Petalonema alatum is a sheathed, heterocystous cyanobacterium in the family Scytonemataceae, forming flattened trichomes enclosed in conspicuous, wing-like (alate) sheaths. It inhabits damp soil, cave walls, and moist rock surfaces in terrestrial and freshwater environments. Its distinctive sheath morphology aids in identification and reflects adaptations for survival in periodically desiccating habitats.