The　red　alga　Porphyridium　purpureum　has　been　known　to　produce　polyunsaturated　fatty　acids,　especially　arachidonic　acid　(ARA),　under　stressful　conditions.　However,　there　is　no　consistent　conclusion　about　the　response　of　ARA　in　this　alga　to　nitrogen　(N)　stress.　Also,　no　research　has　been　done　to　clearly　elucidate　the　underlying　molecular　mechanisms　of　N　stress.　In　this　work,　P.　purpureum　CoE1　was　cultivated　under　nitrogen　limitation　conditions　and　the　putative　Δ5-desaturase　related　gene　FADSD5　was　isolated.　The　results　showed　that　the　fatty　acids　in　P.　purpureum　CoE1　were　significantly　higher　in　the　N　limited　cultures　(54.3 mg g−1)　than　in　the　N-replete　cultures　(45.3 mg g−1)　at　the　18th　day　(t-test,　p　©　2021,　The　Author(s),　under　exclusive　licence　to　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.