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 propertie s (absorption and fluorescence), pig ment contents, N/C ratio and somatic g rowth rate. The 2 limitations induced large variations in phycoerythrin (PE ) and chlorophyll a (chl a) contents, with a significantly higher variation in PE than chi a under nitrogen stress. Under N limitation, there is a sig nificant relationship between the PE/chl a ratio and the growth rate. In th e case of light perturbation, the ratio PE/chl a was slightly modified duri ng photoacclimation. Emission and excitation spectra of PE and chi a showed that the coupling efficiency between these pigments is less affected by ni trogen than by Light changes. It is suggested that transfer of energy betwe en PE and chi a is regulated differently when growth is limited by light or nitrogen. Under nitrogen deprivation, the pool of PE is preferentially deg raded to reduce the energy transmitted to chi a, and to support the N deman d. Under a Light change, the energy harvested by PE and transferred to chi a is regulated by the coupling efficiency between the 2 pigments, and secon darily by synthesis or degradation of the PE quota. For each limitation, di fferent relationships are shown between optical characteristics based on fl uorescence ratios and growth rate.