Salt stress in a halophilic bacterium: alterations in oxidative metabolism and oxy-intermediate scavenging systems

Halobaeterium halobium, an archaebacterium that grows optimally in 4 M NaCl, was subjected to hypo-osmotic stress by growth in media containing either 1, 1.25, 1.65, or 2 M NaCl. Exponential growth occurred over a 48-h period in 2 M NaCl as well as in the standard 4 M NaCl growth medium. In growth media containing NaCl in a concentration below 2 M, exponential growth was observed after a 6-h lag. After 12 h of growth in the lowest NaCl concentrations, extracts from H. halobium showed nearly a 100-fold increase in the specific activity of catalase compared with bacteria grown at higher NaCl concentrations; superoxide dismutase activity increased 10-fold, whereas peroxidase activity increased 4- to 5-fold compared with the activities at the higher salt levels. Specific activities remained elevated throughout the 48-h period. In contrast, malate dehydrogenase activity was not affected by alterations in the salt environment. Respiration of H. halobium growing in the most hyposaline medium was significantly elevated at 12 h and remained high over the ensuing 36 h. Metabolic rates correlated with increases observed in oxy-intermediate scavenging enzymes. No additional isozymes of superoxide dismutase were induced by salt stress. However, the catalase activity emanated from a newly induced mesohalic catalase distinct from the constitutive catalase–peroxidase.Key words: Halobaeterium halobium, metabolism, oxy-intermediate, superoxide dismutase, catalase.