*Article* **Genome Reduction and Secondary Metabolism of the Marine Sponge-Associated Cyanobacterium** *Leptothoe*

**Despoina Konstantinou 1, Rafael V. Popin 2, David P. Fewer 2, Kaarina Sivonen <sup>2</sup> and Spyros Gkelis 1,\***


**\*** Correspondence: sgkelis@bio.auth.gr; Tel.: +30-2310-998083

**Abstract:** Sponges form symbiotic relationships with diverse and abundant microbial communities. Cyanobacteria are among the most important members of the microbial communities that are associated with sponges. Here, we performed a genus-wide comparative genomic analysis of the newly described marine benthic cyanobacterial genus *Leptothoe* (Synechococcales). We obtained draft genomes from *Le. kymatousa* TAU-MAC 1615 and *Le. spongobia* TAU-MAC 1115, isolated from marine sponges. We identified five additional *Leptothoe* genomes, host-associated or free-living, using a phylogenomic approach, and the comparison of all genomes showed that the spongeassociated strains display features of a symbiotic lifestyle. *Le. kymatousa* and *Le. spongobia* have undergone genome reduction; they harbored considerably fewer genes encoding for (i) cofactors, vitamins, prosthetic groups, pigments, proteins, and amino acid biosynthesis; (ii) DNA repair; (iii) antioxidant enzymes; and (iv) biosynthesis of capsular and extracellular polysaccharides. They have also lost several genes related to chemotaxis and motility. Eukaryotic-like proteins, such as ankyrin repeats, playing important roles in sponge-symbiont interactions, were identified in spongeassociated *Leptothoe* genomes. The sponge-associated *Leptothoe* stains harbored biosynthetic gene clusters encoding novel natural products despite genome reduction. Comparisons of the biosynthetic capacities of *Leptothoe* with chemically rich cyanobacteria revealed that *Leptothoe* is another promising marine cyanobacterium for the biosynthesis of novel natural products.

**Keywords:** cyanobacteria; symbionts; marine sponges; comparative genomics; natural products; biosynthetic gene clusters
