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Today's Lecture: "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

Figure 1.18 Geological and evolutionary timetable

Time: Archaeal and Proterozoic ages (over 1.5 billion years ago) Cyanobacteria evolved the ability to utilize H2O as an e donor

·H2O 2H+ + 1/2 O2 ·O2 was released in the atmosphere ·Soluble Fe2+ is oxidized to insoluble Fe3+ ·BIFs

Banded Iron Formation, Dales Gorge, Western Australia

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.

(Algae, Cyanobacteria)

Structure of chlorophyll a

(Purple bacteria)

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

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

Figure 9.2 Photosynthetic unit

Structure of Carotenoids

Photoprotective agents Transfer energy to the reaction centers

Accessory Pigments: Phycobilisome of Cyanobacteria

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

Chromophores of Phycobilisomes

620 nm

550 nm

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).

Reaction center of purple phototrophic bacterium

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).

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

Map of the photosynthetic gene cluster of the purple phototrophic bacterium, Rhodobacter capsulatus. Genes are arranged in superoperons where transcripts of pigment biosynthesis operons extend through to include transcription of polypeptides of the photosynthetic complexes. The bch genes, which encode bacteriochlorophyll synthesis proteins, are shown in green, while crt genes, which encode proteins that synthesize carotenoids, are shown in red. Genes encoding reaction center polypeptides (puh and puf genes) are shown in blue, and genes encoding light-harvesting I polypeptides (B870 complex) (puf genes) are shown in yellow.

Electron Flow in Anoxygenic Photosynthesis

Reducing power to reduce CO2 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.

Comparative Electron Flow in Anoxygenic Photosynthetic Bacteria

Reducing power to reduce CO2 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.

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 ironsulfur protein; Fd, ferredoxin; Fp, flavoprotein; P680 and P700 are the reaction center chlorophylls of PS II and PS I, respectively.

The Calvin Cycle (Cyanobacteria)

The Reverse Citric Acid Cycle (Chlorobium)

The Hydroxypropionate Pathway (Chloroflexus)

From: Jannasch, 1995. Geophysical Monograph 91:273

Box 21.2 Cellular Absorption Spectra of Photosynthetic Bacterial Groups

Microbial mat communities

Cyanobacteria Purple sulfur bacteria

Green sulfur bacteria

Sippewisset Salt Marsh, Cape Cod, MA

Phototrophs in microbial mat

Box 21.3 Scytonemin, an Ancient Prokaryotic Sunscreen Compound

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

Organization of the chlorosome (Green Sulfur Bacteria and Chloroflexus)

Very efficient for absorbing light at low intensities

Green Non-Sulfur Bacteria Chloroflexus and Herpetosiphon

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

Aerobic Anoxygenic Phototrophic Bacteria

Strictly aerobic Produce Bchl a Capable of photophosphorylation Require organic Carbon for growth -Proteobacteria

98 42 42 64 90

NAP1 Erythrobacter longus Erythrobacter litoralis

Erythrobacter citreus Citromicrobium bathyomarinum Erythromicrobium ramosum


Porphyrobacter neustonensis Erythromonas ursincola


From: Kolber et al., 2000. Nature 407:177



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

The Cyanobacterial Heterocyst

Nitrogen Fixation in Trichodesmium

From: Berman-Frank et al., 2001. Science 294:1534

Prochlorotrix and Prochlorococcus

Small-scale diversity in 16S rRNA genes may represent adaptive radiation of species


From: Moore et al., 1998. Nature 393:464


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