Nitric oxide (NO) is a vitally important molecule in many organisms, including Chlamydomonas reinhardtii. Given that NO can be toxic, its generation and degradation have to be tightly regulated. The intracellular NO can be the substrate for NO reductases producing nitrous oxide (N ₂O), and the cytochrome P450, CYP55, catalyzes this NO conversion in the darkness. Here we show that nitrite-dependent NO accumulation in dark cells is directly dependent on O ₂ levels: under hypoxia, very little NO is detected, whereas under normoxia, substantial amount of NO is measured. Following dark incubation, NO level is negatively correlated with the CYP55 expression. Moreover, we found that the transcription of CYP55 is under the complex influence of NO- and O ₂-dependent pathways. Finally, removal of extracellular Ca ²⁺ by the application of EGTA leads to an increase in CYP55 transcript levels both in hypoxic and aerobic dark cells, demonstrating the importance of membrane Ca ²⁺ influx in the regulation of this gene. Taken together, our results suggest that CYP55 expression might be the regulatory point involved in controlling of NO accumulation in the dark-incubated Chlamydomonas.