Schematic of cellular regulation of nitrogen assimilation in the algae Chlamydomonas reinhardtii . The number of identified transporters is in indicated on the respective membrane (i.e. 3x = three ammonium transporters on the chloroplast membrane). Dashed lines in the figure legend represent the regulatory elements involved in nitrogen metabolism. Additional key notations: Amt = ammonium transporter, Nit = nitrate transporter, NR = nitrate reductase, NiR = nitrite reductase, CA = carbonic anhydrase, CCM = carbon concentration mechanisms.