Bioenergetic Aspects of Halophilism

SUMMARY Examinination of microbial diversity in environments of increasing salt concentrations indicates that certain types of dissimilatory metabolism do not occur at the highest salinities. Examples are methanogenesis for H2 + CO2 or from acetate, dissimilatory sulfate reduction with oxidation of acetate, and autotrophic nitrification. Occurrence of the different metabolic types is correlated with the free-energy change associated with the dissimilatory reactions. Life at high salt concentrations is energetically expensive. Most bacteria and also the methanogenic archaea produce high intracellular concentrations of organic osmotic solutes at a high energetic cost. All halophilic microorganisms expend large amounts of energy to maintain steep gradients of NA+ and K+ concentrations across their cytoplasmic membrane. The energetic cost of salt adaptation probably dictates what types of metabolism can support life at the highest salt concentrations. Use of KCl as an intracellular solute, while requiring far-reaching adaptations of the intracellular machinery, is energetically more favorable than production of organic-compatible solutes. This may explain why the anaerobic halophilic fermentative bacteria (order Haloanaerobiales) use this strategy and also why halophilic homoacetogenic bacteria that produce acetate from H2 + CO2 exist whereas methanogens that use the same substrates in a reaction with a similar free-energy yield do not.

Dissimilatory metabolism of nitrate by the rumen microbiota

A mixed culture of bovine rumen microorganisms was incubated anaerobically, under conditions likely to support growth, with added nitrate and in the presence of several potential hydrogen donors known to be intermediates in the rumen fermentation. Reduction of nitrate via nitrite to ammonia was observed but there were marked differences in the relative rates and extent of nitrate and nitrite reduction in the presence of different hydrogen donors. The hydrogen donors were ranked in decreasing order of overall effectiveness for the complete reduction of nitrate and nitrite: formate > hydrogen > glucose [Formula: see text] lactate > succinate. Methane production in the cultures was markedly depressed in the presence of nitrate. The identification by gas chromatography of nitrous oxide in the gas phase of cultures containing nitrate and incubated with hydrogen, glucose, or lactate showed that the microbiota was capable of denitrification. The quantitative significance of denitrification as a pathway of nitrate dissimilation by rumen microorganisms, however, is probably small.