Relationship of Critical Temperature to Macromolecular Synthesis and Growth Yield in Psychrobacter cryopegella

Abstract

Most microorganisms isolated from low-temperature environments (below 4°C) are eury-, not steno-, psychrophiles. While psychrophiles maximize or maintain growth yield at low temperatures to compensate for low growth rate, the mechanisms involved remain unknown, as does the strategy used by eurypsychrophiles to survive wide ranges of temperatures that include subzero temperatures. Our studies involve the eurypsychrophilic bacterium Psychrobacter cryopegella, which was isolated from a briny water lens within Siberian permafrost, where the temperature is ؊12°C. P. cryopegella is capable of reproducing from ؊10 to 28°C, with its maximum growth rate at 22°C. We examined the temperature dependence of growth rate, growth yield, and macromolecular (DNA, RNA, and protein) synthesis rates for P. cryopegella. Below 22°C, the growth of P. cryopegella was separated into two domains at the critical temperature (T critical ‫؍‬ 4°C). RNA, protein, and DNA synthesis rates decreased exponentially with decreasing temperatures. Only the temperature dependence of the DNA synthesis rate changed at T critical . When normalized to growth rate, RNA and protein synthesis reached a minimum at T critical , while DNA synthesis remained constant over the entire temperature range. Growth yield peaked at about T critical and declined rapidly as temperature decreased further. Similar to some stenopsychrophiles, P. cryopegella maximized growth yield at low temperatures and did so by streamlining growth processes at T critical . Identifying the specific processes which result in T critical will be vital to understanding both low-temperature growth and growth over a wide range of temperatures.Evolutionary adaptation to low-or high-temperature conditions often leads to the commitment to exist only in those conditions (28). However, eurythermal organisms tolerate and grow at a wide range of temperatures while stenothermal organisms are restricted to a narrow range (3). Hence, a eurypsychrophilic bacterium would be capable of growing over a wide range of low temperatures, from Ϫ10 to 0°C at the lower limit to 20 to 30°C at the upper limit. One such eurypsychrophilic bacterium is Psychrobacter cryopegella, isolated from a low-temperature (Ϫ12°C), saltwater lens in Siberian permafrost (4). This bacterium displays its maximum growth rate (i.e., has an optimal growth temperature [T opt ]) at 22°C yet is able to reproduce at temperatures as low as Ϫ10°C and as high as 30°C.Of major concern to organisms living over a wide range of temperatures is the fact that reaction rates decrease exponentially as temperature decreases. This also holds for bacterial growth rates (essentially a collection of chemical reactions). However, when bacterial growth rate () is examined on an Arrhenius plot (log versus 1/T), two linear domains often appear below T opt . The point at which the slope changes is designated the critical temperature (T critical ). The existence of T critical has been reported for eurypsychrophiles, mesophiles, and thermophiles (5,6,9,12,13,15,22,35)....

Bacterial genome replication at subzero temperatures in permafrost

“…Here, we have demonstrated bacterial growth on 13 C-acetate at temperatures down to À 20 1C. Our findings of subzero DNA synthesis after 6 months are in agreement with earlier reports indicating microbial growth/metabolism in permafrost within 100-160 days at similar temperature (Rivkina et al, 2000), growth of bacterial isolates at À 5 to À 15 1C (Bakermans and Nealson, 2004;Mykytczuk et al, 2012Mykytczuk et al, , 2013 and acetate incorporation into bacterial lipids in permafrost down to À 20 1C (Rivkina et al, 2000). Furthermore, the active bacterial phyla observed in this SIP study (Acidobacteria, Actinobacteria, Chloroflexi, Gemmatimonadetes and many Alphaproteobacteria) have also been shown to produce ribo-tags and mRNA in a metatranscriptome study of the active layer of a Svalbard fen (Tveit et al, 2013).…”

“…This may be the result of the use of 13 C-acetate to discern activity. Alternatively, because spore-forming bacteria may remain in a dormant, non-metabolically active state in subzero environments (Bakermans and Nealson, 2004;Johnson et al, 2007), it is not surprising that Gram þ bacteria would not be detected by the clonal libraries derived from the 13 C-DNA.…”

Temperature characteristics of bacterial sulfate reduction in continental shelf and slope sediments

“…Environmental conditions in organic matter-limited deep-sea sediment impose nutritional constraints that can impede microbial growth. It is conceivable that psychrotolerant bacteria transported downslope have adaptive mechanisms to maximize and maintain a high growth yield at low temperature (Bakermans and Nealson, 2004), but it is not clear how allochthonous, mesophilic SRB derived from the shelf may proliferate when relocated to greater depths. It is noteworthy, however, that in laboratory studies both psychrophilic and mesophilic sulfate-reducing bacteria have shown comparable growth rates when grown on lactate (Knoblauch and Jørgensen 1999, Sass et al, 1998, suggesting that proliferation of mesophiles in permanently cold environments is conceivable.…”

Psychrobacter arcticus 273-4 Uses Resource Efficiency and Molecular Motion Adaptations for Subzero Temperature Growth

“…Gene expression studies of both psychrotolerant and strict mesophiles growing at suboptimal temperatures have detected increased expression of proteins for destabilization of nucleic acid secondary structures, maintenance of membrane fluidity, chaperones, uptake of compatible solutes, and energy metabolism (2,36,47). However, reports of the global modifications to metabolism necessary for growth of psychrophiles at temperatures below 0°C remain rare (4,5). Analysis of the amino acid sequences and structures of psychrophilic enzymes has given rise to the flexibility concept, i.e., that a psychrophilic enzyme can exhibit increased catalytic activity at low temperature with limited loss of thermostability by adaptation for reduced numbers of stabilizing interactions between key amino acid residues (20,40).…”