Download Patterns of Photosynthesis

Chapter 20
Metabolic Diversity:
Phototrophy, Autotrophy,
Chemolithotrophy, and
Nitrogen Fixation
I. The Phototrophic Way of Life
 20.1 Photosynthesis
 20.2 Chlorophylls and Bacteriochlorophylls
 20.3 Carotenoids and Phycobilins
 20.4 Anoxygenic Photosynthesis
 20.5 Oxygenic Photosynthesis
20.1 Photosynthesis
 Photosynthesis is the most important biological process
on Earth
 Phototrophs are organisms that carry out photosynthesis
 Most phototrophs are also autotrophs
 Photosynthesis requires light-sensitive pigments called
chlorophyll
 Photoautotrophy requires ATP production and CO2
reduction
 Oxidation of H2O produces O2 (oxygenic photosynthesis)
 Oxygen not produced (anoxygenic photosynsthesis)
Classification of Phototrophic Organisms
Figure 20.1
Patterns of Photosynthesis
Figure 20.2
Patterns of Photosynthesis
Figure 20.2
20.2 Chlorophylls and Bacteriochlorophylls
 Organisms must produce some form of chlorophyll (or
bacteriochlorophyll) to be photosynthetic
 Chlorophyll is a porphyrin
 Number of different types of chlorophyll exist
 Different chlorophylls have different absorption spectra
 Chlorophyll pigments are located within special
membranes
 In eukaryotes, called thylakoids
 In prokaryotes, pigments are integrated into cytoplasmic
membrane
Structure and Spectra of Chloro- and Bacteriochlorophyll
Figure 20.3a
Structure and Spectra of Chloro- and Bacteriochlorophyll
Absorption Spectrum
Figure 20.3b
Structure of All Known Bacteriochlorophylls
Figure 20.4
Structure of All Known Bacteriochlorophylls
Figure 20.4
Structure of All Known Bacteriochlorophylls
Figure 20.4
Photomicrograph of Algal Cell Showing Chloroplasts
Figure 20.5a
Chloroplast Structure
Figure 20.5b
20.2 Chlorophylls and Bacteriochlorophylls
 Reaction centers participate directly in the conversion
of light energy to ATP
 Antenna pigments funnel light energy to reaction
centers
 Chlorosomes function as massive antenna complexes
 Found in green sulfur and nonsulfur bacteria
Arrangement of Light-Harvesting Chlorophylls
Figure 20.6
The Chlorosome of Green Sulfur and Nonsulfur Bacteria
Electron Micrograph of Cell of Green Sulfur Bacterium
Figure 20.7
The Chlorosome of Green Sulfur and Nonsulfur Bacteria
Model of Chromosome Structure
Figure 20.7
20.3 Carotenoids and Phycobilins
 Phototrophic organisms have accessory pigments in
addition to chlorophyll, including carotenoids and
phycobiliproteins
 Carotenoids
 Always found in phototrophic organisms
 Typically yellow, red, brown, or green
 Energy absorbed by carotenoids can be transferred to a
reaction center
 Prevent photo-oxidative damage to cells
Structure of -carotene, a Typical Carotenoid
Figure 20.8
Structures of Some Common Carotenoids
Figure 20.9
Structures of Some Common Carotenoids
Figure 20.9
20.3 Carotenoids and Phycobilins
 Phycobiliproteins are main antenna pigments of
cyanobacteria and red algae
 Form into aggregates within the cell called
phycobilisomes
 Allow cell to capture more light energy than chlorophyll
alone
Phycobiliproteins and Phycobilisomes
Figure 20.10
Absorption Spectrum with an Accessory Pigment
Figure 20.11
20.4 Anoxygenic Photosynthesis
 Anoxygenic photosynthesis is found in at least four
phyla of Bacteria
 Electron transport reactions occur in the reaction
center of anoxygenic phototrophs
 Reducing power for CO2 fixation comes from
reductants present in the environment (i.e., H2S, Fe2+,
or NO2-)
 Requires reverse electron transport for NADH production
in purple phototrophs
Membranes in Anoxygenic Phototrophs
Chromatophores
Lamellar Membranes in the Purple Bacterium
Ectothiorhodospira
Figure 20.12
Structure of Reaction Center in Purple Bacteria
Arrangement of Pigment Molecules
in Reaction Center
Figure 20.13
Structure of Reaction Center in Purple Bacteria
Molecular Model of the Protein Structure
of the Reaction Center
Figure 20.13
Example of Electron Flow in Anoxygenic Photosynthesis
Purple bacterium
Figure 20.14
Arrangement of Protein Complexes in Reaction Center
Figure 20.15
Map of Photosynthetic Gene Cluster in Purple Phototroph
Bacteriochlorophyll synthesis
Carotenoid synthesis
Figure 20.16
Phototrophic Purple and Green Sulfur Bacteria
Purple Bacterium, Chromatium okenii
Green Bacterium, Chlorobium limicola
Figure 20.17
Electron Flow in Purple, Green, Sulfur and Heliobacteria
Bacteriochlorophyll a
Bacteriochlorophyll g
Figure 20.18
20.5 Oxygenic Photosynthesis
 Oxygenic phototrophs use light to generate ATP and
NADPH
 The two light reactions are called photosystem I and
photosystem II
 “Z scheme” of photosynthesis
 Photosystem II transfers energy to photosystem I
 ATP can also be produced by cyclic photophosphorylation
The “Z scheme” in Oxygenic Photosynthesis
Figure 20.19
II. Autotrophy
 20.6 The Calvin Cycle
 20.7 Other Autotrophic Pathways in Phototrophs
20.6 The Calvin Cycle
 The Calvin Cycle
 Named for its discoverer Melvin Calvin
 Fixes CO2 into cellular material for autotrophic growth
 Requires NADPH, ATP, ribulose 1,5-bisphophate
carboxylase (RubisCO), and phosphoribulokinase
 6 molecules of CO2 are required to make 1 molecule
of glucose
Key Reactions of the Calvin Cycle
Figure 20.21a
Key Reactions of the Calvin Cycle
Figure 20.21b
Key Reactions of the Calvin Cycle
Figure 20.21c
The Calvin Cycle
Figure 20.22
20.7 Other Autotrophic Pathways in Phototrophs
 Green sulfur bacteria use the reverse citric acid cycle
to fix CO2
 Green nonsulfur bacteria use the hydroxyproprionate
pathway to fix CO2
Autotrophic Pathways in
Phototrophic Green Sulfur Bacteria
Figure 20.24a
Autotrophic Pathways in
Phototrophic Green Nonsulfur Bacteria
(Malonyl-CoA)
Figure 20.24b
Iginicoccus hospitalis, an archaeum, uses
a new CO2 fixation pathway?
III. Chemolithotrophy
 20.8
The Energetics of Chemolithotrophy
 20.9
Hydrogen Oxidation
 20.10 Oxidation of Reduced Sulfur Compounds
 20.11 Iron Oxidation
 20.12 Nitrification
 20.13 Anammox
20.8 The Energetics of Chemolithotrophy
 Chemolithotrophs are organisms that obtain energy
from the oxidation of inorganic compounds
 Mixotrophs are chemolithotrophs that require organic
carbon as a carbon source
 Many sources of reduced molecules exist in the
environment
 The oxidation of different reduced compounds yields
varying amounts of energy
Energy Yields from Oxidation of Inorganic Electron Donors
20.9 Hydrogen Oxidation
 Anaerobic H2 oxidizing Bacteria and Archaea are
known
 Catalyzed by hydrogenase
 In the presence of organic compounds such as
glucose, synthesis of Calvin cycle and hydrogenase
enzymes is repressed
Two Hydrogenases of Aerobic H2 Bacteria
Figure 20.25
20.10 Oxidation of Reduced Sulfur Compounds
 Many reduced sulfur compounds are used as electron donors
 Discovered by Sergei Winogradsky
 H2S, S0, S2O3- are commonly used
 One product of sulfur oxidation is H+, which results in a
lowering of the pH of its surroundings
 Sox system oxidizes reduced sulfur compounds directly to
sulfate (e.g. Paracoccus pantotrophus, etc., 15 genes)
 Usually aerobic, but some organisms can use nitrate as an
electron acceptor
Sulfur Bacteria
Figure 20.26
Oxidation of Reduced Sulfur Compounds
Steps in the Oxidation of Different Compounds
Figure 20.27a
Oxidation of Reduced Sulfur Compounds
Figure 20.27b
20.11 Iron Oxidation
 Ferrous iron (Fe2+) oxidized to ferric iron (Fe3+)
 Ferric hydroxide precipitates in water
 Many Fe oxidizers can grow at pH <1
 Often associated with acidic pollution from coal mining
activities
 Some anoxygenic phototrophs can oxidize Fe2+
anaerobically using Fe2+ as an electron donor for CO2
reduction
Iron-Oxidizing Bacteria
Acid Mine Drainage
Figure 20.28
Iron-Oxidizing Bacteria
Cultures of Acidithiobacillus ferrooxidans shown
in dillution series
Figure 20.28
Iron Bacteria Growing at Neutral pH: Sphaerotilus
Figure 20.29
Electron Flow During Fe2+ Oxidation
(Cu-containing protein)
Figure 20.30
Ferrous Iron Oxidation by Anoxygenic Phototrophs
Growing culture
Inoculated
Sterile
Fe2+ Oxidation in Anoxic Tube Cultures
Figure 20.31
Ferrous Iron Oxidation by Anoxygenic Phototrophs
Gas vesicles
Iron
precipiatates
Phase-contrast Photomicrograph
Of an Iron-Oxidizing Purple Bacterium
Figure 20.31
20.12 Nitrification
 NH3 and NO2- are oxidized by nitrifying bacteria during the
process of nitrification
 Two groups of bacteria work in concert to fully oxidize
ammonia to nitrate
 Key enzymes are ammonia monooxygenase,
hydroxylamine oxidoreductase, and nitrite oxidoreductase
 Only small energy yields from this reaction
 Growth of nitrifying bacteria is very slow
Oxidation of Ammonia by Ammonia-Oxidizing Bacteria
Figure 20.32
Oxidation of Nitrite to Nitrate by Nitrifying Bacteria
Figure 20.33
20.13 Anammox
 Anammox: anoxic ammonia oxidation
- NH4+ + NO2- → N2 + 2 H2O
 Performed by unusual group of obligate anaerobes
 Anammoxosome is a membrane-enclosed compartment
where anammox reactions occur
 Lipids that make up the anammoxsome are not the typical
lipids of Bacteria
 Protects cell from reactions occuring during anammox
 Hydrazine (H2N=NH2), a very strong reductant, is an
intermediate of anammox
 Anammox is very beneficial in the treatment of sewage
and wastewater
Anammox
Phase-contrast Photomicrograph of
Brocadia Anammoxidans cells
Transmission Electronic Micrograph of a Cell
Figure 20.34a-b
Anammoxosome
Reactions in the anammoxosome
Figure 20.34c
Autotrophy in Anammox Bacteria
 Autotrophy
- NO2-, actually hydrazine (N2H4), is an electron donor for the
reduction of CO2
 Lack Calvin cycle enzyme
 Use an acetyl-CoA pathway
Reactions of the Acetyl-CoA Pathway
Figure 21.18
IV. Nitrogen Fixation
 20.14 Nitrogenase and Nitrogen Fixation
 20.15 Genetics and Regulation of Nitrogen Fixation
20.14 Nitrogenase and Nitrogen Fixation
 Only certain prokaryotes can fix nitrogen
 Some nitrogen fixers are free living and others are
symbiotic
 Reaction is catalyzed by nitrogenase
 Sensitive to the presence of oxygen
 A wide variety of nitrogenases use different metal
cofactors
 Nitrogenase activity can be assayed using the acetylene
reduction assay (C2H2 → C2H4)
FeMo-co, the Iron-Molybdenum Cofactor from Nitrogenase
Dinitrogenase
Alternative nitrogenase
- vanadium
- no Mo and Va
Figure 20.35
Nitrogenase Function
Pyruvate ferredoxin oxidoreductase
- contains Iron-sulfur center
Pyruvate flavodoxin oxidoreductase
- contains FMN
Klebsiella pneumoniae
Figure 20.36
Induction of Slime Formation by O2 in Nitrogen-Fixing Cells
Extensive slime
Very little
slime
Under micro-oxic
conditions
Under air
Figure 20.37
Reaction of Nitrogen Fixation in S. thermoautotrophicus
Requires less ATP
Does not reduce acetylene
Figure 20.38
The Acetylene Reduction Assay for Nitrogenase Activity
Figure 20.39
20.15 Genetics and Regulation of Nitrogen Fixation
 Highly regulated process because it is such an energydemanding process
 nif regulon coordinates regulation of genes essential to
nitrogen fixation
 Oxygen and ammonia are the two main regulatory
effectors
The nif regulon in K. pneumoniae
Dinitrogenase: α2β2
Dinitrogenase reductase: α2
NtrC: active in the presence of limited amount of fixed nitrogen compounds
and promotes transcription of regulate the expression of nifAL
NifA: a positive regulator
NifL: a negative regulator containing FAD, a redox coenzyme
Figure 20.40