LAUSR

Studying marine Actinobacteria across ecological niches is essential for discovering novel natural products and understanding microbial interactions. In this study, we investigated the diversity of Actinobacteria associated with five Arctic marine invertebrates using both selective culture-based techniques and culture-independent methods. Additionally, we investigated bacteria–bacteria interactions in an advanced high-throughput co-cultivation assay. We isolated 25 Actinobacteria and classified them into 15 genera, with 53% of the isolates recovered from the sponge Halichondria panicea. In contrast, metabarcoding revealed a high diversity of Actinobacteria, with Chlamys islandica exhibiting the highest uniqueness of Amplicon Sequence Variants (ASVs), as 21.76% of its ASVs were found exclusively in this species. Similarly, not only did Dendrobeania sp. and Tricellaria ternata display notable levels of unique ASVs at 19.91% and 18.06%, respectively, they also shared 17.74% of ASVs, demonstrating a greater similarity in their microbial communities than between more distantly related hosts. A variety of microbial interactions were observed on solid medium, including both cooperative and antagonistic relationships, using the co-cultivation assay. These included inter- and intra-Actinobacteria interactions, as well as interactions with human pathogenic bacteria. The duration of co-cultivation and the physical proximity of bacterial partners influenced the extent of these interactions.