AUTHOR=Califano Gianmaria , Kwantes Michiel , Abreu Maria Helena , Costa Rodrigo , Wichard Thomas 

TITLE=Cultivating the Macroalgal Holobiont: Effects of Integrated Multi-Trophic Aquaculture on the Microbiome of Ulva rigida (Chlorophyta)

JOURNAL=Frontiers in Marine Science

VOLUME=7

YEAR=2020

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2020.00052

DOI=10.3389/fmars.2020.00052

ISSN=2296-7745

ABSTRACT=<p><italic>Ulva</italic> is a ubiquitous macroalgal genus of commercial interest. Integrated Multi-Trophic Aquaculture (IMTA) systems promise large-scale production of macroalgae due to their high productivity and environmental sustainability. Complex host–microbiome interactions play a decisive role in macroalgal development, especially in <italic>Ulva</italic> spp. due to algal growth- and morphogenesis-promoting factors released by associated bacteria. However, our current understanding of the microbial community assembly and structure in cultivated macroalgae is scant. We aimed to determine (i) to what extent IMTA settings influence the microbiome associated with <italic>U. rigida</italic> and its rearing water, (ii) to explore the dynamics of beneficial microbes to algal growth and development under IMTA settings, and (iii) to improve current knowledge of host–microbiome interactions. We examined the diversity and taxonomic composition of the prokaryotic communities associated with wild versus IMTA-grown <italic>Ulva rigida</italic> and surrounding seawater by using 16S rRNA gene amplicon sequencing. With 3141 Amplicon Sequence Variants (ASVs), the prokaryotic richness was, overall, higher in water than in association with <italic>U. rigida</italic>. Bacterial ASVs were more abundant in aquaculture water samples than water collected from the lagoon. The beta diversity analysis revealed distinct prokaryotic communities associated with <italic>Ulva</italic> collected in both aquacultures and coastal waters. Aquaculture samples (water and algae) shared 22% of ASVs, whereas natural, coastal lagoon samples only 9%. While cultivated <italic>Ulva</italic> selected 239 (8%) host-specific ASVs, wild specimens possessed more than twice host-specific ASVs (17%). Cultivated <italic>U. rigida</italic> specimens enriched the phyla <italic>Cyanobacteria</italic>, <italic>Planctomycetes</italic>, <italic>Verrucomicrobia</italic>, and <italic>Proteobacteria</italic>. Within the <italic>Gammaproteobacteria</italic>, while <italic>Glaciecola</italic> mostly dominated the microbiome in cultivated algae, the genus <italic>Granulosicoccus</italic> characterized both <italic>Ulva</italic> microbiomes. In both wild and IMTA settings, the phylum <italic>Bacteroidetes</italic> was more abundant in the bacterioplankton than in direct association with <italic>U. rigida</italic>. However, we observed that the Saprospiraceae family within this phylum was barely present in lagoon water but very abundant in aquaculture water. Aquaculture promoted the presence of known morphogenesis-inducing bacteria in water samples. Our study suggests that IMTA significantly shaped the structure and composition of the microbial community of the rearing water and cultivated <italic>U. rigida.</italic> Detailed analysis revealed the presence of previously undetected taxa associated with <italic>Ulva</italic>, possessing potentially unknown functional traits.</p>