Environmentally controlled production of pagoamide A in marine macroalgae by an intracellular bacterial symbiont

Marine algae are a rich source of diverse molecules, most of which are thought to be produced by the alga itself. We recently reported the discovery of pagoamide A from a cultured marine macroalga collected from American Samoa. Here, we found that the production of pagoamide A is conditional upon environmental temperature. Using comparative metagenomic, metatranscriptomic, and metabolomic analyses of algal cultures, we identified a nonribosomal peptide synthetase biosynthetic gene cluster (NRPS BGC) in the algal microbiome that varies in abundance between producing and non-producing conditions and whose architecture and biosynthetic logic match pagoamide A (named pag). pag belongs to a bacterium that we named "Candidatus Bryopsidiphilus pagoamidifaciens BP1," a new genus in the family Amoebophilaceae and a relative of amoeba, arthropod, and nematode endosymbionts. Ca. B. pagoamidifaciens lives intracellularly in its Bryopsis sp. algal host, harbors a reduced genome (1.7 Mbp), has lost most genes essential for free living, and is enriched in genes containing eukaryotic domains. By quantitatively monitoring longitudinal algal cultures under varying conditions for 9 weeks, we found that the abundance of both Ca. B. pagoamidifaciens and pagoamide A undergoes dramatic fluctuations in response to temperature changes. Finally, we discovered three additional strains of Ca. B. pagoamidifaciens that vary in their NRPS BGCs and eukaryotic domain-containing genes from algal samples of diverse geographical origins. Our findings suggest that symbiont-derived production of algal molecules is more common than previously anticipated and provide a unique case of environmental control of both symbiont and chemical levels in marine algae.