- The microalga Parietochloris incisa (Trebouxiophyceae, Chlorophyta) was isolated from an alpine environment. It was found to accumulate unusually high amounts of arachidonic acid (AA)-rich TAG. We have hypothesized that microalgal PUFA-rich TAG might have a role as a depot of PUFA, which could be mobilized for the construction of chloroplastic membranes under sudden changes in environmental conditions. We have thus studied the changes in lipid and fatty acid composition during recovery from nitrogen starvation at 24 and 12 degrees C. At both temperatures, TAG was mainly consumed to support growth, however, there was a significant increase in the content of AA in the chloroplastic lipids, predominantly, monogalactosyldiacylglycerol (MGDG) at 24 degrees C, but much less so at 12 degrees C. Similar results were obtained using radiolabeled precursors. These and other findings point to the existence of three modes of operation for the construction of chloroplastic lipids that the alga can utilize to support growth under changing environmental conditions. When environmental conditions do not support growth, the prokaryotic pathway predominates. When sudden changes occur, the eukaryotic pathway is enhanced and can be even further augmented by influx of acyl moieties from TAG to maximize the exploitation of growth conditions that may possibly be transitory.