This thesis presents various cases of acclimation of a phytoplankton species (Isochrysis sp.) to changes in growth conditions, with special emphasis on the lipid response to those changes. In this context we have studied lipids as both products (triacylglycerol) and actors (glycolipids) of photosynthesis. First, we studied the influence of several types of nitrogen limitation on the neutral lipid content of algae. We showed that the relationship between nitrogen availability and lipid content was not monotonous and presented a hysteretic behaviour. Isochrysis sp. cells accumulated neutral lipids only transiently during the transition from high growth to low growth. We were also interested in the relationship between the circadian cycle and the accumulation of energy reserves as fats and sugars. Reserves were accumulated during the day and consumed during the night to provide energy for cell growth and division. Nitrogen deficiency under alternative light did not lead to significant accumulations of neutral lipids. Through these experiences we could understand the relationship between environment, growth, and accumulation of reserves. These results allowed us to identify the environmental conditions that enhance oil production for biofuel use. We also studied the influence of growth conditions on the composition of glycolipids that comprises chloroplast membranes. Changes in MGDG, DGDG and SQDG contents and in their fatty acids unsaturation were linked to changes in photosynthetic characteristics when growth conditions are modified (L/D cycle, N-limitation). Glycolipids are likely to be important actors of photosynthetic acclimation to environmental changes.
