Aerobic Proteobacteria: Methylotrophs & Methanotrophs

Aerobic Chemo-organotrophic Proteobacteria

• Organisms to be discussed include Methylotrophs, Methanotrophs, Acetic Acid Bacteria and Free-Living Aerobic Nitrogen-Fixing Bacteria.

Methylotrophs

• Methylotrophs utilize CH₄ and other one-carbon (C1) compounds as electron donors and a carbon source.
• These organisms are widespread in soil and water, are obligate aerobes and morphologically diverse.
• They can grow using carbon compounds lacking C-C bonds.
• Most, but not all, methylotrophs are also methanotrophs.

Methanotrophs

• Methanotrophs are a subset of methylotrophs capable of using CH₄ as a substrate for growth.
• They possess the key enzyme methane monooxygenase.
• This enzyme incorporates an atom of oxygen from O₂ into methane to produce methanol.
• Methanotrophs contain large amounts of sterols.
• There are two major groups of Methanotrophs: Type I and Type II.
• They contain extensive internal membrane systems for methane oxidation.

Type I Methanotrophs

• Type I Methanotrophs assimilate C1 compounds via the ribulose monophosphate cycle.
• They are part of the Gammaproteobacteria class.
• Membranes are arranged as bundles of disc-shaped vesicles.
• They lack a complete citric acid cycle.
• Type I Methanotrophs are obligate methylotrophs.

Type II Methanotrophs

• Type II Methanotrophs assimilate C1 compounds via the serine pathway.
• They are part of the Alphaproteobacteria class.
• Feature paired membranes that run along the periphery of the cell.

Ecology of Methanotrophs

• Methanotrophs are widespread in aquatic and terrestrial environments
• These organisms convert methane back into cell material and CO₂.
• They play an important role in global carbon cycling.
• Some marine mussels have symbiotic relationships with methanotrophs.
• Some marine mussels live near hydrocarbon seeps where methane is released.
• Gills consume methane at high rates in the presence of O₂.
• Bacterial symbionts are found in vacuoles within the cells near the gill surface.
• These symbionts are methanotrophs.
• Methanotrophs assimilate the methane and distribute it as organic compounds.
• This process is conceptually similar to what happens with sulfide-oxidizing chemolithotrophs and hydrothermal vent tube worms.

Acetic Acid Bacteria

• Key genera of Acetic Acid Bacteria include Acetobacter and Gluconobacter.
• These organisms carry out incomplete oxidation of alcohols and sugars and leads to the accumulation of organic acids as end products.
• They are motile rods, aerobic and Gram-negative.
• Acetic Acid Bacteria have a high tolerance to acidic conditions.
• Most strains grow well at pH values lower than 5.
• They consist of petrichously and polarly flagellated organisms.
• Acetobacter group is petrichously flagellated.
• Gluconobacter group is polarly flagellated.
• All group phylogenetically within the Alphaproteobacteria class.
• Acetobacter and Gluconobacter differ in that Acetobacter can further oxidize acetic acid to CO₂.
• Acetobacter have a complete citric acid cycle.
• Typically found in alcoholic juices (wine, beer).
• Used in the production of sorbose.
• Acetic Acid Bacteria oxidize sorbitol to sorbose.
• Used in biotransformation.
• Sorbose is used in the formation of ascorbic acid (Vitamin C).
• Acetic Acid Bacteria can synthesize cellulose.
• Used in the commercial production of vinegar.
• Colonies can be identified on CaCO3 agar plates containing ethanol.

Free-Living Aerobic Nitrogen-Fixing Bacteria

• Diazotrophs fix N₂ into NH₃ which can be assimilated as a source of N in cells.
• A variety of soil microbes are capable of fixing N₂ aerobically.
• Azotobacter was discovered by Martinus Beijerinick.
• Belong phylogenetically to Alpha, Beta or Gamma Proteobacteria.
• Major genera capable of fixing N₂ nonsymbiotically are Azotobacter, Azospirillium, and Beijerinckia.
• Large, with many isolates being almost the size of yeast; diameter 2-4 um or more.
• Bacteria are pleomorphic in shape, motile by petrichous flagella.
• Extensive capsules or slime layers are produced when fixing nitrogen on carbohydrate containing media.
• Azotobacter is an obligate aerobe.
• Azotobacter's nitrogenase is O₂ sensitive.
• The capsular slime helps to protect the nitrogenase from O₂.
• Azotobacter can grow on many different carbohydrates, alcohols and organic acids.
• Metabolism of carbon compounds is strictly oxidative.
• Acids or other fermentative products are never produced.
• All members fix nitrogen but can also grow on simple forms of combined nitrogen: ammonia, urea, nitrate.
• Azotobacter can form resting structures called cysts.
• Cysts show negligible endogenous respiration and are resistant to desiccation, mechanical disintegration and UV and ionizing radiation.
• In contrast to endospores, cysts are not especially heat resistant and not completely dormant.
• Additional genera of free-living N₂ fixers include acid-tolerant microbes Azomonas:
  • These do not produce cysts
  • Have a large coccus to rod shape
  • Primarily aquatic
• Beijerinckia and Derxia grow well on acidic soils.
• Azospirillum contains rod-spirillum shaped bacteria.
• Azospirillum forms non-specific symbiotic associations with plants, especially corn.