Table 1.

Main physiological and molecular effects/responses observed for microalgae exposed to stressful conditions encountered in polar environments.

Stress Exposure	Main Effects/Responses	Reference
Physiological changes
Low salt concentration	Increase in specific growth rate, biochemical composition shifting towards a lower POC:PON relationship with higher protein content, but reduced fatty acid and carbohydrate content.	[27,44]
High salt concentration	Slowing down of cell division and growth rate, reduced cell size and motility, triggering “palmelloid” formation in Chlamydomonas, production of osmoregulatory compounds (e.g., glycerol and proline), increase in ion transmembrane transport and lipids. In the sea-ice diatom, Nitzschia lecointei, small changes in growth rate, effect on cellular metabolite pool sizes	[46,47,48]
High levels of ultraviolet radiation (UVR)	Increase in photoprotective pigments	[49,50]
	Reduction in photosynthetic rate	[51]
	Inactivation of specific enzymes
	affecting species diversity and richness.
	Increase in saturated fatty acids, decrease in polyunsaturated fatty acids (PUFAs), small increase in C18 PUFAs	[52]
Low light exposure	Protection from UVR
	Limitation in primary production
Nutrients: Fe limitation	Influence in microalgal growth and composition	[53]
	Increase in lipid production	[54]
Low pH	Increase in large diatoms, early senescence	[55]
	Increase in photosynthetic rate and growth rate
	Affecting membrane potential, energy partitioning and enzyme activity	[56]
Low temperature	Maintenance of membrane fluidity thanks to unsaturated fatty acids	[57]
	Maintenance of sufficient rates of enzyme-catalyzed reactions for key metabolic processes
	Evolution of cold shock and antifreeze proteins
	Photosynthetic electron transport chain adaptations
Molecular changes
Genome comparison between cold-adapted and temperate species	In the cold-adapted genome there were highly divergent alleles which were also differentially expressed across various environmental conditions. Main genes identified for cold adaptation were ice-binding proteins IBPs, proton-pumping proteorhodopsins and chlorophyll a/c light-harvesting complex LHC.	[58]
Low temperature, high light	Genes encoding proteins of PSII (psbA, psbC) and for carbon fixation (rbcL) were down-regulated	[59]
	Chaperones (hsp70) and genes for plastid protein synthesis and turnover (elongation factor EfTs, ribosomal protein rpS4, ftsH protease) were up-regulated
Low temperature, low light	Down-regulation of psbA, psbC, and rbcL	[59]
Low temperature, high salinity	Ionic transporters and antiporters, heat shock proteins, genes related to oxidative stress, and three key genes involved in proline synthesis	[60]
Low temperature	Expression of various DEAD-box RNA helicase genes, such as CiRH5, CiRH25, CiRH28, and CiRH55, were found significantly up-regulated under freezing treatment	[61]
	Increase in genes encoding proteins involved in protein translation and transport, including protein transport protein SEC61, signal recognition particle protein, protein involved in vacuolar protein sorting. Increase in Heat shock protein 70, matrix metalloproteinase M11, X-Pro dipeptidyl-peptidase, and protein binding 26S proteasome regulatory complex, nitrate reductase, ferredoxin-nitrite reductase, and nitrate/nitrite transporter.	[62]
Increased temperature	Up-regulation of cytoprotective genes, down-regulation of genes related to photosynthesis, increase in fucoxanthin chlorophyll a/c-binding proteins	[63]
High light	Transcripts related to photosynthesis were affected	[64]
	Identification of an antifreeze protein gene (Cn-AFP)	[65]
High salinity	Increased expression of genes participating in the metabolism of carbohydrates, such as starch, sucrose, soluble sugar, and glucose	[66]