October 1992

Document Type


Degree Name



Dept. of Environmental Science and Engineering


Oregon Graduate Institute of Science & Technology


Halophilic methanogenic archaeobacteria populate saline and hypersaline environments, where they convert methylated compounds to methane. Methylamines are thought to be the dominant methanogenic substrates that can be utilized by halophilic methanogens. Methanohalophilus mahii and Methanohalophilus halophilus, two moderately halophilic methanogens from such environments, were the focus of this study into the physiology and osmotics of halophilic methanogens. These methanogens grew at broader salinity ranges than previously reported. In fact, Methanohalophilus mahii could grow in medium without added salt, contrary to published studies. Saline environments have low water-activity, and microorganisms must accumulate solutes to maintain a homeostatic level of cytoplasmic hydration. In Methanohalophilus mahii and Methanohalophilus halophilus, the cytosolic concentrations of K[superscript]+ and Na[superscript]+ were low relative to the total external solute concentration, and no other major intracellular cations were detected. This suggested that halophilic methanogens balance external osmotic pressure mainly by accumulating organic molecules, as do most halophiles. Significant concentrations of glycine betaine were observed in three moderately halophilic methanogens and in the extremely halophilic Methanohalobium evestigatum. Endogenous N,N-dimethylglycine was identified as a novel compatible solute and the predominant cytosolic osmolyte in Methanohalophilus. Exogenous sarcosine (monomethylglycine), dimethylglycine, and glycine betaine (trimethylglycine) stimulated growth of Methanohalophilus mahii in saline medium. In some saline habitats (e.g., salterns and intertidal ponds), halophilic methanogens are exposed to osmotic stress caused by rain storms or evaporation. Dilution stress must be particularly critical for many halophilic methanogens because their cell walls can not bear significant turgor pressure. The cytosolic water activity, measured in Methanohalophilus mahii growing in 2 M NaCl indicated the absence of a detectable turgor pressure in this archaeobacterium. Methanohalophilus mahii displayed unexpected capabilities to survive both hypertonic osmotic shocks and 10-fold hypotonic shocks.





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