The comparative study of various unicellular algae, characterised by different carbon chain lengths and different numbers of double bonds per fatty acid (FA) chain, exhibited some similarity in the mechanisms of their response to changes in light conditions, in terms of FA metabolism. In all cases, the optimisation of photosynthetic process resulted in some increase in the relative content of the most unsaturated FA, i.e. C16:3Ω3 and C18:3Ω3 acids in Chlorella cells, C16:4Ω3 and C18:3Ω3 in Dunaliella and Chlamydomonas, C20:5Ω3 in Porphyridium, and C18:2Ω6 in Synechocystis sp. As a rule, these FA were esterified to monogalactosyldiacylglycerols (MGDG), the predominant lipids of thylakoid membranes. Such an increase in the relative content of the polyunsaturated FA usually occurred during the period when the photosynthesis, as well as the biosynthesis of FA de novo, were transiently inhibited following shifts in environmental conditions even at their optimisation. The increase in the relative content of the most unsaturated FA could be performed via desaturation of their immediate precursors. In turn, the deterioration of life conditions (decrease in the light intensity, ageing of cells or cultures, and others) resulted in the accumulation of these precursors. As a result, the cell could change its FA composition without alteration of the whole multistage process but only at the rate of this end reaction of polyunsaturated FA biosynthesis. In the majority of algae, these polyunsaturated acids were Ω3-homologues, regardless of the difference in their structures, but in some cyanobacteria (e.g. Synechocystis) the relative content of Ω6-FA increased. The acceleration of Ω3-FA biosynthesis could be observed, regardless of changes in the total index of unsaturation. This FA desaturation was shown to correlate with the activity of photosystem I (PSI). The specificity of this reaction enables us to assume it to be an adaptive response which provides alterations to lipid-protein interactions in the membrane that may be important for the self-assembly of active chlorophyll-protein complexes for photosynthetic akpparatus.