Culturable and non-culturable actinobacterial diversity in seaweeds
Seaweed-associated Actinomycetota remains one of the least explored marine niches, both in diversity and biotechnological potential. Actinobacterial secondary metabolism, as the major microbial source of bioactive compounds with applicability in the pharmaceutical, industrial and ecological fields, continues to be a driving force into the study of the distribution, dynamics and metabolome of these microorganisms. In this study, we combine traditional classic cultivation and metagenomic analysis to survey actinobacterial diversity living in association with two seaweeds native  from the northern Portuguese coast, Chondrus crispus and Codium tomentosum. A total of 380 Actinomycetota strains were isolated from the specimens collected and taxonomically identified. According to the 16S rRNA sequencing, these isolates are distributed across 12 orders, 17 families and 25 genera affiliated to the Actinomycetia class, being composed by around 60% of Streptomyces. Metagenomic results showed that Actinomycetota is present in both Chondrus crispus and Codium tomentosum datasets, with 11 and 2% of relative abundance, respectively. In total, 12 orders, 16 families and 17 genera affiliated to this phylum were identified, with little overlapping with the cultivation results. Acidimicrobiales was identified as the dominant actinobacterial order in both seaweeds, but no strain affiliated to this taxonomic group was successfully isolated.  Our findings suggest that seaweeds from the Atlantic coast represent a rich hub of Actinomycetota diversity. Also, the synergistic use of both culture dependent and independent approaches to study the seaweed-associated prokaryotic community proved to be beneficial, enabling the recovery of not only abundant but also rare taxonomic members.