# Reporting Key Features in Cold-Adapted Bacteria
## 2 Metabolic Features Related to Energy Generation in Cold Environments
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Metabolic Features**} & \\ \cline{2-5}
**Bacterial Species** & \begin{tabular}{c} **TCA** \\ **Representation or** \\ **Shortened** \\ \end{tabular} & \
\begin{tabular}{c} **Cytochrome** \\ **Repression** \\ \end{tabular} &
\begin{tabular}{c} **Presence of** \\ **Alternative** \\ **Pathways** \\ \end{tabular} & **Increase of** \\ \hline _Exiguobacterium sibiricum_ & NI & X & X & \({}^{\circ}\) & \({}^{\circ}\) \\ _Nesterenkonia sp. AN1_ & NI & NI & X & \({}^{\circ}\) & \({}^{\circ}\) \\ _Planococcus halocyophilus Or1_ & X & X & X & \({}^{\circ}\) & Decrease & [12; 13; 14] \\ _Pseudoalteromonas haloplanktis_ & X & X & NI & X & [15] \\ _Pseudomonas extremaustralis_ & X & X & X & \({}^{\circ}\) & NI & [16] \\ _Psychrobacter articus 273-4_ & X & NI & X & \({}^{\circ}\) & \({}^{\circ}\) \\ _Splingopyxis alaskensis_ & X & X & \({}^{\circ}\) & \({}^{\circ}\) & \({}^{\circ}\) \\ \hline \hline \end{tabular}  
X: presence of the feature; NI: not informed; \({}^{\circ}\): Glyoxylate and methylglyoxal pathways; \({}^{\circ}\): Projonyl-CoA catabolism;  
\({}^{\circ}\): Ethanol oxidation; TCA: tricarboxylic acid cycle.  
\end{table} 
Table 1: Cold adaptive bacterial responses revealed by genomic, transcriptomic and proteomic studies. methylglyoxal, formed as by-products of several metabolic pathways under subzero conditions, allows the removal of reactive electrophilic species through the recycling of reactive carbonyls to lactate, which is further used for cellular metabolism [12]. These results were in line with previous work, showing that, in this bacterium, energy metabolism was repressed at \(-15\)\({}^{\circ}\)C. However, the expression of succinic semialdehyde dehydrogenase, alcohol dehydrogenase and several oxidoreductase are increased, a fact that was attributed to the maintenance of energy metabolism and ATP levels [13]. A similar finding was reported in a transcriptomic analysis of _Pseudomonas extremaustralis_, a bacterium isolated from a temporary pond in Antarctica, in which the genes involved in primary metabolism were downregulated while those involved in ethanol oxidation--_exaA_, _exaB_ and _exaC_--encoding a pyrroloquinoline quinone (PQQ)-dependent ethanol dehydrogenase, cytochrome c550 and an aldehyde dehydrogenase respectively, were upregulated [16]. Surprisingly, genetic and physiological approaches showed that this secondary pathway was essential for growth under low temperatures, as mutant strains in different ethanol oxidation related genes were unable to grow under cold conditions [16]. This result shows that, despite the lower energy yield in comparison with central metabolism, ethanol oxidation becomes essential under cold conditions where TCA and cytochrome coding genes are repressed [16]. In the marine bacterium _Sphingopyxis alaskensis_, which inhabits permanently cold waters (4-10 \({}^{\circ}\)C), the fatty acid metabolism influences energy generation since five enzymes of the fatty acid degradation pathway (\(\beta\)-oxidation) were more abundant at low temperatures, rendering acetyl-CoA, which can enter to the TCA [18]. These authors also observed that fatty acid enzymes have a role in the propionate pathway, where propionyl-CoA is the central metabolic intermediate. Propionyl-CoA can also be obtained from the catabolism of some aminoacids, and can be carboxylated to S-methylmalonyl-CoA and subsequently isomerized to succinyl-CoA, another intermediate of TCA [18]. Similarly, RNAseq studies of the well-characterized model strain _Pseudomonas putida_ KT2440, growing at 10 \({}^{\circ}\)C, showed that the 2-methylcitrate pathway and branched aminoacid degradation were upregulated [21], allowing catabolism of propionate and propionyl-CoA, which are further transformed into 2-methylcitrate and later processed to succinate and pyruvate [22; 23]. _P. putida_ KT2440 is a derivative strain of the soil isolate _P. putida_ mt-2 [24], which can grow at low temperatures [21; 25; 26].