AEROBIC ORGANOTROPHIC PROTEOBACTERIA
AEROBIC ORGANOTROPHIC PROTEOBACTERIA
### Dr Sherline Brown
Table of Contents
Aerobic Organotrophic Proteobacteria
Methylotrophs and Methanotrophs
2.1 Overview of Methylotrophs
2.2 Characteristics of Methylotrophs
2.3 Methanotrophs
2.3.1 Types of Methanotrophs
Acetic Acid Bacteria
3.1 Characteristics of Acetic Acid Bacteria
3.2 Ecological Roles of Acetic Acid Bacteria
Free-Living Aerobic Nitrogen-Fixing Bacteria
4.1 Overview of Diazotrophs
4.2 Characteristics of Free-Living Nitrogen-Fixers
4.3 Major Genera of Free-Living Nitrogen-Fixing Bacteria
Aerobic Organotrophic Proteobacteria
1. Introduction to Aerobic Organotrophic Proteobacteria
Proteobacteria are a major group of Gram-negative bacteria that include a wide variety of pathogens, nitrogen-fixers, and other microbes involved in important biogeochemical cycles.
2. Methylotrophs and Methanotrophs
2.1 Overview of Methylotrophs
Definition: Methylotrophs are organisms that utilize one-carbon (C1) compounds, such as methane (CH4) and other compounds that lack C-C bonds, as their electron donors and sources of carbon.
Habitat: They are commonly found in soil and aquatic environments.
Classification: Most methylotrophs are obligate aerobes and exhibit morphologically diverse structures.
2.2 Characteristics of Methylotrophs
They can grow using C1 compounds without C-C bonds.
Most are also known as methanotrophs, which specifically utilize CH4 as a substrate for growth.
2.3 Methanotrophs
Definition: Methanotrophs are a specific group of methylotrophs that consume methane.
Key Enzyme: They possess methane monooxygenase, an enzyme that catalyzes the incorporation of oxygen from O2 into methane, resulting in the production of methanol.
Unique Feature: Methanotrophs contain significant amounts of sterols, which is unusual among bacteria.
Ecological Importance: They play a critical role in methane oxidation processes, which are essential for global carbon cycling.
2.3.1 Types of Methanotrophs
Type I Methanotrophs
Classification: Belong to Gammaproteobacteria.
Carbon Assimilation: They assimilate C1 compounds via the ribulose monophosphate cycle.
Membrane Structure: Their membranes are arranged in bundles of disc-shaped vesicles.
Metabolism: They lack a complete citric acid cycle and are obligate methylotrophs.
Type II Methanotrophs
Classification: Belong to Alphaproteobacteria.
Carbon Assimilation: They utilize the serine pathway for C1 compound assimilation.
Membrane Structure: Their paired membranes run along the periphery of the cell.
Acetic Acid Bacteria
3. Introduction to Acetic Acid Bacteria
Key Genera: Includes Acetobacter and Gluconobacter.
Function: These bacteria carry out the incomplete oxidation of alcohols and sugars, leading to the production of organic acids as end products.
Morphological Characteristics: They are typically motile rods, possess high tolerance to acidic conditions, and are classified as Gram-negative bacteria.
3.1 Characteristics of Acetic Acid Bacteria
pH Tolerance: They thrive at pH levels lower than 5, demonstrating their ability to endure acidic environments.
Flagella: They consist of peritrichously flagellated organisms (Acetobacter) or polarly flagellated ones (Gluconobacter).
Phylogenetics: All are phylogenetically associated with Alphaproteobacteria.
Metabolic Ability: Acetobacter can oxidize acetic acid to CO2 because it contains a complete citric acid cycle, unlike Gluconobacter.
3.2 Ecological Roles of Acetic Acid Bacteria
Habitats: Commonly found in alcoholic fermentation products like wine and beer.
Commercial Use: Utilized in the production of sorbose, which is an intermediate for synthesizing ascorbic acid (Vitamin C).
Cellulose Production: Acetic acid bacteria can synthesize cellulose.
Identification: Their colonies can be identified on calcium carbonate agar plates where they form clear zones due to the dissolution of the calcium carbonate.
Free-Living Aerobic Nitrogen-Fixing Bacteria
4. Overview of Diazotrophs
Definition: Diazotrophs are microorganisms capable of fixing nitrogen gas (N2) into ammonia (NH3), a form of nitrogen that can be assimilated by cells.
Notable Member: Azotobacter, discovered by Martinus Beijerinck, is a well-studied diazotroph belonging to various groups within the Proteobacteria.
4.1 Characteristics of Free-Living Nitrogen-Fixers
They show a variety of phylogenetic distributions across Alpha, Beta, and Gamma Proteobacteria.
Significant in agricultural soils and ecosystems for facilitating nitrogen availability.
Key genera include Azotobacter, Azomonas, and Beijerinckia.
4.2 Major Genera of Free-Living Nitrogen-Fixing Bacteria
Table 15.12: Genera of Free-Living Aerobic Nitrogen-Fixing Bacteria
Genus | Characteristics | Phylogenetic Group |
|---|---|---|
Azotobacter | Large rod; produces cysts; primarily found in neutral to alkaline soils | Gammaproteobacteria |
Azomonas | Large rod; does not produce cysts; primarily aquatic | Gammaproteobacteria |
Azospirillum | Microaerophilic rod; forms associations with plants | Alphaproteobacteria |
Beijerinckia | Pear-shaped rod with large lipid bodies; produces extensive slime; inhabits acidic soils | Alphaproteobacteria |
4.3 Additional Characteristics of Free-Living Nitrogen-Fixing Bacteria
Typical size is in the range of 2-4 um or larger.
They are pleomorphic, meaning they can exhibit various shapes and structures.
Often motile, utilizing peritrichous flagella for movement.
Known for producing substantial capsules or slime layers during nitrogen fixation when grown on carbohydrate-containing media.
Example of slime production is illustrated with images demonstrating Derxia gummosa and Beijerinckia sp.
Notably, Azotobacter is an obligate aerobe, but its nitrogenase is sensitive to O2; hence, the slime provides protection against oxygen.
They are capable of utilizing varied carbohydrates, alcohols, and organic acids for growth.
Unlike fermenters, their metabolism is strictly oxidative without producing acids or fermentative by-products.
All members can fix nitrogen and can thrive on simpler nitrogen forms like ammonia, urea, and nitrate.
Azotobacter can form resting structures known as cysts, which have resistance to desiccation and radiation, though they are not as heat resistant as endospores.
Additional genera such as Azomonas, Beijerinckia, and Derxia can thrive in acidic soils, with Azospirillum known for forming beneficial associations with corn.