The degree of synchrony of light/dark-adapted Chlamydomonas reinhardtii cell-wall-less cells was analysed by flow cytometry. The cultures exhibited the classical growth curve reported for synchronously dividing cells. However, analysis of the nuclear DNA content revealed a partial asynchrony of cell division. This allowed us to determine that cell division lasts only 1 h 15 min. The changes in the distribution of nuclei in G1 and S phases did not exhibit the DNA peaks expected for highly synchronous cells. However, these peaks were observed for the G2+M nuclei and reflected the number of cell divisions. Modification of culture conditions including temperature, photoperiod, quantum irradiance and culture density affected the degree of synchrony but did not completely abolish the asynchrony. An overlap between the divisions of cells dividing one, two or three times could account for the partial asynchrony observed. In fact, it is possible that cells perform their divisions synchronously but that the timing and/or the duration of the cell divisions is different for the three kinds of cells. The synchrony seems to be perturbed when the cells move from the G1 to the S phase. The transduction of the release signal, allowing cells to pass through the restriction point of the G1 phase, might be too slow and insufficiently homogeneous between the cells to allow perfect synchronization. Finally, if highly synchronized cells are needed to study Chlamydomonas reinhardtii cell cycle, other synchronization procedures should be considered.

The green alga Chlamydomonas reinhardtii can use the ureides allantoin and allantoate as sole nitrogen sources. Once the uptake systems for allantoin and allantoate were induced, the uptake and growth rates were identical for the two ureides. However, the enzymatic activities involved in the degradation of the two ureides (allantoinase and allantoicase) were regulated differently. Allantoinase seems to be constitutive, since it was detected in all the nitrogen sources studied, while allantoicase behaved as an inducible enzyme, since it was present only in cells cultured in ureides or any metabolic precursor of these compounds. Neither allantoinase nor allantoicase activities were repressed by ammonium in the presence of ureides. Allantoicase activity was not induced under nitrogen starvation conditions, while it was induced in cells that had been cultured with allantoin or allantoate in the dark. Allantoin uptake showed a pattern similar to that of allantoate under all nutritional and environmental conditions tested. Inhibition of allantoin and allantoate uptake by N-ethylmaleimide suggests that thiol (SH–) groups are involved in both uptake systems. The use of both allantoin and allantoate was similarly inhibited by the metabolic poisons tested (cyanide, azide, 2,4-dinitrophenol and 3′-(3,4-dichlorophenyl)-1′,1′-dimethyl urea) but only at very high concentrations. The possibility that uptake of allantoin and allantoate might take place through two independent systems is discussed.