Positive Interactions Under Ocean Warming and Acidification: Crustose Coralline Algae Holobionts Enhance Gorgonian Larval Settlement Under Climate Change.

Crustose coralline algae (CCA) and their bacterial communities can emit chemical cues favoring coral larval settlement. Indeed, larvae of Eunicella singularis (white gorgonian) preferentially settle on CCA. Here, we investigated the effect of two Mediterranean CCA holobionts, Macroblastum dendrospermum and Lithophyllum stictiforme, on E. singularis larvae settlement and their bacterial communities, after warming and acidification treatments. We exposed CCA to temperature and pH expected for 2100 (SSP5-8.5) and to a marine heatwave event. Larval settlement increased 1.8-2.7 times in the presence of CCA exposed to warming and acidification compared to non-exposed CCA. High abundance of bacteria belonging to the Pirellulaceae family was observed in all CCA, while a higher abundance of monosaccharides was found in exudates of exposed CCA. Based on CCA-related 16S rDNA metabarcoding and metabolomics results, we hypothesize that the enhanced larval settlement was driven by the Pirellulaceae breakdown and utilization of CCA polysaccharides, in combination with polysaccharide release through the CCA cell walls likely augmented by decalcification. Furthermore, CCA acted as sources of bacterial taxa that may establish and persist in the adult E. singularis holobiont, independently of climate change effects. We conclude that CCA are key for E. singularis recruitment success, especially under future climate conditions, and contribute to their microbiome development.