chapter 20 metabolic diversity: phototrophy, autotrophy, chemolithotrophy, and nitrogen fixation
TRANSCRIPT
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)
Figure 20.1
Classification of Phototrophic Organisms
Figure 20.2
Patterns of Photosynthesis
Figure 20.2
Patterns of Photosynthesis
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
Figure 20.3a
Structure and Spectra of Chloro- and Bacteriochlorophyll
Figure 20.3b
Structure and Spectra of Chloro- and Bacteriochlorophyll
Absorption Spectrum
Figure 20.4
Structure of All Known Bacteriochlorophylls
Figure 20.4
Structure of All Known Bacteriochlorophylls
Figure 20.4
Structure of All Known Bacteriochlorophylls
Figure 20.5a
Photomicrograph of Algal Cell Showing Chloroplasts
Figure 20.5b
Chloroplast Structure
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
Figure 20.6
Arrangement of Light-Harvesting Chlorophylls
Figure 20.7
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
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
Figure 20.8
Structure of -carotene, a Typical Carotenoid
Figure 20.9
Structures of Some Common Carotenoids
Figure 20.9
Structures of Some Common Carotenoids
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
Figure 20.10
Phycobiliproteins and Phycobilisomes
Figure 20.11
Absorption Spectrum with an Accessory Pigment
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
Figure 20.12
Membranes in Anoxygenic Phototrophs
Chromatophores Lamellar Membranes in the Purple BacteriumEctothiorhodospira
Figure 20.13
Structure of Reaction Center in Purple Bacteria
Arrangement of Pigment Moleculesin Reaction Center
Figure 20.13
Structure of Reaction Center in Purple Bacteria
Molecular Model of the Protein Structure of the Reaction Center
Figure 20.14
Example of Electron Flow in Anoxygenic Photosynthesis
Purple bacterium
Figure 20.15
Arrangement of Protein Complexes in Reaction Center
Figure 20.16
Map of Photosynthetic Gene Cluster in Purple Phototroph
Bacteriochlorophyll synthesis
Carotenoid synthesis
Figure 20.17
Phototrophic Purple and Green Sulfur Bacteria
Purple Bacterium, Chromatium okenii Green Bacterium, Chlorobium limicola
Figure 20.18
Electron Flow in Purple, Green, Sulfur and Heliobacteria
Bacteriochlorophyll a Bacteriochlorophyll g
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
Figure 20.19
The “Z scheme” in Oxygenic Photosynthesis
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
Figure 20.21a
Key Reactions of the Calvin Cycle
Figure 20.21b
Key Reactions of the Calvin Cycle
Figure 20.21c
Key Reactions of the Calvin Cycle
Figure 20.22
The Calvin Cycle
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
Figure 20.24a
Autotrophic Pathways in Phototrophic Green Sulfur Bacteria
Figure 20.24b
Autotrophic Pathways in Phototrophic Green Nonsulfur Bacteria
(Malonyl-CoA)
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
Figure 20.25
Two Hydrogenases of Aerobic H2 Bacteria
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
Figure 20.26
Sulfur Bacteria
Figure 20.27a
Oxidation of Reduced Sulfur Compounds
Steps in the Oxidation of Different Compounds
Figure 20.27b
Oxidation of Reduced Sulfur Compounds
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
Figure 20.28
Iron-Oxidizing Bacteria
Acid Mine Drainage
Figure 20.28
Iron-Oxidizing Bacteria
Cultures of Acidithiobacillus ferrooxidans shown in dillution series
Figure 20.29
Iron Bacteria Growing at Neutral pH: Sphaerotilus
Figure 20.30
Electron Flow During Fe2+ Oxidation
(Cu-containing protein)
Fe2+ Oxidation in Anoxic Tube CulturesFigure 20.31
Ferrous Iron Oxidation by Anoxygenic Phototrophs
Sterile
Inoculated
Growing culture
Figure 20.31
Ferrous Iron Oxidation by Anoxygenic Phototrophs
Phase-contrast Photomicrograph Of an Iron-Oxidizing Purple Bacterium
Gas vesicles
Iron precipiatates
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
Figure 20.32
Oxidation of Ammonia by Ammonia-Oxidizing Bacteria
Figure 20.33
Oxidation of Nitrite to Nitrate by Nitrifying Bacteria
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
Figure 20.34a-b
Anammox
Phase-contrast Photomicrograph of Brocadia Anammoxidans cells
Transmission Electronic Micrograph of a Cell
Figure 20.34c
Anammoxosome
Reactions in the anammoxosome
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)
Figure 20.35
FeMo-co, the Iron-Molybdenum Cofactor from Nitrogenase
Dinitrogenase
Alternative nitrogenase
- vanadium
- no Mo and Va
Figure 20.36
Nitrogenase Function
Pyruvate ferredoxin oxidoreductase - contains Iron-sulfur center
Pyruvate flavodoxin oxidoreductase - contains FMN
Klebsiella pneumoniae
Figure 20.37
Induction of Slime Formation by O2 in Nitrogen-Fixing Cells
Under micro-oxic conditions
Under air
Very little slime
Extensive slime
Figure 20.38
Reaction of Nitrogen Fixation in S. thermoautotrophicus
Requires less ATPDoes not reduce acetylene
Figure 20.39
The Acetylene Reduction Assay for Nitrogenase Activity
20.15 Genetics and Regulation of Nitrogen Fixation
Highly regulated process because it is such an energy-
demanding process
nif regulon coordinates regulation of genes essential to
nitrogen fixation
Oxygen and ammonia are the two main regulatory
effectors
Figure 20.40
The nif regulon in K. pneumoniae
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
Dinitrogenase: α2β2
Dinitrogenase reductase: α2