Anaerobic Respiration

Anaerobic Respiration Overview

• 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.

Unique Characteristics of Anaerobic Respiration

• Prokaryotic Specificity: Anaerobic respiration is unique to prokaryotes.

• Alternative Electron Acceptors:

  • Nitrate (NO3): Reduced to nitrite (NO2–).

  • Sulfate (SO42–): Reduced to sulfite (SO32–).

Electron Acceptors and Donors

• 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 Reduction Process

• 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).

Energy Production in Anaerobic Respiration

• 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.

Denitrification and Soil Fertility

• 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.

Important Terms

• 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.