Algae can absorb nitrogen from different compounds, with ammonium being their preferred source. They use specific transport systems for this process. Algae can also break down purines to create urea and ammonium. The pathway for assimilating nitrate has been studied in detail, with mutants identified that affect different steps in the process. Genes responsible for this pathway are clustered together in the genome and are regulated in response to various factors. The mechanisms for sensing and responding to nitrate and ammonium signals are being investigated further.