Chapter 20-Microbial Metabolic Diversity

Microbial Diversity 

REDOX
Oxidative/reduction reaction

One molecule gives another molecule an e-
Carbon and Energy Sources
Photoautotrophs
Carbon source is carbon dioxide
Energy source is sunlight
Reduction of CO2 to organic compounds
Photoheterotrophs: Use organic carbon as their carbon source
Energy is through a series of oxidation/reductions
Photosynthesis
Photosynthesis: The conversion of light energy to chemical energy.
Photosynthesis involves reactions in which ATP is generated and reactions in which ATP is consumed in the reduction of NADH.


Photoautotroph
Two distinct set of reactions
ATP production and
CO2 reduction to organic compounds
Energy is supplied from ATP
Electrons for the reduction of CO2 come from NADH or NADPH
Produced by electrons originating from various electron donors
Variety of Pigments
Chlorophylls a and b
Bacteriochlorophyll
Carotenoids
Anthocyanins
Phycobilins
Light reaches phototrophic organisms in units of energy called quanta.

Photosynthetic Pigments
In the photosynthetic membrane, chlorophyll or bacteriochlorophyll are associated with proteins to form complexes consisting of 50 to 300 molecules.

Chlorophyll: light sensitive, Mg++ containing porphyrin of photosynthetic organisms that initiates the process of photophosphorylation. Pigment of oxygenic phototrophs.

Bacteriochlorophyll: the chlorophyll pigment of anoxygenic phototrophs.

Photophosphorylation: the production of ATP in photosynthesis


Prokaryotes
No chloroplasts
Pigments are integrated into the internal membrane system
Within invaginations of the cytoplasmic membrane
Or within cytoplasmic membrane itself
Chlorosomes: specialized non-unit membrane enclosed structures.
Chlorophylls
Main pigments in most photoautotrophs
Carotenoids and Phycobilins
Carotenoids:
Most widespread accessory pigments
Hydrophobic pigments embedded in membrane
Photoprotective role: Quench toxic oxygen
Transfers energy to the reaction center which is used to make ATP
Phycobilins
Cyanobacteria and red algae
Accessory Pigments

Light-Dependent Reactions
Pigments absorb light energy, give up electron, which enter electron transfer chains.
Water molecules split, ATP and NADH form, and oxygen is released.
Pigments that give up electrons get replacement electrons.
Photosystem Function: Harvester Pigments
Most pigments in photosystem are harvester pigments

When excited by light energy, these pigments transfer energy to adjacent pigment molecules. Each transfer involves energy loss.

Antenna: chlorophyll molecules harvest light energy and transfer it on to the reaction center of the pigments.

Exciton: mobile forms of energy (photons) that migrate through the antenna pigments to the reaction center

Oxygenic Photosynthesis
In oxygenic photosynthesis, water donates electrons to drive autotrophy (CO2), and oxygen is produced as a by-product.

Two separate light reaction center are involved, photosystems I and II.

Anoxygenic Photosynthesis
Anoxygenic Photosynthesis: Photosynthesis in which O2 is not produced
Photosynthesis begins when exciton energy strikes the bacteriochlorophyll a molecules.
The absorption of energy excites the pigments, converting them to strong electron donors with a low reduction potential.
The energy is then released when electrons are transported through the membrane.

Reverse Electron Flow
Electrons from the quinone pool must be forced backward to reduce NAD to NADH
Light-Independent Reactions
Synthesis part of photosynthesis
Can proceed in the dark
Takes place in the stroma
Calvin-Benson cycle
Autotrophic Fixation: The Calvin Cycle
Most phototrophic and other autotrophic organisms accomplish fixation of CO2 by the Calvin cycle, in which the enzyme ribulose bisphosphate carboxylase (RubisCO) plays a key role.

Calvin-Benson Cycle
Overall reactants
Carbon dioxide
ATP
NADPH oxidized to
Overall products
Glucose
ADP
NADP+

Other Types of Energy Production
Chemolithotrophs oxidize inorganic chemicals as their sole sources of energy and reducing power.

Most chemolithotrophs are also able to grow autotrophically.

Mixotrophic: able to obtain energy from the oxidation of an inorganic compound. They require an organic compound as a carbon source.

Chemolithotrophs: ATP generation is similar to that in chemo-organotrophs, except that the electron donor is inorganic.

Chemolithotrophy-Misc.
Sources of inorganic electron donors: geological, biological or anthropogenic in nature.
The hydrogen bacteria can oxidize H2 compounds
The sulfur bacteria can oxidize reduced sulfur compounds such as H2S and S0
The iron bacteria are chemolithotrophs that use ferrous iron (Fe2+)
Anaerobes reduce CO2 to acetate, usually with H2 as the electron donor

Energy Yields from Oxidation of Inorganic Electron Donors
Sulfur Bacteria
Iron-Oxidizing Bacteria
Electron Flow During Fe2+ Oxidation
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
Oxidation of Nitrite to Nitrate by Nitrifying Bacteria
Anammox
Anammox: anoxic ammonia oxidation
Performed by unusual group of obligate aerobes
Anammoxosome is compartment where anammox reactions occur
Protects cell from reactions occuring during anammox
Hydrazine is an intermediate of anammox
Anammox is very beneficial in the treatment of sewage and wastewater
Anammox

Anaerobic Respiration
The use of an alternate electron acceptor other than O2 for reduction.
Assimilative Metabolism: When an inorganic compound such as NO3, SO4 or CO2 is reduced for biosynthesis
Dissimilative metabolism : The reduced product is excreted into the environment

Fermentation
When inorganic electron acceptors are not present in anoxic environments, carbon is catabolized( broken down) by fermentation.
Fermentations are classified in terms of either the substrate fermented or the fermentation products formed.
Products: Amino acids, organic acids, purines and pyrimidines, alcohols, sugars

Lactic and Mixed-Acid Fermentations

Lactic acid fermentation can occur by homofermentative and heterofermentative pathways

Mixed-Acid Fermentations
Generate acids
Acetic, lactic, and succinic
Sometimes also generate neutral products
E.g., butanediol
Characteristic of enteric bacteria
The Butyric Acid and Butanol/Acetone Fermentation-Clostridia
Entner-Doudoroff Pathway-pathway of sugar carabolism for pseudomonas.
Some Clostridium species ferment amino acids using a complex biochemical pathway known as the Strickland reaction
Propionic Acid fermentation-P. acnes

Non-Substrate-Level Phosphorylation Fermentations
Fermentations of certain compounds do not yield sufficient energy to synthesize ATP
Catabolism of the compound can then be linked to ion pumps that establish a proton or sodium motive force

Nitrification
Nitrate is commonly used as an electron acceptor in anaerobic respiration. Its use requires the enzyme nitrate reductase, which reduces nitrate to nitrite.
Nitrification: the use of ammonia and nitrite as electron donors.
NH4 + NO2 N2 + 2 H20
Ammonia + nitrite Nitrous gas
Denitrification: use of nitrate (NO4) in anaerobic respiration to produce N2
Nitrogen fixation —the reduction of N2 to NH3 (ammonia)
The most widespread inorganic nitrogen compounds in nature are ammonia and nitrate, both formed in the atmosphere.

Methanotrophy, Methylotrophyand Methanogenisis
Methanogenesis is the biological production of CH4 (methane) either from CO2 plus H2 or from methylated compounds

Methanotrophy is the use of CH4 as a carbon and energy source.

Methylotrophs use C1 compounds (or other organic compounds lacking C–C bonds) for energy metabolism and biosynthesis.
Polysaccharides, Organic Acids, Fats
Polysaccharides are abundant in nature and can be broken down, usually by phosphorolysis, into hexose (6) or pentose (5) monomers and used as energy sources
Starch and cellulose are common polysaccharides.
Organic acids are frequently metabolized through the citric acid cycle.
Fats are metabolized via hydrolysis by lipases or phospholipases to free fatty acids.
The fatty acids are oxidized by beta oxidation to acetyl-CoA units, which are subsequently oxidized to CO2 by the citric acid cycle