# Reporting Key Features in Cold-Adapted Bacteria
## 3 Compounds and Mechanisms Involved in Stress Resistance and Cold Adaptation
### Polyhydroxyalkanoates Metabolism  
Several bacteria indigenous to cold environments can synthesize polyhydroxyalkanoates (PHAs)--reserve polymers that have an important physiological role. These polymers are accumulated under unbalanced growth conditions, such as an excess of carbon source with respect to other nutrients such as nitrogen or phosphorus, acting as dynamic reservoirs of carbon and reducing equivalents [32]. PHAs endow bacteria with enhanced survival and resistance to a variety of environmental stress conditions while also having ecological relevance (e.g., [33; 34; 35; 36]). Genome analysis of _C. psychrerythtraca_ revealed the capacity to produce PHA linked with its significant ability to produce and degrade fatty acids. Multiple gene duplications of acyl-CoA dehydrogenase and enoyl-CoA hydratase gene families were detected, probably indicating versatility in PHAs characteristics that it can synthesize [27]. Later proteomic analysis showed a significant increase in PHA depolymerase after 24 h at \(-10\)\({}^{\circ}\)C, suggesting polymer utilization under freezing conditions [30]. In _S. alaskensis_, proteome analysis revealed an increased abundance of enzymes related to PHA synthesis at low temperatures, probably indicating that it may compensate for reduced rates of enzyme activity and nutrient transport, as an adaptive strategy to ensure that this pathway remains functional in the cold [18]. Among the proteins involved in PHA biosynthesis, phasins are the major PHA granule-associated protein that has multiple functions, and it has been proposed that it could play an active role in stress protection and fitness enhancement [37]. The PhaP phasin of _S. alaskensis_, along with a 3-hydroxybutyrate dehydrogenase that is presumably secreted, were found increased at low temperatures [18]. These findings suggested that the contribution of PHA metabolism to cold adaptation in _S. alaskensis_ involves both the synthesis of PHA de novo and the secretion of enzymes suitable for scavenging extracellular PHA [18]. _P. extremaustralis_ shows important traits related to the role of PHAs for low temperature adaptability. This bacterium produces a different kind of PHAs, short chain length PHAs (sclPHAs) as polyhydroxybutyrate (PHB) and medium chain length PHAs (mclPHAs) [38]. In particular, PHB production was found to be fundamental for cold growth and freezing survival [39]. PHB accumulation increased motility and the survival of planktonic cells in the biofilms developed by this bacterium under cold conditions, suggesting that the capability to accumulate PHB could constitute an adaptive advantage for the colonization of new ecological niches in those environments [40]. Interestingly, PHB production is not a common characteristic in _Pseudomonas_ species, and genetic analysis of _P. extremaustralis_ showed that PHB genes are located in a genomic island [38; 41], highlighting the importance of lateral gene transfer events for adaptability. The relevance of the metabolism of this polymer for cold adaptation was found by analyzing a PHB synthase mutant strain of _P. extremaustralis_ (_phbC_). The mutant was unable to grow at 10 \({}^{\circ}\)C and showed higher lipid peroxidation and a noticeable decrease in NADH/NAD ratio and NADPH content in comparison with the wild-type strain after a temperature downshift in which a rapid degradation of PHA was observed [39]. Protection conferred for PHA can lie in different non-excluding mechanisms. PHA metabolism is related to global stress responses, since an association between PHA degradation and the accumulation of the alarmone (p)ppGpp was found with a concomitant increase of the intracellular level of the master regulator RpoS [42; 43]. In addition, a cryoprotective role was attributed to the PHB monomer, 3-hydroxybutyrate and it was observed that under freezing conditions PHB granules remain highly flexible, suggesting that they might protect bacterial cells against injury from intracellular and extracellular ice [44; 45].