| Summary: | Leaf-cutting ants live in mutualistic symbiosis with their garden fungus <i>Leucoagaricus gongylophorus</i> that can be attacked by the specialized pathogenic fungus <i>Escovopsis</i>. <i>Actinomyces</i> symbionts from <i>Acromyrmex</i> leaf-cutting ants contribute to protect <i>L. gongylophorus</i> against pathogens. The symbiont <i>Streptomyces</i> sp. Av25_4 exhibited strong activity against <i>Escovopsis weberi</i> in co-cultivation assays. Experiments physically separating <i>E. weberi</i> and <i>Streptomyces</i> sp. Av25_4 allowing only exchange of volatiles revealed that <i>Streptomyces</i> sp. Av25_4 produces a volatile antifungal. Volatile compounds from <i>Streptomyces</i> sp. Av25_4 were collected by closed loop stripping. Analysis by NMR revealed that <i>Streptomyces</i> sp. Av25_4 overproduces ammonia (up to 8 mM) which completely inhibited the growth of <i>E. weberi</i> due to its strong basic pH. Additionally, other symbionts from different <i>Acromyrmex</i> ants inhibited <i>E. weberi</i> by production of ammonia. The waste of ca. one third of <i>Acomyrmex</i> and <i>Atta</i> leaf-cutting ant colonies was strongly basic due to ammonia (up to ca. 8 mM) suggesting its role in nest hygiene. Not only complex and metabolically costly secondary metabolites, such as polyketides, but simple ammonia released by symbionts of leaf-cutting ants can contribute to control the growth of <i>Escovopsis</i> that is sensitive to ammonia in contrast to the garden fungus <i>L. gongylophorus</i>.
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