The synergistic negative effects of combined acidification and warming on the coral host and its symbiotic association with Symbiodiniaceae indicated by RNA-Seq differential expression analysis

Global warming and ocean acidification represent major threats to coral reefs, the combination of these stressors may have concomitant impacts on coral holobionts. However, the molecular mechanisms of the impacts and synergistic effects of acidification and warming on coral holobionts are rarely known, particularly from the point of coral-Symbiodiniaceae symbioses. In this study, using branching Acropora valida and massive Galaxea fascicularis as representatives in a laboratory system simulating acidification (pH 7.7) and/or warming (32 °C), the response of coral host, Symbiodiniaceae and their symbiotic association were investigated by high-throughput transcriptome sequencing (RNA-Seq) with pH 8.1 and 26 °C as controls. Based on differentially expressed genes (DEGs) analysis, acidification and/or warming show greater impacts on the gene expression of coral host than its symbiotic Symbiodiniaceae. Synergistic effects of combined acidification and warming are suggested by comparison with single stress, especially the synergistic negative effects on coral-Symbiodiniaceae symbioses are suggested, because the expression of most of the genes related to photosynthesis, nutrient metabolism and transfer, and the symbionts recognition are downregulated indicating the instability of the coral-Symbiodiniaceae symbioses. This study provides molecular evidence for the synergy of acidification and warming on coral holobionts. In particular, the synergistic negative effects on the nutrients and symbionts recognition-based coral-Symbiodiniaceae symbioses are highlighted, which is helpful for predicting the response of coral holobionts to future global climate changes.