Characterization of physiological and transcriptome changes in the ancient Siberian permafrost bacterium Psychrobacter arcticum 273-4 with low temperature and increased osmotica

The Siberian permafrost is an extreme, yet stable environment due to its continuously frozen state. Average temperatures of -10 to -12°C concentrate solutes to an aw = 0.90 (5 osm), and limit nutrient diffusion into cells. The isolation of viable Psychrobacter arcticum 273-4 and Exiguobacterium sibericum 255-15 from ancient permafrost suggests that these bacteria have maintained some level of metabolic activity for thousands of years and are likely cryo-adapted. Past studies of cold-acclimated bacteria have focused primarily on organisms not capable of sub-zero growth. Siberian permafrost isolates E. sibericum 255-15 and P. arcticum 273-4, which grow at subzero temperatures, were used to study cold-acclimated physiology. Changes in membrane composition and exopolysaccharides were defined as a function of growth at 24, 4 and -2.5°C in the presence and absence of 5% NaCl. As expected, there was a decrease in fatty acid saturation and chain length at the colder temperatures and a further decrease in the degree of saturation at higher osmolarity. A shift in carbon source utilization and antibiotic resistance occurred at 4 versus 24°C growth, perhaps due to changes in the membrane transport. Both isolates had excellent survival after one year of being frozen at -20°C and moderate ice nucleation activity.; To gain knowledge of the features for adaptation to low temperature and water activity, the genome of a Siberian permafrost isolate was sequenced. Psychrobacter arcticum 273-4, a gamma-Proteobacterium grows at temperatures as low as -10°C and displays marked physiological changes under low temperature vs. mesophilic growth and in the presence of increased osmotica (salt). Differential gene expression analyses at 4°C and 22°C, using microarrays specific for the predicted genes of P. arcticum 273-4, reveal that only a few genes known to be low-temperature and salt-responsive in mesophiles and in some psychrotrophs are differentially expressed in P. arcticum. Total respiratory activity increased in the presence of salt, due to increased number of Na + dependent dehydrogenase and decarboxylase transcripts suggesting that P. arcticum has adapted for energy generation in the low temperature and low water activity of the permafrost environment. Growth in salt-amended ½ TSB leads to induction of capsule genes and a capsule is visible around cells grown at both 22 and 4°C, which could allow P. arcticum to adhere to soil particles within the permafrost. Capsule production, lipid modification and compatible solute accumulation to balance internal osmolytes may allow P. arcticum to remain metabolically active within the low water activity of the Siberian permafrost. (Abstract shortened by UMI.)...