Definition: Anaerobic respiration is a metabolic process used by prokaryotes that does not require oxygen.
Electron Acceptors: Prokaryotes can utilize various terminal electron acceptors besides O2, such as:
Metals
Oxidized ions of nitrogen
Sulfur compounds
Habitats: Occurs in environments where oxygen is limited, including wetland soils and the human digestive tract.
Prokaryotic Specificity: Anaerobic respiration is unique to prokaryotes.
Alternative Electron Acceptors:
Nitrate (NO3): Reduced to nitrite (NO2–).
Sulfate (SO42–): Reduced to sulfite (SO32–).
Key Electron Acceptors:
Oxygen (O2): Reduced to water (H2O).
Nitrate (NO3):
Reduced to nitrite (NO2–) by E. coli and enteric bacteria.
Reduced to N2O and N2 by Pseudomonas, Bacillus, and Paracoccus.
Sulfate (SO42–): Reduced to hydrogen sulfide (H2S) by sulfate-reducing bacteria (Desulfovibrio, etc.).
Carbon Dioxide (CO2): Converted to acetate (CH3COO–) by acetogens.
Iron (Fe3+): Reduced to ferrous iron (Fe2+).
Nitrate Reductase Activity:
Nitrate Reduction Test: Uses sulfanilic acid and alpha-naphthylamine to detect nitrate reduction.
Products: Nitrate (NO3) is reduced to nitrite (NO2) and further to nitric oxide (NO), and subsequently to nitrous oxide (N2O) and nitrogen (N2).
ATP Yield: Anaerobic respiration generally yields less ATP compared to aerobic respiration due to lower reduction potentials of alternative electron acceptors.
Electron Transport Chain (ETC):
Shorter, which pumps fewer protons (H+) across the membrane.
Example: Paracoccus denitrificans has a more complex and branched ETC than aerobic organisms.
Process:
Reduction of nitrate (NO3) to nitrogen gas (N2), resulting in the escape of nitrogen from the soil.
This process can lead to reduced soil fertility as nitrogen becomes unavailable for plant uptake.
Dissimilatory Nitrate Reduction:
Nitrate is reduced to nitrogen gas, making nitrogen unavailable for assimilation by the cell.
Dinitrification:
The conversion of nitrate (NO3) into nitrogen gas (N2), releasing gaseous compounds into the atmosphere.