Table 2.

Main physiological and molecular effects/responses observed for bacteria exposed to the stressful conditions they could encounter in polar environments.

Stress Exposure	Main Effects/Responses	Reference
Physiological changes
Low temperature	Reduction of primary metabolism	[111]
	Modulation of PUFA amount for membrane fluidity maintenance, modulation of proteins and carotenoids	[91,112,113,114]
	Peptidoglycan thickening	[115,116]
	LPS lacking O-chain component	[117]
	Increase in short chain and/or unsaturated fatty acids	[117,118]
	Increase in enzymes with high catalytic activity	[91,119]
	Increase in antifreeze (AFPs) and ice nucleating (INPs) proteins	[120,121]
	Increase in cryoprotective compounds, including compatible solutes, extracellular polymeric substances (EPSs) and polyhydroxyalkanoates (PHAs)	[116]
Molecular changes
Genomic observations	Common genetic traits include those related to oxidative stress, metabolism and energy and nutrient acquisition, cell wall membrane structure and fatty acid biosynthesis, production of cold-shock protein (CSP) and chaperones, production of exopolysaccharides or other extracellular substances, biosynthesis or transport of compatible solutes (i.e., glycine betaine, ectoine, and trehalose), and presence of genes involved in antioxidant activity, such as superoxide dismutase, glutathione peroxidase, glutathione reductase, catalase, aconitase, thioredoxin and ascorbic acid	[122,123]
	Glycogen synthesis genes	[124]
	Presence of gene cluster encoding for glycogen synthase and 4-oxoacyl-ACP reductase, putative secondary metabolite biosynthesis gene clusters for terpene, nonribosomal peptide synthetase (NRPS), and different polyketide synthases (T1PKS and T3PKS)	[125]
	Presence of genes related to exopolysaccharide and polyunsaturated fatty acid biosynthesis, or involved in nutrient acquisition, production of proteins associated with ice-binding and light-sensing processes	[122]
	Reduced G+C content and peculiar composition for certain aminoacids to increase protein flexibility	[126]
Transcriptome analyses at low temperature	Up-regulation of transcripts involved in oxidative stress, CSP and chaperone production, metabolism and energy management, membrane fluidity assessment	[127]
	Up-regulation of genes involved in ethanol oxidation (exaA, exaB and ExaC) encoding for a pyrroloquinoline quinone (PQQ)-dependent ethanol dehydrogenase, a cytochrome c550 and an aldehyde dehydrogenase	[128]
	Induction of transcripts encoding for antioxidants (suggested by the up-regulation of sodA, bcp and bpoA2), and of genes encoding for key enzymes of the glioxylate cycle (isocitrate lyase and malate synthase)	[129]
	Expression for several protein families (i.e., membrane and regulatory proteins, metabolic proteins, especially those involved in NADH and NADPH generation), DNA metabolism and translation apparatus components resulted up-regulated	[130]
	Down-regulation of genes involved in transcription, translation, energy production, and most biosynthetic pathways was evidenced, while genes for specific biosynthesis processes (proline, tryptophan, and methionine), chaperones clpB and hsp33, RNases and peptidases were generally up-regulated	[131]
	Up-regulation of genes for translation, ribosomal structure and biogenesis and a down-regulation of lipid transport and metabolism	[132]
Proteome analysis at low temperature	Up-regulation was detected for proteins involved in metabolite transport, protein folding, membrane fluidity and aminoacid biosynthesis (protein MetF, ScoB and MmsA). Proteins related to energy production and conversion were down-regulated	[132]
Multi-omics (Genomic and Phenomic) analysis	High metabolic versatility, capacity to enhance the uptake of compounds with peculiar role in cryoprotection (i.e., spermine, glutathione, ornithine and other compounds related to glutathione metabolism). Enrichment in genes involved in lipid transport and metabolism and an up-regulation of protein synthesis metabolism	[2]