The transmembrane electrical potential and intracellular pH in methanogenic bacteria

Area of Science:

• Microbiology
• Biochemistry
• Cell Physiology

Background:

• Archaea, including Methanospirillum hungatei GP1 and Methanobacterium thermoautotrophicum, possess unique cellular mechanisms for maintaining internal environments.
• Understanding the bioenergetics of archaeal cells, particularly the electrical potential and proton gradients, is crucial for comprehending their survival and function.

Purpose of the Study:

• To quantify the electrical potential and proton gradient across the cytoplasmic membrane of Methanospirillum hungatei GP1 and Methanobacterium thermoautotrophicum.
• To investigate the role of ion transport and proton gradients in the bioenergetics of these methanogenic archaea.

Main Methods:

• Utilized distribution assays with weak acids (DMO, butyrate, propionate) and methylamine to assess the pH gradient (delta pH).
• Employed the distribution of triphenylmethylphosphonium cation with tetraphenylboron anion to estimate electrical potential (delta psi).
• Measured 86Rb+ uptake in the presence of valinomycin to corroborate electrical potential measurements in M. thermoautotrophicum.

Main Results:

• No significant delta pH (alkaline interior) was detected in either organism under normal growth conditions using multiple weak acid probes.
• Internal pH varied with external pH, remaining more neutral, with electrical potentials estimated at 119 mV for M. thermoautotrophicum and 79 mV for M. hungatei.
• Electrical potential measurements using 86Rb+ uptake in M. thermoautotrophicum yielded similar values (143 mV to 120 mV) across different external pH ranges.

Conclusions:

• The absence of a substantial delta pH suggests that methanogenic archaea may not rely heavily on proton gradients for energy transduction.
• Observed electrical potentials correlate with K+ gradients, indicating active monovalent cation antiport and proton efflux mechanisms at the cytoplasmic membrane.
• Potential proton pumping activity at internal vesicles is proposed as a contributing factor to the observed bioenergetic parameters.