Involved in the biosynthesis of polyamines which are thought to support the growth of thermophilic microorganisms under high-temperature conditions. It seems that long-chain and branched-chain of polyamines effectively stabilize DNA and RNA, respectively. Catalyzes the decarboxylation of N1-(3-aminopropyl)agmatine to yield spermidine and urea. Does not act on agmatine. H2O + N(1)-aminopropylagmatine = spermidine + urea Binds 2 manganese ions per subunit. Amine and polyamine biosynthesis; spermidine biosynthesis. The biosynthetic pathway, by which spermidine is synthesized from agmatine via N1-aminopropylagmatine has been characterized in the hyperthermophilic archaeon Pyrococcus kodakarensis and the thermophilic Gram-negative bacterium Thermus thermophilus. Belongs to the arginase family.