Multiple and interrelated factors influence diversity of giant kelp (Macrocystis pyrifera) associated bacterial communities along the Strait of Magellan.

The giant kelp Macrocystis pyrifera forms abundant underwater forests that hosts diverse and dynamic bacterial communities. In the sub-Antarctic Strait of Magellan, while spatial variability is known to influence these associations, the combined effects of temporal variability and intra-thallus differentiation remains poorly understood. In this study, 16S rRNA gene metabarcoding was used to characterize bacterial diversity and composition by comparing communities across two sampling dates (March, late austral summer, and August, austral winter), two different kelp blade types (apical vegetative and basal reproductive sporophylls), and three sampling sites along the Strait of Magellan (Bahía Buzo, San Gregorio and Buque Quemado) which differ in their environmental settings. Bacterial communities were primarily structured by spatial variation among sites, both in terms of richness and composition. Beta diversity analyses revealed strong site-level differentiation in both sampling periods, while blade-type effects on community composition were only evident in August, indicating a date-related modulation of host-associated bacterial assemblages. Alpha diversity showed few differences related with blade types, whereas spatial variation, particularly in bacterial richness, was more pronounced and varied across dates. Furthermore, the global core microbiota was very small, consisting of only a few ubiquitous taxa consistently associated with Macrocystis pyrifera: Persicirhabdus (Verrucomicrobiia), Thalassotalea (Gammaproteobacteria), and an unclassified member of the family Flavobacteriaceae. Beyond this minimal shared core, core composition varied with blade type, sampling site and date, highlighting the combined influence of these factors on kelp-associated bacterial communities from a remote and understudied sub-Antarctic region.