Dinitrogen fixation by a thermophilic methanogenic bacterium

Area of Science:

• Microbiology
• Biochemistry
• Evolutionary Biology

Background:

• Methanogenic bacteria typically utilize ammonium (NH+4) for nitrogen.
• Previous studies hinted at nitrogen fixation in impure methanogen cultures, but lacked conclusive evidence.
• The nitrogenase enzyme, responsible for nitrogen fixation, was not previously known in Archaea.

Purpose of the Study:

• To investigate the nitrogen-fixing capabilities of Methanococcus thermolithotrophicus.
• To determine if M. thermolithotrophicus can utilize atmospheric dinitrogen (N2) as its sole nitrogen source.
• To characterize the properties of the nitrogenase enzyme in this organism.

Main Methods:

• Culturing M. thermolithotrophicus in nitrogen-free media.
• Monitoring growth yields and transfers in N2-supplemented versus N2-depleted conditions.
• Assessing nitrogenase activity through acetylene reduction assays.

Main Results:

• M. thermolithotrophicus demonstrated robust growth with N2 as the sole nitrogen source, outperforming control cultures with limited NH+4.
• Growth yields were approximately one-third lower with N2 compared to NH+4, indicating significant energy requirements for N2 reduction.
• Acetylene reduction to ethylene confirmed active nitrogenase enzyme function in N2-grown cells.
• The organism's nitrogenase remained active at high temperatures (64°C).

Conclusions:

• M. thermolithotrophicus is a nitrogen-fixing archaebacterium, expanding the known diversity of this metabolic process.
• The discovery has significant implications for understanding the evolution of nitrogenase and microbial metabolism.
• The high-temperature activity suggests a novel, heat-stable nitrogenase enzyme.