Specific growth rate plays a critical role in hydrogen peroxide resistanceof the marine oligotrophic ultramicrobacterium Sphingomonas alaskensis strain RB2256

The marine oligotrophic ultramicrobacterium Sphingomonas alaskensis RB2256 has a physiology that is distinctly different from that of typical copiotro phic marine bacteria, such as Vibrio angustum S14. This includes a high lev el of inherent stress resistance and the absence of starvation-induced stre ss resistance to hydrogen peroxide. In addition to periods of starvation in the ocean, slow nutrient-limited growth is likely to be encountered by oli gotrophic bacteria for substantial periods of time. In this study we examin ed the effects of growth rate on the resistance of S. alaskensis RB2256 to hydrogen peroxide under carbon or nitrogen limitation conditions in nutrien t-limited chemostats. Glucose-limited cultures of S. alaskensis RB2256 at a specific growth rate of 0.02 to 0.13 h(-1) exhibited 10,000-fold-greater v iability following 60 min of exposure to 25 mM hydrogen peroxide than tells growing at a rate of 0.14 h(-1) or higher. Growth rate control of stress r esistance was found to be specific to carbon and energy limitation in this organism. In contrast, V. angustum S14 did not exhibit growth rate-dependen t stress resistance. The dramatic switch in stress resistance that was obse rved under carbon and energy limitation conditions has not been described p reviously in bacteria and thus may be a characteristic of the oligotrophic ultramicrobacterium, Catalase activity varied marginally and did not correl ate with the growth rate, indicating that hydrogen peroxide breakdown was n ot the primary mechanism of resistance. More than 1,000 spots were resolved on silver-stained protein gels for cultures growing at rates of 0.026, 0.0 76, and 0.18 h(-1). Twelve protein spots had intensities that varied by mor e than twofold between growth rates and hence are likely to be important fo r growth rate-dependent stress resistance. These studies demonstrated the c rucial role that nutrient limitation plays in the physiology of S. alaskens is RB2256, especially under oxidative stress conditions.