Chemolithotrophic Proteobacteria and Nitrification

Chemolithotrophic Proteobacteria

• Chemolithotrophs are prokaryotes
• Capable of oxidizing inorganic electron donors
• Can utilize CO₂ as their sole carbon source in many instances
• Capable of oxidizing reduced sulfur, nitrogen compounds, and hydrogen

Nitrifying Bacteria

• Key genera include Nitrosomas and Nitrobacter
• Microorganisms grow chemolithotrophically using reduced nitrogen compounds
• Found across all 4 classes of Proteobacteria
• Nitrification is the oxidation of NH₄⁺ to NO3- by nitrifying bacteria
• Reaction: NH₄⁺ → NO2⁻ (Nitrosomas)
• Reaction: NO2⁻ → NO3⁻ (Nitrobacter)
• Process occurs under oxic conditions

Nitrosifying Bacteria

• NH3 + O2 + 2 e¯ + 2 H+→NH2OH + H₂O
• NH2OH + H2O + ½ O2→NO2⁻ + 2 H2O + H+
• Sum: NH3 + 1½ O2 → NO2⁻ + H2O
• ΔGº′ = −288 kJ/reaction

Nitrifying Bacteria

• NO2⁻ + ½ O2→NO3⁻
• ΔGº′ = −74.1 kJ/reaction
• Many species possess internal membrane systems that house key enzymes in nitrification
• Ammonia monooxygenase oxidizes NH3 to NH₂OH (Hydroxylamine)
• Hydroxylamine oxidoreductase converts hydroxylamine to nitrite
• Nitrite oxidoreductase oxidizes NO₂⁻ to NO3⁻
• Nitrosifying bacteria typically have genus names beginning with "Nitroso”
• Genus name of nitrate producers begin with "Nitro”
• First bacteria shown to grow chemolithotrophically
• Widespread in soil and water
• Present in highest numbers in sewage treatment facilities
• Develop well in streams that have had an input of sewage or other wastewater
• This is due to high ammonia content
• Most are obligate chemolithotrophs and obligate aerobes
• Nitrobacter is an exception
• Nitrobacter can grow chemoorganotrophically on acetate or pyruvate

Sulfur- and Iron-Oxidizing Bacteria

• Key genera include Thiobacillus, Acidithiobacillus, Achromatium, and Beggiatoa
• Ability to grow chemolithotrophically on reduced sulfur compounds is a property of a diverse group of Proteobacteria
• Two broad ecological classes exist:
  • Those living at neutral pH (neutrophiles)
  • Those living at acid pH (acidophiles)
• Acidophiles can grow chemolithotrophically using ferrous iron Fe²⁺

Thiobacillus

• Gram-negative and rod-shaped bacteria
• Best studied of the sulfur chemolithotrophs
• Species can be found in Alpha, Beta and Gamma classes
• Sulphur compounds best used as donors include H₂S, Sº, and S₂O3²⁻
• Generate large amounts of sulfuric acid when use as donors
• Several Thiobacilli are acidophilic

Acidithiobacillus ferrooxidans

• Acidophilic species
• Major biological agent for the oxidation of ferrous iron
• Used in the oxidation of the iron ore pyrite (FeS₂)
• Bioleaching releases the iron from the ore, which can be beneficial
• Ecologically disastrous due to environment acidification and contamination with heavy metals from pyrite such as cadmium and lead
• Used in the microbial leaching of ores, especially if the metal concentration is low
• Utilized in the leaching of low grade copper ores
• 25% of copper mined worldwide is by leaching

Sulfur Oxidizing Bacteria

• Some obligate chemolithotrophs possess Calvin cycle enzymes in special structures called carboxysomes
• This increases the rate at which they can fix CO₂
• Carboxysomes are often present in the cells of obligate chemolithotrophs
• Other sulfur chemolithotrophs are facultative chemolithotrophs
• Grow either chemolithotrophically (autotrophs) or chemoorganotrophically

Beggiatoa

• Filamentous and gliding sulfur oxidizing bacteria
• Found in habitats rich in H₂S
  • Examples: sulfur springs, decaying seaweed beds, mud layers of lakes, sewage polluted waters, and hydrothermal vents
• Most species obtain energy from the oxidation of inorganic sulfur compounds
• Lack enzymes of the Calvin cycle
• Require organic compounds as carbon sources and are called mixotrophs
• Most grow mixotrophically using reduced sulfur compounds as electron donors and organic compounds as carbon sources
• Found to inhabit the rhizosphere of plants (rice and other swamp plants) living in flooded (anoxic) soils near the oxic-anoxic boundary
• Oxidizes H₂S, hence detoxifying the effect
• Can cause major settling problems in sewage treatment facilities, which is called bulking
• Occurs if waste water improperly treated is high in organic matter

Hydrothermal Vent Communities

• Tube worms can be 2 meters long
• Lack a mouth, gut, and anus
• Possess trophosomes, spongy tissue packed with S granules and S-oxidizing chemoautotrophs
• Bacteria grow on H₂S, thiosulphate and CO₂

Hydrogen Oxidizing Bacteria

• Key genera include Ralstonia and Paracoccus
• A wide variety of bacteria can grow with H₂ as the sole electron donor and O₂ as the electron acceptor
• Use the reduction of O₂ with H₂ as their energy metabolism
• Both gram-negative and gram-positive representatives are known
• One or more hydrogenase enzymes function to bind H₂
  • Use it to produce ATP or for reducing power for autotrophic growth
• Most are facultative chemolithotrophs and can grow chemoorganotrophically
• Some can grow on carbon monoxide (CO) as electron donor (carboxydotrophs)
• Use the Calvin cycle to fix CO₂ generated from the oxidation of CO
• Carboxydotrophic bacteria are important ecologically
• CO levels in the air have not risen dramatically
• Carboxydotrophic bacteria at the surface of soils act as a sink
• Most significant sink of CO in nature