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.

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