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　Delta　5-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　＜　0.001),　which　was　attributed　to　the　upregulated　abundance　of　the　putative　Delta　5-desaturase　related　protein,　Delta　5-Des.　The　study　also　indicated　that　the　expression　of　the　putative　Delta　5-desaturase　related　gene,　FADSD5,　increased　with　cell　growth,　demonstrating　considerable　potentials　for　ARA　biosynthesis　in　P.　purpureum　CoE1.　These　results　might　guide　the　direction　in　illuminating　the　biosynthetic　pathway　of　fatty　acids　with　molecular　evidence　and　enable　genetic　modifications　of　P.　purpureum　CoE1　for　enhancing　the　ARA　accumulation.