetically different. The gametes are called isogametes and the organisms are isogamous. REPRODUCTION CONT’D  Anisogamy ;The motile gametes taking part in fusion are morphologically different in size. The smaller one is referred to as the male and the large one is the female.  Oogamy. One gamete is large and non-motile and is referred to as the egg- the female gamete  The other gamete is usually small and motile-male gamete- sperm REPRODUCTION CONT’D Conditions for sexual reproduction  Takes place after considerable accumulation of food material and  the climax of vegetative activity is over slowed.  Suitable pH is necessary.  Optimum temperature is necessary.  Presence of appropriate light. LIFE-CYCLE PATTERNS Haplontic type/zygotic meiosis.  In such cases the somatic phase (plant) is haploid (Gametophyte) while the diploid phase (sporophyte) is represented by zygote.  During germination the zygote (2n) divides meiotically producing haploid (n) zoospores, which develop into individual plant.  Here the unicellular or filamentous gametophyte (n) alternates with one-celled zygote or sporophyte (2n). Life-cycle patterns cont’d  The haploid filamentous plants are known as haplonts which reproduce asexually by zoospores or aplanospores producing the individals like parents. Haplontic type Life-cycle patterns cont’d  Diplontic type/Sporic meiosis.  It is reverse of haplontic type. The somatic phase (plant) is diploid (sporophyte 2n) while the haploid phase (gametophyte n) is restricted to gametes which are produced by meiotic division.  After a gametic union, a diploid zygote is formed, which develop into a diploid (sporophyte) plant by mitotic division. Diplontic/sporic type Life-cycle patterns cont’d Isomorphic type:  In this type there are two exactly similar somatic phases (morphologically identical plants) showing alternation of generations.  Here the one phase is diploid (sporophyte, 2n) while the other haploid (gametophyte, n). Life-cycle patterns cont’d  In such cases, the zygote develops into a diploid multicellular plant (sporophyte).  through meiosis, zoospores (meiospore) are formed.  These zoospores (n) develop into haploid plants (gametophyte n).  The haploid plants produce gametes (n) which after fusion develop into zygote (2n) which grows into a sporophyte phase Isomorphic type Life-cycle patterns Heteromorphic type : cont’d  This pattern of life cycle is exactly like the isomorphic type only with the difference that the alternating haploid (n) and diploid (2n) somatic phases (plant) are morphologically different.  Diploid multicellular sporophytic plants produces haploid zoospores (meiospores) by meiosis. Life-cycle patterns cont’d  These zoospores develop into gameophytes.  Each gametophytic plant (n) produce gametes which after their union form a zygote which later develops into a diploid sporophytic plant by mitotic division. Heteromorphic type CLASSIFICATION OF ALGAE The primary classification of algae is based on certain morphological and physiological features.  Pigment constitution of the cell.  Chemical nature of stored food materials  Nature and the number of flagella.  Chemical composition of the cell wall.  Presence or absence of a definitely organized nucleus in the cell. THE END