Luna M. van der Loos1,2, Sophie Steinhagen3, Anne Willems2, Olivier De Clerck1
1Phycology Research Group, Department of Biology, Ghent University, Ghent, Belgium
2Laboratory of Microbiology, Department Biochemistry and Microbiology, Ghent University, Ghent, Belgium
3Department of Marine Sciences-Tjärnö, University of Gothenburg, SE-452 96 Strömstad, Sweden
The green seaweed Ulva is a model system to study seaweed-bacteria interactions. Although it becomes increasingly clear that microbes are of vital importance to their algal host, the impact of environmental drivers on the dynamics of these interactions is little understood. Assessing microbiome changes across an environmental gradient may provide information on the redundancy of the associated bacteria and the relative importance of stochastic vs. deterministic mechanisms of the community assembly. We investigated the stability and variability of Ulva-associated bacterial communities across the stable Atlantic-Baltic Sea salinity gradient. Using the full-length 16S rRNA gene, generated with Oxford Nanopore sequencing, we taxonomically characterized the bacterial communities of 15 Ulva sensu lato species along 2000 km coastline in a total of 481 samples. In addition, we functionally characterized a subset of 92 samples with Illumina shotgun metagenomic sequencing, resulting in 656 MAGs (metagenome-assembled genomes). Our results demonstrated that Ulva-associated bacterial composition and functional gene composition were strongly structured by both salinity and host species. The largest shift in the bacterial consortia coincided with the horohalinicum (5-8 PSU, known as the transition zone from freshwater to marine conditions). We defined distinct low and high salinity bacterial communities, as well as identified a small core community (contributing to 14% of the reads per sample, on average). Our results contradict earlier statements that Ulva-associated bacterial communities are taxonomically highly variable across individuals and largely stochastically defined. Characteristic bacterial communities associated with distinct salinity regions may therefore facilitate the host’s adaptation across the environmental gradient.