Phylogenetic diversity of Gram-positive bacteria cultured from Antarctic deep-sea sponges

Gram-positive bacteria, specifically actinobacteria and members of the order Bacillales, are well-known producers of important secondary metabolites. Little is known about the diversity of Gram-positive bacteria associated with Antarctic deep-sea sponges. In this study, cultivation-based approaches were applied to investigate the Gram-positive bacteria associated with the Antarctic sponges Rossella nuda, Rossella racovitzae (Porifera: Hexactinellida), and Myxilla mollis, Homaxinella balfourensis, Radiella antarctica (Porifera: Demospongiae). In total, 46 Gram-positive strains were cultured. Phylogenetic analysis revealed that 24 strains were affiliated with the Actinobacteria, including six genera Streptomyces, Nocardiopsis, Pseudonocardia, Dietzia, Brachybacterium, and Brevibacterium. The other 22 strains were affiliated with the Firmicutes, and among them two (V17-1 and V179-1) only shared 92-95% 16S rRNA gene sequence identity with the nearest type strain. To our knowledge, this is the first report on the isolation of strains belonging to genera Dietzia and Brevibacterium from Antarctic sponges. All of the 46 strains were PCR screened for genes encoding polyketide synthases (PKS), and a selection of 36 isolates were used in subsequent bioassay analyses. Eighty-eight percentage of the isolates that possess a PKS gene were active against at least one test organism. The study confirms the existence of diverse bacteria in Antarctic sponges and their potential for producing active compounds.