Growth rate, absorption and fluorescence, elemental and pigments composition of Cryptomonas sp. under light and nitrogen stresses.

Changes in cellular contents of pigments and chromoproteins as phycoerythrin (PE), are known to be regulated by three major processes: photo-acclimation, chromatic adaptation and nitrogen availability. PE which is an accessory pigment for photosynthesis, has also been suggested to play a role as N source, if necessary. Carbon and Nitrogen metabolic pathways are internally coupled and both governed by nutrients and photon fiuxes. 'rherefore, spectral absorption and fluorescence, pigments and elemental composition of continuous cultures of Ctyptomonas sp., submitted to light dowrishift and nitrogen puise-statvation, have been investigated in relation to their growth rate, to provide new insights on the adaptive processes involved. As expected, the growth rates ranging from O to 0.4 (d-l), are clearly affected by both limitations, but N and light deficiencies have respectively negative and positive effects on the celis diameter. The external N perturbations affects directly and strongly the N/C ratio and the cellular qi~otas of PE (6 to 10 folds), of Chl a (4 to 6 folds) and of nitrogen (2 to 3 folds), not of carbon. The light limitation entails the increase of PE and Chl a contents, but it slightly modifies the PE/Chl a ratio, contrarily to N limitation. The energy transfer between PE and Chl a, which is observed through the fluorescence emission and excitation spectra, seems to be differently regulated by the light and nitrogen limitations.