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Geological and evolutionary timetable

Phototrophs

·Definitions, general characteristics ·The phototrophic way of life: Chlorophyll and pigments Anoxygenic and oxygenic photosynthesis ·Phototrophic bacteria: Anoxygenic phototrophs (purple, green, green non sulfur bacteria) Oxygenic phototrophs (cyanobacteria) ·Case studies: Aerobic anoxygenic phototrophic bacteria (-proteobacteria) Prochlorococcus Nitrogen fixation in Trichodesmium

Energy and reducing power synthesis in anoxygenic phototrophs. Anoxygenic phototrophs obtain their energy from light (hv).

Energy and reducing power synthesis in oxygenic phototrophs. In oxygenic phototrophs, light also drives the oxidation of water to oxygen.

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(Algae, Cyanobacteria)

Structure of chlorophyll a

Organisms have different chlorophylls that absorb at different wavelengths Better use of the energy of the electromagnetic spectrum Two unrelated organisms can coexist in a habitat by using different wavelengths

(Purple bacteria)

Structure of bacteriochlorophyll a. Both Chl a and BChl a are magnesium tetrapyrroles.

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Photosynthetic unit

Structure of Carotenoids

Photoprotective agents Transfer energy to the reaction centers

Antenna pigments are involved in capturing photons in low light intensity conditions

Accessory Pigments: Phycobilisome of Cyanobacteria

Chromophores of Phycobilisomes

620 nm

550 nm

Pigments absorb shorter (higher energy) wavelength of light. Allow growth of cyanobacteria at low light intensities

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Chromatic Adaptation of a Phycobilisome

Accessory pigments allow the organism to capture more of the available light

The absorption spectrum of a cyanobacterium that has a phycobiliprotein (phycocyanin) as an accessory pigment. Note how the presence of phycocyanin broadens the wavelengths of usable light energy (between 600 and 700 nm).

Electron Flow in Anoxygenic Photosynthesis

Reaction center of purple phototrophic bacterium

Reducing power to reduce CO 2 to (CH2O) n

E0'= -0.32 V

Reduction Potential

E0'= 0 V

Note how light energy converts a weak electron donor, P870, into a very strong electron donor, P870*, and that following this event, the remaining steps in photosynthetic electron flow are much the same as that of respiratory electron flow. RC, reaction center; Bchl, bacteriochlorophyll; Bph, bacteriopheophytin; QA, QB, intermediate quinones; Q pool, quinone pool in membrane; Cyt, cytochrome.

Arrangement of protein complexes in the photosynthetic membrane of a purple phototrophic bacterium. The light-generated proton gradient is used in the synthesis of ATP by the ATP synthase (ATPase). This is called cyclic photophosphorylation because electrons are moved around a closed circle.

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Map of photosynthetic gene clusters of the purple phototrophic bacterium, Rhodobacter capsulatus

Comparative Electron Flow in Anoxygenic Photosynthetic Bacteria

Map of the photosynthetic gene cluster of the purple phototrophic bacterium, Rhodobacter capsulatus . Genes are arranged in superoperons where transcripts of pigment biosynthesis( bch and crt) operons extend through to include transcription of polypeptides of the photosynthetic complexes. Green: bacteriochlorophyll genes ( bch) Red: carotenoid genes ( crt) Blue: reaction center polypeptides( pu f and puh, bind pigments in the RC and LH complexes) Yellow: light-harvesting I polypeptides (B870 complex) Expression of photosynthetic components is highly coordinated O2 is the signal governing transcription: it represses pigment synthesis so that photosynthsis in anoxigenic phototrophs occurs only under anoxic conditions

Reducing power to reduce CO 2 to (CH2O) n

A comparison of electron flow in purple bacteria, green sulfur bacteria, and heliobacteria. Note how reverse electron flow in purple bacteria is necessitated by the fact that the primary acceptor (quinone, Q) is more positive in potential than the NAD+/NADH couple. In green and heliobacteria ferredoxin (Fd), whose E0' is actually more negative than that of NADH, is produced by light-driven reactions. P870 and P840 are reaction centers of purple and green bacteria, respectively, and consist of Bchl a. The reaction center of heliobacteria (P798) contains Bchl g. The reaction center of Chloroflexus is similar to that of purple bacteria.

The Reverse Citric Acid Cycle (Chlorobium)

The Hydroxypropionate Pathway (Chloroflexus)

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Electron flow in reaction center of a cyanobacterium

Electron flow in oxygenic (green plant) photosynthesis, the "Z" scheme. Two photosystems (PS) are involved, PS I and PS II. Ph, Pheophytin; Q, quinone; Chl, chlorophyll a; Cyt, cytochrome; PC, plastocyanin; FeS, nonheme iron-sulfur protein; Fd, ferredoxin; Fp, flavoprotein; P680 and P700 are the reaction center chlorophylls of PS II and PS I, respectively.

The Calvin Cycle (Cyanobacteria)

RubisCO

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From: Jannasch, 1995. Geophysical Monograph 91:273

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Cellular Absorption Spectra of Photosynthetic Bacterial Groups

Scytonemin, an Ancient Prokaryotic Scytonemin, Sunscreen Compound

UV blocking agent Deposited in the extracellular sheath of some Cyanobacteria Sheathed Cyanobacteria are found in fossil stromatolites

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Organization of the chlorosome (Green Sulfur Bacteria and Chloroflexus)

Very efficient for absorbing light at low intensities Green sulfur bacteria can at the lowest light intensities of any known phototroph

Green Non-Sulfur Bacteria Chloroflexus and Herpetosiphon

From: Adrian et al., 1997. Nature 408:580

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Anabaena

Heterocysts

Heterocysts in the cyanobacterium Anabaena . Heterocysts are the sole site of nitrogen fixation in heterocystous cyanobacteria.

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The Cyanobacterial Heterocyst

Formed from a typical vegetative cell when fixed nitrogen is depleted from the environment Differentiation process; Photosystem II is shut off; Photosystem I remains operational for ATP production Nitrogen is fixed as ammonium (nitrogenase), incorporated into glutamine, and transported to adjoining cells

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