Division ochrophyta--the chromistan (golden-brown) algae




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DIVISION OCHROPHYTA--THE CHROMISTAN (GOLDEN-BROWN) ALGAE



  1. Basic characteristics

    1. an diverse group, ranging from flagellate unicells to giant kelps; related to closely to oomycetes, less so to a number of colorless protozoans, then alveolates and haptophytes

    2. flagellar apparatus – the defining feature of the group

      1. typically two flagella

      2. leading flagellum with two rows of tripartite hairs (mastigonemes); formed in the endoplasmic reticulum either near the plastid or near the nucleus

      3. trailing whiplash flagellum; often with a basal swelling involved in phototropic responses

      4. usually with a helical structure above the basal plate

      5. usually four microtubules, two associated with the anterior flagellum, two with the posterior

      6. one of the anterior roots is associated with the cytoskeleton in general, one of the posterior roots is associated with a feeding apparatus

    3. plastid structure and photosynthetic apparatus

      1. pigments

        1. chlorophyll a in reaction centers and antennae

        2. chlorophyll c acts as a free-floating (lacks the phytol tail of chlorophylls a and b) accessory pigments

        3. other accessory pigments include beta-carotene, fucoxanthin (brown forms), and vaucheriaxanthin (in green forms)

      2. storage products are beta-1, 3 glucans (chrysolaminarin), stored in vacuoles outside of the plastid

      3. plastids structure

        1. within periplastidal endoplasmic reticulum, which usually joins with the nuclear envelope

        2. thylakoids usually in stacks of three, with a girdle lamella running just below the chloroplast envelope




  1. Classification – Ochrophyta exhibits at least five major lineages, traditionally divided into at least nine classes (often 9 divisions); again we will follow the system used by Graham




    1. Bacillariophyceae (the diatoms)




      1. large group of fresh and salt water unicellular forms; extreme estimates suggest that there are over 10,000,000 species

      2. basic characteristics

        1. coccoid habit dominates, the only flagellate cells are the male gametes of centric diatoms

          1. flagellate cells, when present, have a single flagellum with mastigonemes, but without the two central microtubules of the axoneme

        2. plastid morphology

          1. basic features are those of the division (chloroplast ER continuous with the nuclear envelope, presence of a girdle lamella, chlorophyll c, golden brown color, etc.)

          2. some difference in whether the cell has numerous discoid plastids or a few parietal plates that may be lobed or not

        3. each cell is enclosed in a two-part siliceous shell (frustule) whose parts overlap--this is the defining characteristic of the group

          1. frustule features

            1. the upper valve of the frustule is called the epitheca, the lower is the hypotheca

              1. the epitheca consists of an epivalve and epicingulum (girdle), the lower of an hypovalve and hypocingulum

              2. the cingulum consists of a number of separate bands, the valvocopula next to the valve, and additional girdle elements

            2. all parts of the valve can be penetrated by pores called areolae, usually with a complex structure

              1. if the hole goes straight through it is called a poroid areola, if constricted at one side it is called loculate

              2. one side or both may be partially occluded by a velum

                1. cribrum - velum with an organized series of pores

                2. rota - solid plate attached by spokes

                3. vola - flap-like growth from the sides

            3. may also be penetrated by pore-fields of varying structure

            4. some valves with rib-like costae

            5. some valves with processes

              1. rimoportulae (labiate processes) – fairly common, usually associated with mucus secretion

              2. fultoportulae (strutted processes) – restricted to a few centric genera

            6. many pennate forms with a raphe system

              1. longitudinal slit, usually with an s-shaped or zigzag cross-section

                1. in some forms the raphe is on a raised structure (keel); this type is usually called a canal raphe

              2. the raphe is involved in movement of the diatom; the mechanism is not clear, but seems to involve the exocytosis of polysaccharide fibrils that bind to the substrate

          2. frustule formation

            1. wall composed of silicic acid deposited on an organic matrix in a silicon deposition vesicle (SDV)

            2. in some forms silica deposition begins with an annular ring with radiating ribs, in others with a solid central rib (sternum); in raphid forms silicon deposition begins in a long tube to one side of what will become the raphe (this side is called the primary side); the tube gradually curves around to form the slit of the raphe on the secondary side

            3. when the frustule formation is complete, the inner membrane of the deposition vesicle becomes the new cell membrane; the old cell membrane and the outer membrane of the vesicle are lost

            4. because new frustules are always made inside of the old frustule, one of the new cells has a slightly smaller diameter than the original cell leading to a gradual reduction in the diameter of the cells; this can be compensated in part by increasing the number of girdle bands, but ultimately leads to smaller cells

            5. silica deposition is inhibited by the presence of germanium acting as a competitive inhibitor of the enzymes; copper also inhibits diatoms and copper sulphate is widely used as an algicide

        4. sexual life cycles

          1. basic life-cycle is diplontic

          2. sexual reproduction is triggered in part by the reduction in the size of frustules: when the cell reaches certain size and it meets another cell in the same state, the two undergo meiosis; gametes are formed and fuse; the resulting zygote swells to form a large auxospore with a differently-structured wall; the auxospore germinates by first one mitotic division, resulting in the formation of the epitheca, and then a second, resulting in the formation of the hypotheca; one of the nuclei from each of these divisions degenerates

      3. other features of diatoms

        1. mucilage secretion

          1. can be through pore fields, processes, or raphes

          2. used for attachment to the substratum (Gomphonema) or to other diatoms (Biddulphia) or for movement

            1. movement usually associated with secretion through raphe systems

            2. motive force may come from microtubule/kinesin or dynein interactions

        2. toxin production - some diatoms (memebers of Pseudonitzschia) secrete domoic acid, a cause of amnesic shellfish poisoning (symptoms include nausea, vomiting, and diarrhea after 3 hours, dizziness, hallucinations, short-term memory loss, decreased reaction to pain, and seizures in extreme cases) where it acts on glutamate receptors in the brain

        3. nutrition

          1. diatoms require relatively high concentrations of silica; where exists in the ocean, diatoms predominate (20% of the ocean area); these areas are characterized by diatomaceous sediments

          2. diatom growth triggered by iron addition in high nitrate, low chlorophyll regions

          3. some (Rhizosolenia) contain endosymbiotic cyanobacteria

          4. some can function as heterotrophs

      4. classification - traditionally based on valve symmetry, which may not hold up as more molecular data is added; we will follow the basic scheme in Tomas's Identifying Marine Diatoms and Dinoflagellates

        1. Biddulphiales (Centrales) -- radially symmetric valves

          1. Coscinodiscineae -- valves usually with a ring of marginal processes and radial symmetry

            1. Thalassiosira, Cyclotella, Skeletonema -- with strutted processes

            2. Melosira

            3. Coscinodiscus

            4. Actinocyclus

            5. Actinoptychus

            6. Asteromphalus

          2. Rhizosoleniineae -- valves unipolar, without a ring of processes

            1. Rhizosolenia

          3. Biddulphiineae -- valves bipolar, without a ring of processes

            1. Biddulphia, Odontella

            2. Chaetoceros, Bacteriastrum

            3. Cerataulina

        2. Bacillariales (Pennales) -- valves with bilateral symmetry

          1. Fragilariineae -- usually with a sternum, without a raphe

            1. Fragilaria, Synedra, Tabellaria

            2. Rhaphoneis

            3. Thalassionema

          2. Bacillariineae -- valves with a sternum and a raphe

            1. Eunotia -- arced valves with a short raphe

            2. Achnanthes, Cocconeis -- one valve without a raphe

            3. Navicula, Pinnularia, Frustulia, Stauroneis

            4. Nitzschia, Bacillaria

            5. Epithemia, Surirella




    1. Chrysophyceae -- the golden-brown algae




      1. basic characteristics

        1. unicellular or colonial organisms, primarily freshwater

        2. plastids with chlorophylls c1 and c2 and fucoxanthins; plastid morphology follows the basic ochrophyte plan

        3. flagellate stages with typical ochrophyte flagellar apparatus:

          1. two flagella, a long one with mastigonemes and a short, trailing one without

            1. mastigonemes on the leading flagellum - help pull cell through water(?)

            2. short flagellum with a basal swelling thought to have a function in light sensing (fluoresces green when illuminated with blue or near UV light)

          2. flagella terminate in a helical transition structure

          3. each basal body has two set of microtubules

            1. one of the sets on the leading flagellum appears to contain an MTOC

            2. one of the sets on the trailing flagellum loops around to form part of a feeding apparatus in mixotrophic forms

          4. basal bodies are connected by fibrous bands; there is a banded rhizoplast extending to the nuclear envelope

        4. many genera form siliceous scales in special silica deposition vesicles

        5. all form statospores or stomatocysts

          1. special resting stages almost completely covered in silica; there is an organic plug allowing the cell to exit at germination; the wall is formed in a silica deposition vesicle

          2. can be formed in conjunction with either asexual or sexual reproduction

          3. fossil cysts date back 80 million years; can be used to estimate paleoclimates

        6. exhibit a range of nutritional modes, from photoautotrophy to mixotrophy to heterotrophy -- possibly related to high requirements for phosphorus, iron, and vitamins

      2. classification and diversity -- traditionally, the taxonomy of this group has been based on habit; this is a notoriously poor method, so we will just look at a few examples spanning the range of diversity

        1. Chromulina and Ochromonas -- wall-less flagellate forms. both with two golden-brown plastids

        2. Dinobryon -- loricate flagellates forming branched colonies

        3. Uroglena and Uroglenopsis -- wall-less flagellates in a colonial slime; Uroglenopsis colonies resemble Volvox

        4. Chrysamoeba -- an amoeboid form



    1. Synurophyceae -- a second group of golden-brown algae that was included in the Chrysopheae until recently




      1. basic characteristics

        1. plastids similar to those of chrysophyceans, but without chlorophyll c2

        2. flagellar apparatus is similar to that of chrysophyceans, except that the flagella, both extend forward, so there is no trailing flagellum, the flagellar root system includes a banded rhizoplast

          1. at mitosis, two new long flagella are produced; the short flagellum is formed from the old long flagellum later

        3. flagellate cells retain the photoreceptor, but lack an eyespot

        4. no member of the group is phagotropic

        5. surface usually covered in overlapping siliceous scales

      2. classification and diversity -- again,  a small group, but with two important freshwater genera

        1. Synura -- motile colonies

        2. Mallamonas -- scaled, unicellular flagellates

    1. Dictyochophyceae -- silicoflagellates, a very small group of flagellates, fairly common in colder waters

      1. basic characteristics

        1. single flagellum, with mastigonemes

        2. ochrophyte plastid with chlorophylls a and c, and fucoxanthin

        3. siliceous external skeleton in the form of a basket

      2. classification and diversity

        1. only a few living species all in the genus Dictyocha

        2. extensive fossil record dating back to the Cretaceous (100 million years)




    1. Raphidophyceae (chloromonads)




      1. basic characteristics

        1. relatively large flagellates with the typical set of ochrophyte flagella, but without parabasalar swellings or eyespots

        2. naked (without scales, walls, or thecae)

        3. numerous trichocysts and, possibly, mucocysts

        4. usually with numerous green to golden-brown plastids; brown forms contain fucoxanthin, green forms do not

      2. classification and diversity -- because this is a small group, we will only note differences between marine and freshwater forms

        1. marine raphidophyceans -- tend to be golden-brown, with pyrenoids in the plastids

          1. Heterosigma -- bloom-forming species, sometimes toxic

          2. Chattonella -- another bloom-forming species

        2. freshwater raphidophytes -- tend to be green, without pyrenoids

          1. Gonyostomum -- large form common in acidic waters rich in organic matter (Lake Louise); has a large triangular cavity in the front of the cell

          2. Merotrichia, Vacuolaria


    1. Tribophyceae (Xanthophyceae) -- the yellow-green algae




      1. small group of mostly freshwater and terrestial algae, similar in overall appearance to chlorophytes

      2. basic characteristics

        1. numerous discoid plastids, possibly containing pyrenoids, chlorophylls a and c, and xanthophylls, but no fucoxanthin or violaxanthin (see table 15-1)

        2. flagellate stages with the typical ochrophyte flagellar apparatus, but neither flagellum bears mastigonemes

      3. classification and diversity -- a small group, with the traditional classification based on habit; we follow Graham's lead and just point out a few important genera

        1. Chloridella -- similar in form to Chlorella

        2. Characiopsis -- similar in form to Characium and Pseudocharaciopsis

        3. Ophiocytium -- fairly common curved cylinders

        4. Stipitococcus -- small loricate forms found as epiophytes on filamentous algae

        5. Tribonema -- robust filamentous form with walls composes of two pieces,similar in appearance to those of Microspora

        6. Vaucheria -- large coenocytic filaments forming coarse mats; zoospores are large and multinucleate; sexual reproduction is oogamous



    1. Eustigmatophyceae -- recently identified group of  yellow-green algae




      1. basic characteristics

        1. small unicellular coccoid algae

        2. flagellate cells have a very short trailing flagellum and a large orange-red eyespot outside of the plastid

        3. plastids more or less normal for the ochrophytes: girdle lamella usually missing, chloroplast ER does not connect with the nuclear envelope, usually pyrenoid on a stalk, violaxanthin usually present as accessory pigment

      2. classification -- a small group (but possibly many more, hard to tell from green and yellow-green algae) with a single order (Eustigmatales)

        1. Eustigmatos

        2. Pseudocharaciopsis

        3. Nannochloropsis



    1. Phaeophyceae -- the brown algae

      1. large group of marine macrophytes with over 250 genera and 1500 species

      2. basic characteristics

        1. plastids and photosynthetic apparatus

          1. one to many plastids per cell

          2. plastid construction and pigment composition follow the typical ochrophyte plan with large amounts of fucoxanthin

          3. reserve products include laminarin, mannitol, sucrose, and glycerol

        2. flagellate stages (only zoospores and gametes) with typical ochrophyte flagella

        3. cell walls

          1. cellulose (1-10% of dry weight)

          2. alginates  -- polymer of mannuronic and guluronic acids

          3. fucoidin and other sulfated sugars

        4. physodes

          1. vesicles filled with fucosan (phaeophycean tannin) a colorless liquid that turns black on exposure to air

          2. fucosans apparently inhibit the growth of unicellular algae keeping the thallus relatively free of epiphytes

        5. life histories -- basically haplodiplontic with heteromorphic generations; diploid dominates

          1. gametophyte reproduction

            1. sexual reproduction can be isogamous or oogamous

            2. gametes are produced by mitosis in special gametangia (plurilocular gametangia or sporangia)

              1. products of gametophyte plurilocs sometimes develop into multicellular gametophytes without benefit of fertilization (parthenogenetically); if this is the common mode of reproduction the algae, the alga is called monophasic

            3. most groups studied has some sort of pheromone produced by the female that influences sperm develop and attracts the sperm to the egg

          2. sporophyte reproduction

            1. also produce plurilocular sporangia; the products in this case are zoospores that develop into another sporophyte generation

            2. other structures (unilocular sporangia) are used to produce haploid spores by meiosis; in this case, walls are not formed between the cells, giving the appearance of a single (unilocular) chamber

        6. modes of growth

          1. primitive version is apparently a diffuse growth occuring throughout the thallus

          2. advanced versions involve the formation of definite meristems; the occurrence and position of these has been used to define the orders

            1. apical meristems with division in one plane resulting in filamentous growth

            2. rows of marginal meristem cells giving rise to a broad blade

            3. trichothallic growth - division of  a cell at the base of a long uniseriate filament (hair); below the meristematic cell, division can be in multiple planes

            4. intercalary meristems, between stipes and blades, adding new tissue to both

            5. meristoderms that increase the girth

        7. habit

          1. brown algae can be filamentous as individual filaments or packed together into pseudoparenchymatous thalli or composed of true parenchymatous thalli arising through the division of cells in multiple planes of division

          2. evolutionarily advanced forms may display a marked amount of differentiation of tissues and structures

            1. thallus can be divided into holdfast, stipe, and blade, with the stipe and/or the blade capable of forming air bladders (gas vesicles or pneumocysts) filled with atmospheric gases, sometimes enriched with CO

            2. within the stipe and blade can be three well-defined tissues

              1. meristoderm -- photosynthetic, meristematic tissue, usually covered by a layer of mucilage

              2. cortex -- mass of larger cells

              3. medulla -- composed of several different cell types

                1. cells that extend from one side of the thallus to the other; develop from outgrowths of inner cortex cells

                2. hypae -- longitudinally arranged cells with sieve plates at the junctions; transport soluble storage products via active transport(?) from sources to sinks; appear trumpet-shaped as they are stretched by the growth of the thallus

      3. classification and diversity

        1. Phaeophyceae is normally divided into 14 orders; however, because these are primarily marine macrophytes, we will only treat a few of the major orders displaying the range of form

        2. Ectocarpales -- forms with a thallus consisting of individual (branched) filaments or filaments bunched together to give a compact thallus; reproduction isogamous (this version of the order lumps together a number of the more primitive orders)

          1. Ectocarpus -- both generations composed of uniseriate branched filaments, with plurilocs on the gametophyte, and plurilocs and unilocs on the sporophyte

          2. Scytosiphon -- gametophyte is a large tube-shaped organism, with true parenchymatous growth, attached to the substrate by a basal disc; sporophytes are inconspicuous crusts

        3. Sphacelariales -- branched multiseriate thalli with true tissues growing from a definite and conspicuous apical cell; isomophic alternation of generations

          1. Sphacelaria

        4. Dictyotales -- complex thalli with true tissues developing from apical cells; isomophic alternation of generations; non-motile meiospores

          1. Dictyota -- dichotomously branched thalli

          2. Padina -- flattened, fan-like thalli

        5. Desmarestiales -- pseudoparenchymatous habit derived from a trichothallic growth pattern; oogamous sexual reproduction

          1. Desmarestia -- well-known from polar regions; apparently one form stores sulfuric acid at pH 1 in vacuoles

        6. Laminariales -- also known as the kelps; large sporophytes differentiated into holdfast, stipe, and blades, with intercalary meristems; thallus morphology is that describe previously as an example of the evolutionary extreme; sporophytes produce only unilocs, sometimes in localized patches (sori), sometimes on specializes blades (sporophylls); gametophytes are microscopic

          1. Laminaria -- common in polar and temperate waters; cultivated for food in Asia (undari)

          2. Postelsia -- intertidal form with the appearance of small flattened palm-trees

          3. Nereocystis -- annual reaching up to 30 m in length

          4. Macrocystis -- responsible for large forests off the coast of California, harvested for chemicals since World War II; female gametophytes are unicellular

          5. Ecklonia -- forms underwater forests in warm temperate waters

          6. Egregia

        7. Fucales -- complex thallus arising from apical growth; diplontic life-cycle with gametes formed in conceptacles (chambers) located in receptacles (branch tips)

          1. Fucus -- common intertidal form with flattened thallus with dichotomous or pinnate fronds; medullary region full of water-holding polysaccharides; photosynthesis continues when exposed

          2. Ascophyllum -- similar to Fucus, but without a midrib; sometimes used as animal feed or as a source of thickeners in cooking

          3. Sargassum -- highly differentiated thallus with air bladders in the axils of the fronds; usually attached, but some forms can continue growth while free-floating, forming large rafts with a distinctive animal fauna



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