Effects of osmotic stress on Methanococcus thermolithotrophicus:: 13C-edited 1H-NMR studies of osmolyte turnover

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 H-2 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, formats utilization rates increased. Production of methane from C-13 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 N-epsilon-acetyl-beta-lysine, had 13C turnover rates that increased with external NaCl concentration. Upon hyperosmotic stress, there was a net synthesis of alpha-glutamate lover a 30-min time-scale) with smaller amounts of beta-glutamate and little if any of the zwitterion N-epsilon-acetyl-beta-lysine. This is a marked contrast to adapted growth in high NaCl where N-epsilon-acetyl-beta-lysine is the dominant osmolyte. Hypoosmotic shock selectively enhanced beta-glutamate and N-epsilon-acetyl-beta-lysine turnover. These results are discussed in terms of the osmoadaptation strategies of M. thermolithotrophicus. (C) 1999 Elsevier Science B.V. All rights reserved.