Analysis of photosynthetic responses and adaptation to nitrogen starvationin Chlorella using in vivo chlorophyll fluorescence

Nitrogen (N) starvation resulted in degreening, inhibition of photosyntheti c oxygen evolution and dark respiration, reduced survival, and increased ag e-specific mortality in both Chlorella fusca and Chlorella vulgaris. Analys is of in vivo chlorophyll (Chl) fluorescence induction kinetics revealed th e presence of N-starvation-induced changes at the level of degreened thylak oids in both species. These changes included decreased yield of the photoch emistry of photosystem 2 (PS2), and a declined photosynthetic efficiency. S ynthesis of secondary carotenoids represented a biochemical change in carot enogenesis that had a photoprotective effect in degreened C, fusca. This in ferred photoprotection was reflected in the delayed inhibition of oxygen ev olution and improved survival of C. fusca under N-starvation. The effect wa s further elucidated by comparison with C. vulgaris which was not able to s ynthesize secondary carotenoids under the same conditions.