Nitrate assimilation is a key process for nitrogen (N) acquisition in green microalgae. Among Chlorophyte algae, Chlamydomonas reinhardtii has resulted to be a good model system to unravel important facts of this process, and has provided important insights for agriculturally relevant plants. In this work, the recent findings on nitrate transport, nitrate reduction and the regulation of nitrate assimilation are presented in this and several other algae. Latest data have shown nitric oxide (NO) as an important signal molecule in the transcriptional and posttranslational regulation of nitrate reductase and inorganic N transport. Participation of regulatory genes and proteins in positive and negative signaling of the pathway and the mechanisms involved in the regulation of nitrate assimilation, as well as those involved in Molybdenum cofactor synthesis required to nitrate assimilation, are critically reviewed.

Inorganic nitrogen is an essential nutrient for photosynthetic organisms. Its efficient use in nature involves adaptation of the organisms to the availability of the nitrogen supply, to changing environmental conditions, and to the provision of carbon and other nutrients. The unicellular alga Chlamydomonas provides a useful model to identify not only each of the components participating in the assimilative process in a species, but also the regulatory networks modulating their activity. A remarkable fact is the ample array of transporters for inorganic nitrogen compounds operating in this single cell: 13 putative nitrate/nitrite transporters and eight putative ammonium transporters. However, for nitrate, only a few of them participate as the main suppliers of nitrogen for cell growth, and others probably function to adapt nitrogen utilization efficiency to conditions depending not only on the nitrogen source available but also on other nutrients and environmental conditions. This paper summarizes recent findings in Chlamydomonas to provide an integrated perspective.