High-Density Cultivation of Terrestrial Nostoc Strains Leads to Reprogramming of Secondary Metabolome.

Terrestrial symbiotic cyanobacteria of the genus exhibit a large potential for the production of bioactive natural products of the nonribosomal peptide, polyketide, and ribosomal peptide classes, and yet most of the biosynthetic gene clusters are silent under conventional cultivation conditions. In the present study, we utilized a high-density cultivation approach recently developed for phototrophic bacteria to rapidly generate biomass of the filamentous bacteria up to a density of 400 g (wet weight)/liter. Unexpectedly, integrated transcriptional and metabolomics studies uncovered a major reprogramming of the secondary metabolome of two strains at high culture density and a governing effect of extracellular signals in this process. The holistic approach enabled capturing and structural elucidation of novel variants of anabaenopeptin, including one congener with potent allelopathic activity against a strain isolated from the same habitat. The study provides a snapshot on the role of cell-type-specific expression for the formation of natural products in cyanobacteria. Terrestrial filamentous cyanobacteria are a largely untapped source of small-molecule natural products. Exploitation of the phototrophic organisms is hampered by their slow growth and the requirement of photobioreactors. The present study not only demonstrates the suitability of a recently developed two-tier vessel cultivation approach for the rapid generation of biomass of strains but also demonstrates a pronounced upregulation of high value natural products at ultrahigh culture densities. The study provides new guidelines for high-throughput screening and exploitation of small-molecule natural products and can facilitate the discovery new bioactive products from terrestrial cyanobacteria.