A blue essential oil (0.33%) was obtained by steam distillation and fractionated under reduced pressure to yield a residue. Chromatography of the residue on silica gel yielded fractions containing terpenes and a dark blue oil, which was rechromatographed to isolate guayazulene and pure marmelerin (1). Additional marmelerin (1) was obtained from the hexane extract of roots. Marmelerin (1) was characterized by spectral data (CD, UV, IR, MS) and a positive Erlich test. Dihydromarmelerin (3) was synthesized by hydrogenating marmelerin (1) over PtO₂ in methanol and characterized spectroscopically. Steroidal alkaloids, well-known metabolites of terrestrial plants but unreported from marine organisms, were investigated in a Plakina sp. sponge collected in shallow waters (~5 m) at Mant Island, Ponape, which overgrows and kills corals. A methanolic extract of the freeze-dried sponge showed antimicrobial activity against Staphylococcus aureus and Candida albicans. Bioassay-directed fractionation using Sephadex LH-20 and G-15 chromatography yielded two steroidal alkaloids: plakinamine A (1, 0.3% dry weight) and plakinamine B (2, 0.1% dry weight as hydrochloride salt). Their structures were elucidated by interpreting spectral data and comparing with model compounds synthesized from ergosterol. Plakinamine A (1) inhibited S. aureus (25 μg/disk) and C. albicans (10 μg/disk); the hydrochloride salt of plakinamine B (2) inhibited S. aureus (10 μg/disk) and C. albicans (2 μg/disk).