Responses of growth rate, pigment composition and optical properties of Cryptomonas sp. to light and nitrogen stresses.

The cryptophyte Cryptomonas sp. was cultured in nitrogen- and light-limited conditions to investigate relationships between inherent optical properties (absorption and fluorescence), pigment contents, N/C ratio and somatic growth rate. The 2 limitations induced large variations in phycoerythrin (PE) and chlorophyll a (chl a) contents, with a significantly higher variation in PE than chl a under nitrogen stress. Under N limitation, there is a significant relationship between the PE/chl a ratio and the growth rate. In the case of light perturbation, the ratio PE/chl a was slightly modified during photoacclimation. Emission and excitation spectra of PE and chl a showed that the coupling efficiency between these pigments is less affected by nitrogen than by light changes. It is suggested that transfer of energy between PE and chl a is regulated differently when growth is limited by light or nitrogen. Under nitrogen deprivation, the pool of PE is preferentially degraded to reduce the energy transmitted to chl a, and to support the N demand. Under a light change, the energy harvested by PE and transferred to chl a is regulated by the coupling efficiency between the 2 pigments, and secondarily by synthesis or degradation of the PE quota. For each limitation, different relationships are shown between optical characteristics based on fluorescence ratios and growth rate. KEY WORDS: Cryptophyceae · Growth rate · Quota · Chlorophyll a · Phycoerythrin · Absorption · Fluorescence · Photoacclimation · Nitrogen limitation · Light limitation