Genomes of 'Ca. Entotheonella' phylotypes

Marine sponges are the source of numerous bioactive natural products that serve as chemical defenses and provide pharmaceutical leads for drug development. For some of the compounds, as-yet unculturable symbiotic bacteria have been established as the actual producers. Among the known sponge symbionts, 'Candidatus Entotheonella' members stand out due to their abundant and variable biosynthetic gene clusters (BGCs) that were previously identified in three analyzed bacterial variants. To obtain broader insights into this unusual producer taxon, we conducted a comparative study on eight sponges by metagenomic and single-bacterial genome sequencing. The data suggest consistently rich sets of biosynthetic genes that are largely unique in 14 'Entotheonella' candidate species and a member of a serendipitously discovered BGC-rich sister lineage named 'Candidatus Proxinella'. Four biosynthetic loci were linked in silico or experimentally to cytotoxins, antibiotics, and the terpene cembrene A. The results support widespread and diverse bacterial roles in the defensive chemistry of sponges and aid the development of sustainable production methods for sponge-derived therapeutics.