Growth yields in bacterial denitrification and nitrate ammonification

Area of Science:

• Microbiology
• Environmental Science
• Biochemistry

Background:

• Denitrification and nitrate ammonification are key anaerobic respiration pathways.
• These processes are considered high-energy-yielding after oxygen depletion.
• Previous understanding suggested significant energy conversion to ATP for cell mass synthesis.

Purpose of the Study:

• To quantify microbial growth yields during denitrification and nitrate ammonification.
• To compare experimental energy conversion efficiencies with theoretical predictions.
• To investigate ATP synthesis efficiency in these anaerobic respiration pathways.

Main Methods:

• Growth yield determinations using pure cultures of specific bacteria (Paracoccus denitrificans, Pseudomonas stutzeri, Wolinella succinogenes, Sulfurospirillum deleyianum).
• Utilizing various electron donors (formate, hydrogen, acetate) with nitrate as the electron acceptor.
• Measuring dry matter produced per mole of electron donor consumed.

Main Results:

• Observed growth yields were lower than predicted based on free energy changes.
• ATP synthesis efficiency during denitrification was significantly lower than expected.
• Nitrate ammonification showed similarly low ATP synthesis efficiency compared to denitrification.
• Specific yields: 2.4-3.0 g dry matter/mol formate or H2; 15-18 g dry matter/mol acetate.

Conclusions:

• ATP synthesis during denitrification is less efficient than previously assumed.
• Denitrification and nitrate ammonification yield less energy for biomass production than theoretical calculations suggest.
• These findings have implications for understanding microbial metabolism and ecological roles in anoxic environments.