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

In vivo NMR studies of the thermophilic archaeon Methanococcus thermolithot rophicus, with sodium formate as the substrate for methanogenesis, were use d to monitor formate utilization, methane production, and osmolyte pool syn thesis and turnover under different conditions. The rate of formate convers ion to CO2 and H-2 decreased for cells adapted to higher external NaCl, con sistent with the slower doubling times for cells adapted to high external N aCl. However, when cells grown at one NaCl concentration were resuspended a t a different NaCl, formats utilization rates increased. Production of meth ane from C-13 pools varied little with external NaCl in nonstressed culture , but showed larger changes when cells were osmotically shocked. In the abs ence 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. U pon 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 contra st to adapted growth in high NaCl where N-epsilon-acetyl-beta-lysine is the dominant osmolyte. Hypoosmotic shock selectively enhanced beta-glutamate a nd N-epsilon-acetyl-beta-lysine turnover. These results are discussed in te rms of the osmoadaptation strategies of M. thermolithotrophicus. (C) 1999 E lsevier Science B.V. All rights reserved.