In vivo NMR studies of the thermophilic archaeon Methanococcus thermolithotrophicus, with
sodium formate as the substrate for methanogenesis, were used to monitor
formate utilization, methane production, and osmolyte pool synthesis and turnover under different conditions. The rate of
formate conversion to CO2 and H2 decreased for cells adapted to higher external NaCl, consistent with the slower doubling times for cells adapted to high external NaCl. However, when cells grown at one NaCl concentration were resuspended at a different NaCl,
formate utilization rates increased. Production of methane from 13C pools varied little with external NaCl in nonstressed culture, but showed larger changes when cells were osmotically shocked. In the absence of osmotic stress, all three solutes used for osmotic balance in these cells, l-alpha-
glutamate, beta-
glutamate, and
Nepsilon-acetyl-beta-lysine, had 13C turnover rates that increased with external NaCl concentration. Upon hyperosmotic stress, there was a net synthesis of alpha-
glutamate (over a 30-min time-scale) with smaller amounts of beta-
glutamate and little if any of the zwitterion
Nepsilon-acetyl-beta-lysine. This is a marked contrast to adapted growth in high NaCl where
Nepsilon-acetyl-beta-lysine is the dominant osmolyte. Hypoosmotic
shock selectively enhanced beta-
glutamate and
Nepsilon-acetyl-beta-lysine turnover. These results are discussed in terms of the osmoadaptation strategies of M. thermolithotrophicus.