Oxygen producing prokaryotes cyanobacteria




Дата канвертавання18.04.2016
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OXYGEN PRODUCING PROKARYOTES – CYANOBACTERIA

1. Most important group of photosynthetic bacteria; one of the most important groups of algae



2. possibly 150 genera, 2000 species, possibly many more, possibly many less

      1. possibly responsible for changing the atmosphere to an oxygen containing atmosphere

      2. as picoplankton, may be the largest group of photosynthetic organisms in the ocean

      3. an important component of the nitrogen cycle, especially in the tropics

      4. produce toxic blooms

      5. sold in health-food stores

    1. important features

      1. normal eubacterial, Gram-negative cell

        1. wall contains peptidoglycan layer covered by a membrane of lipopolysaccharides

        2. may be covered by a sheath or capsule of sugars, proteins, etc.

        3. genetic material consists of nucleoid; translation using 70S ribosomes

        4. generally large for prokaryote cells (internal compartments formed by thylakoids)

      2. basic morphologies

        1. coccoid, bacilloid, spiral forms with and without sheaths

        2. how many planes of division?

        3. production of small cells (baeocytes) by repeated or simultaneous cell division

        4. polarity of the cell

        5. filamentous forms with and without sheath

          1. the filament of cells, excluding any sheath is called a trichome

          2. most important features are the degree of branching and the presence of special cells (heterocysts and akinetes)

      3. photosynthetic apparatus

        1. thylakoids in various arrangements - continuous with cell membrane?

        2. embedded in the thylakoids are the reaction centers and the electron transport chains

        3. attached to the reaction center are the water-soluble phycobilisomes

        4. normally produce oxygen during photosynthesis, but many capable of switching to system similar to that of the sulfur-bacteria, especially in sulfide-rich environments with lots of light (stratified shallow lakes and lagoons)

      4. inclusion bodies in the cytoplasm

        1. cyanophycean starch granules (alpha-1,4-linked glucose)

        2. cyanophycin granules - large granules, often found near the cross-walls of filamentous forms, composed of a polymer of arginine and asparagine (one of very few proteins known not to be made on a ribosome) - functions as a nitrogen reserve

        3. polyphosphate granules (volutin) - phosphate "rocks" which act a reserve for phosphate; stain with toluidine blue

        4. carboxysomes - polyhedral accumulations of RuBP carboxylase (similar to pyrenoids?)

        5. poly-beta-hydroxybutyric acid - storage product in a few cyanobacteria

        6. gas vacuoles in cyanobacteria

          1. composed of bundles of hollow protein cylinders filled with gas

          2. serve a buoyancy tanks to regulate the position of the alga in the water column

            1. if light is low, more vesicles are formed and the alga floats to more brightly lit layers

            2. in brightly lit layers more photosynthesis; more photosynthesis increases the number of small sugars, etc., in the cell, decreasing the osmotic pressure (makes more negative)

            3. as osmotic pressure becomes more negative, water enters the cell; this increases the hydrostatic pressure; as the hydrostatic pressure increases, the gas vesicles collapse and the alga sinks

      5. special cells

        1. akinetes - large resting stages

          1. relatively thick wall

          2. full of cyanophycin granules and cyanophycean starch, but no volutin

          3. formation may be triggered by phosphate deficiency

          4. resistant to environment, surviving years in lake sediments

            1. this may be important since some of the toxic forms are also akinete formers; stirring up the bottom by dredging may trigger a bloom

        2. heterocysts - special cells designed for nitrogen fixation

          1. nitrogen fixation is a high energy process

            1. need to reduce nitrogen to ammonium

              1. requires 16 ATP and 8 electrons for each molecule of N2 reduced

                1. ATP from photosynthesis

                2. electrons supplied by organic molecules via NADPH and reduced ferrodoxin

                3. ferrodoxin is a water soluble, iron and sulfur containing molecule

            2. ammonium is immediately attached to glutamate to make glutamine

          2. enzyme for the reduction step is nitrogenase

            1. enzyme is reasonably well-conserved over a variety of bacteria

              1. two components, both necessary, both oxygen sensitive

                1. Fe component that binds MgATP and transfers electrons to second

                2. MoFe component that reduces dinitrogen

          3. special structures of heterocysts help protect nitrogenase

            1. visually, heterocysts appear as thick-walled structures with a pale color, no inclusions

              1. closer examination reveals reduced thylakoids, no photosystem II, only photosystem (cyclic photophosphorylation), numerous plasmodesmata

                  1. free from oxygen, source of energy, means to pass material to rest of cells

            2. should be noted that non-heterocystous forms may also fix nitrogen

              1. at night, so that oxygen production limited (quickly make nitrogenase at sunset

              2. in anoxic regions of the environment

          4. should also be noted that enzyme not very specific

            1. standard test for nitrogen fixation monitors the reduction of acetylene to ethylene

          5. under some conditions, nitrogenase shunts electrons off to H+, producing hydrogen gas; during the Carter years there was a federal program to see if this could be made into a reliable source of H2

      6. toxins

        1. over 30 species implicated in toxic water blooms

        2. three major categories of toxins

          1. hepatatoxins (microcystin, etc.) can lead to intrahepatic hemorrhage and death within days

          2. anatoxins increase the effects acetylcholine (block cholinesterase), causing paralysis

          3. saxitoxins block sodium channels, preventing transmission of impulses

      7. habitats

        1. important constituent of marine picoplankton

        2. common in freshwater and terrestrial environments

        3. found in some of the most extreme environments

          1. thermal springs

          2. hot deserts

          3. Antarctica

      8. taxonomic concepts

        1. species concept - what constitutes a species in asexual organisms

          1. broad species with ecological variants

          2. narrow species with similar morphology

          3. are strains the only real category

        2. higher level taxonomic relations

          1. Geitlerian vs Drouetian vs Stanierian vs Anagnostidis and Komarek vs ???

          2. many arguments based on disagreement concerning the way the group should be treated

            1. blue-green algae (cyanophytes) vs blue-green bacteria (cyanobacteria)

            2. botanical vs bacteriological codes of nomenclature

          3. currently, molecular phylogenies based on small-subunit ribosomal RNA sequences are redefining (yet again) the major grouping; until sorted out we will stick with a somewhat classical-botanical approach

        3. major taxonomic groups

          1. Chroococcales (Anagnostidis and Komarek)

            1. all of the unicellular cyanobacteria (and old orders Chamaesiphonales, Pleurocapsales)

            2. families distinguished by: shape of cell, planes of division, formation of baeocytes and nanocytes, presence and type of sheath

            3. important genera: Microcystis, Gloeocapsa, Chroococcus, Synechococcus, Synechocystis, Anacystis

          2. Oscillatoriales (Anagnostidis and Komarek)

            1. characterized by uniseriate trichomes with or without sheaths (trichomes and filaments may be bundled), without special cells or true branching (false branching rare)

            2. families and genera are based on the type and color of the sheath, the shape and color of the cells, motility of filaments and hormogonia, false branching

            3. some important genera: Oscillatoria, Phormidium, Lyngbya, Plectonema, Spirulina

          3. Nostocales (Anagnostidis and Komarek)

            1. characterized by uniseriate trichomes with or without sheaths, with heterocysts and/or akinetes, without true branching

            2. families and genera based on: the type and color of sheath, the shape and color of cells, the shape and relative position of heterocysts and akinetes, the type of false branching, tapering of the trichomes and filaments

            3. some important genera: Nostoc, Anabaena, Aphanizomenon, Scytonema/Tolypothrix,Rivularia

          4. Stigonematales (Anagnostidis and Komarek)

            1. characterized by uniseriate or multiseriate trichomes with true banching and heterocysts and possibly akinetes

            2. families and genera distinguished by shape of cells, the nature of the trichome, the type of branching, the shape and relative positions of heterocysts and akinetes

            3. some important genera: Stigonema, Fischerella, Hapalosiphon

        4. Prochlorophytes—not a taxonomic group

          1. first discovered living as symbionts of sea squirts, now known from the picoplankton of some lakes and from blooms of filamentous forms

          2. photosynthetic apparatus

            1. chlorophyll a and b, divynil chlorophylls with carotene and other carotenoids as accessory pigments in stacked thylakoids, and using non-cyclic photophosphorylation

          3. cyanobacteria-like carboxysomes and starch

          4. peptidoglycan wall

          5. once thought to be implicated in the origin of mitochondria and chloroplasts, but now considered to be an interesting group of unrelated cyanobacteria


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