Dr.Ayad M.J. Lecture 8 Algae Rhodophyta

Transcription

1 Rhodophyta 1-2 General characteristics phylum (division) of the kingdom Protista consisting of the photosynthetic organisms commonly known as red algae. Most of the world's seaweeds belong to this group. Members of the division have a characteristic clear red or purplish color imparted by accessory pigments called phycobilins, unique to the red algae and the cyanobacteria. The chloroplasts of red algae are believed to be derived from cyanobacteria that formed an ancient symbiotic relationship with the algae. Red algae have a number of general characteristics that in combination distinguish them from other eukaryotic groups: 1-Absence of flagella and centrioles. 2-Floridean starch as a storage product and the storage of starch in the cytoplasm 3-Phycoerythrin, phycocyanin, and allophycocyanin as accessory pigments 4-Unstacked thylakoids in plastids 5-No chloroplast endoplasmic reticulum 1

2 1 3 Structure Cells of the Rhodophyta possess chloroplasts that, in addition to the phycobilins, contain chlorophyll a, carotenes, and xanthophylls. At great ocean depths, where the wavelength of light available for photosynthesis is very different from that in shallow water, the phycobilins become more active than the chlorophylls in absorbing light; this fact may explain the ability of red algae to exist at depths of up to 879 ft (268 m). The carbohydrate reserves of red algae are in the form of floridean starch, a specialized glucose polymer of different structure than the starch of plants. The red algae, unicellular to multicellular (up to 1 m) mostly free-living but some parasitic or symbiotic, with chloroplasts containing phycobilins. Cell walls made of cellulose with mucopolysaccharides penetrated in many red algae by pores partially blocked by proteins (complex referred to as pit connections). Usually with separated phases of vegetative growth and sexual reproduction. Common and widespread, ecologically important, economically important (source of agar). No flagella. Ultrastructural identity: Mitochondria with flat cristae, sometimes associated with forming faces of dictyosomes. Thylakoids single, with phycobilisomes, plastids with peripheral thylakoid. During mitosis, nuclear envelope mostly remains intact but some microtubules of spindle extend from noncentriolar polar bodies through polar gaps in the nuclear envelope. Synapomorphy: No clear-cut feature available; possibly pit connections Composition: About 4,000 species. 2

3 Figure 1:Left is a General structure of Rhodophyta and right is Different forms of tetrasporangia commonly found in Rhodophyta. ( a ) Tetrahedral ( b ) Cruciate type ( c ) and ( d ) Zonnate 1 4 Life cycle and reproduction They display alternation of generations; in addition to gametophyte generation, many have two sporophyte generations, the carposporophyte-producing carpospores, which germinate into a tetrasporophyte this produces spore tetrads, which dissociate and germinate into gametophytes. The gametophyte is typically (but not always) identical to the tetrasporophyte. Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (freeliving) tetrasporophyte phase. Tetrasporangia may be arranged in a row (zonate), in a cross (cruciate), or in a tetrad. The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form a cystocarp. These case studies may be helpful to understand some of the life histories algae may display: In a simple case, such as Rhodochorton investiens: 3

4 In the Carposporophyte: a spermatium merges with a trichogyne (a long hair on the female sexual organ), which then divides to form carposporangia which produce carpospores. Carpospores germinate into gametophytes, which produce sporophytes. Both of these are very similar; they produce monospores from monosporangia "just below a cross wall in a filament" and their spores are "liberated through apex of sporangial cell." The spores of a sporophyte produce either tetrasporophytes. Monospores produced by this phase germinate immediately, with no resting phase, to form an identical copy of parent. Tetrasporophytes may also produce a carpospore, which germinates to form another tetrasporophyte.[verification needed] The gametophyte may replicate using monospores, but produces sperm in spermatangia, and "eggs"(?) in carpogonium. A rather different example is Porphyra gardneri: Figure2: Life cycle of Rhodophyta 4

5 1 5 Classification Class Bangiophyceae Members of the Bangiophyceae have a simple alternation of heteromorphic generations in which the sporophyte is a small, prostrate filament called a conchocelis that releases meispores called conchospores. The sporophyte is the stage that has pit connections. The gametophyte can be variable in this group and range from filamentous to foliose. Figure 2: Batrachospermum Class Floridiophyceae The Floridiophyceae contains most of the taxa in the phylum. These plants tend to be complex, either filamentous or pseudoparenchymatous and tend to be seaweeds of warmer waters. The polysaccharides common in the cell walls of many in this group are the sources of agar, agarose, and carrageenin, common food additives. Chondrus crispus is the red most commonly harvested on the coast of the eastern US as a source of agar. Corallina is a taxon that impregnates its cell walls with calcium carbonate forming filaments that appear armored and segmented. 5

6 1 7 Important terms Figure 3: Polysiphonia 1-carposporophyte (2n): diploid stage that forms from fertilization and produces asexual carpospores 2-tetrasporophyte (2n): diploid stage that forms from carpospores, and produces haploid tetraspores through meiosis 3-gametophyte (1n): haploid stage that forms from tetraspores, and produces gametes 4-spermatia: non-motile sperm 5-trichogyne: female stalk that catches spermatia Figure 4: Polysiphonia life cycle 6

7 Phaeophyta (Brown Algae) The Phaeophyta or brown algae are mostly marine algae. Phaeophyta are characterized by the pigment fucoxanthin that gives them the brown colour. The cell wall in Phaeophyta is two layered; inner layer consists of cellulose and outer layer mainly of algin and fucoidan. The brown seaweeds serve as important source of the industrial hydrocolloid alginate as well as food in countries like Japan, Korea and China. 1- General characteristics (a) Occurrence: Mostly marine. (b) Pigments: Fucoxanthin is dominant, Chlorophyll a, c and carotene. (c) Pyrenoids: Stalked pyrenoids present outside the chloroplast envelope.. (d) Reserve food material: Laminarin, mannitol and fats. (e) Cell wall: Cellulose, alginic acid and fucinic acid. (f) Structure: Microscopic to branched, filamentous macroscopic parenchymatous plants. (g) Flagella: Zoospores flagellated, flagella unequal, one is tinsel type. (h) Reproduction: Sexual reproduction (isogamous, anisogamous and oogamous). 2-Structure Most of brown algae are lithophytes, which require stable hard substrata for attachment, and a number of the fi lamentous, smaller species are epiphytes. Unicellular, colonolial and unbranched fi laments are absent in 7

8 pheophyceae. The freshwater phaeophyta species are simply filamentous and smaller in size unlike their marine counterparts which have complex gigantic and bulky thalli Their size ranging from small fi lamentous forms like Ectocarpus and Hinskia, which are few millimetres to massive intertidal weeds such as Ascophyllum and Fucus, to subtidal large kelps and the largest seaweed known Macrocystis pyrifera, They have higher morphological and anatomical differentiation compared to the other algae The size range vary greatly, from crustose form which may be 1 2 mm, macroscopic fi lmentous tufts 2 10 mm, subtidal kelp forests that might be as tall as m. Figure 5: General Morphology of Brown Algae 8

9 The cell walls of brown algae are generally gelatinous and consist of two layers. Cellulose makes up the skeleton backbone but is present in small quantiites i.e. 1 8 % of dry weight. The chloroplasts of brown algae are usually discoid and surrounded by an envelope. The outer membranes of the chloroplast endoplasmic reticulum are continuous or discontinuous depending on the species. Microfibrils of DNA occurring in the plastid may be linear or circular attached to the thylakoid membranes. The pigments are located in plastids lacking pyrenoid; their presence may also vary according to algal stage. Presence of Physodes (fucosan granules) is one of the characteristic features of brown algae. In the meristmatic, photosynthetic and reproductive cells, cytoplasm a large number of colourless vesicles with highly refractive acidic fl uid staining red with vanillin and hydrochloric acid are present 3-Reproduction Brown alga reproduces by vegetative, asexual and sexual methods of reproduction: 1- Vegetative Reproduction Several species of brown algae show vegetative reproduction via fragmentation. In members of sphaecelariales propagules are found 2-Asexual Reproduction All brown algae reproduce asexually with exceptions of Tilopetridales, Dictyotales and Fucales. In ectocarpales and spherocarpales asexual reproduction occurs via bifl agellate zoospores that develops in to reproductive organs called sporangia which could be unilocular (onecelled) or many cells plurilocular as observed in Hinskia 9

10 mitchelliae.gametes can also reproduce parthenogenetically to form asexual progenies, for example in Ectocarpus. Asexual reproduction is absent in Laminaria 3-Sexual Reproduction In pheophyceae sexual reproduction takes place by the formation of fl agellate gametes that are formed inside gametangia. Multicellular gametangia are formed only in some of the brown algae, The haploid thalli form ranges from isogamous (both male and female gametes exactly similar), anisogamous (female gamete larger than male) to oogamous (small fl agellated male and large non-fl agellated female gametes), The sexual reproduction is through fusion of fl agellated male and female gametes or fusion of flagellated male and large non-flagellated female gametesthe haploid (gametangial) and diploid (sporangial) thalli may be similar (isomorphic) as in Ectocarpales or different (heteromorphic) in appearance for example in Laminariales, or the gametangial generation may be extremely reduced (Fucales). 3-Life Cycle Brown algal life cycle shows alteration of generations of haploid and diploid organisms: 1-Haploid gametophytes (n) give rise to haploid gametes by mitosis. 2-male and female gametes (n) fusion give rise to zygote (2n) that forms diploid sporophyte 3-The sporophyte (2n) produces meiospores (n) by meiosis which germinates and 4-forms haploid gametophyte. 5-Brown algae life cycle may be isomorphic, heteromorphic and Diplontic. 10

11 3-1 Life Cycle of Ectocarpus The sexual life cycle of Ectocarpus consists of alternating heteromorphic gametophyte and sporophyte generations,both generations of the sexual life cycle are multicellular. The sexual life cycle in the sporophyte generation starts with the formation of a diploid zygote that undergoes a bipolar germination to produce two germs tubes that eventually forms the sporophyte. The initial cell division in the zygote is symmetric. Two kinds of fi laments prostrate and upright are produced in the sporophytic generation. Two specialized reproductive structures; plurilocular and unilocular sporangia are produced on the upright filaments A mitotic event in multi-chambered plurilocular sporangia releases mito-spores, which after their release, forms sporophyte Unlike plurilocular sporangium, unilocular sporangium consists of single chamber that contain meiotically produced meiospores that give rise to multicellular gametophytes. Morphologically, it is hard to distinguish between a male and a female gametophyte. Like zygote, the meio-spores undergo a bipolar germination but the initial cell division of meio-spore is asymmetric that produces different cell types. Only upright fi laments have been observed in the gametophytic generation. Specialized reproductive structures called plurilocular gametangia are produced on the upright fi laments of the gametophyte Plurilocular gametangia produce male and female gametes that are released in the surrounding marine water. Male and female gametes can be distinguished based on the behavior and physiology Unlike male gametes, female gametes settle quickly and release a sex pheromone that attracts male gametes. Once a male gamete fuses with a female gamete, a diploid zygote is produced that marks the onset of the first diploid structure of the sporophyte generation. 11

12 Figure 5: General life cycle of Brown Algae (Ectocarpus) Figure 6:Life cycle of Laminaria 12