Rhodolith holobionts in a changing ocean: Ocean Acidification effects on the free-living coralline algae and their associated microbiota

Rhodoliths, free-living coralline algae (Rhodophyta, Corallinales), form extensive beds worldwide distributed, ecologically important for the functioning of marine environments. Rhodolith beds are large carbon sinks, but the growth of the Rhodolith holobiont might be affected by changes in ocean carbonate chemistry, predicted to occur in the near future. The term holobiont refers to any organism and all of its associated symbiotic microbes (parasites, mutualists, synergists and amensals), including endobionts and epibionts that perform diverse ecological roles. A holobiont occupies and adapts to an ecological niche, and is able to employ strategies unavailable in any one species alone when challenged by environmental perturbations. The impact of increasing acidification of oceans on Rhodolith holobiont growth might be due to dissolution of their calcium carbonated skeleton, effects over photosynthetic rates, as well as changes in their associated microbial community, herein investigated through physiological assays (photosynthesis) and metagenomics (WGS Illumina sequencing). We used a mesocosm experimental system to assess potential effects of OA on dead and live rhodoliths following a 5 week exposure to increased pCO₂. Integrating both taxonomical and functional diversity from multiple players (Eukarya, Bacteria and Archaea) in the acidification context, we have demonstrated that the Rhodolith holobiont harbor an impressive stable microbiome, whereas high pCO₂ affect the seawater microbes. Our study has extended the comprehension of physiological relationships within Rhodolith holobiont by including the microbial component in the response of this coralline algae to higher pCO₂ levels, and endorsed previous works that indicated a parabolic photosynthetic response to pH and pCO₂. The outcomes of this research are an increased understanding of microbes associated with Rhodoliths and additional hints on how the holobiont might thrive in face to global climate changes.