Transcriptome profiling of the meristem tissue of Saccharina japonica (Phaeophyceae, Laminariales) under severe stress of copper.

Copper (Cu) is an essential metal involved in many physiological processes of living organisms. However, beyond a certain threshold, Cu can become highly toxic. For instance, in the summer sporeling production of the economic kelp Saccharina japonica, the excess Cu accidently released from the low-quality alloys of the refrigerating machine was deadly to the seedlings and led to the failure of hatchery operations. However, the molecular basis underlying high toxicity of Cu remains unclear. In this study, juvenile sporophytes were cultured in seawater containing different concentrations of Cu (10, 100, and 200 μg L). Bleaching was observed in the meristem of individuals in the 100 and 200 μg L treatment groups on the third day, indicating that Cu has caused severe harm at these concentrations. RNA-Seq was used to profile transcriptomic changes under different Cu concentrations. Compared with the control, the number of differentially expressed genes (DEGs) was 11,350 (4944 up- and 6406 down-regulated) in the 200 μg L treatment group and 2868 (1075 up- and 1793 down-regulated) in the 100 μg L treatment group, whereas much fewer DEGs were detected in the 10 μg L treatment group. Genes coding for glutathione-S-transferase and vanadium-dependent bromoperoxidase and iodoperoxidase were found to be remarkably regulated, especially in the 200 μg L treatment group. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that only down-regulated DEGs were enriched. There were 45 enriched GO terms and four enriched KEGG pathways common to the 100 and 200 μg L treatment groups, which were associated with diverse essential biological processes such as photosynthesis, protein synthesis, redox activity, and metabolism and biosynthesis of functional biomolecules, among others. Suppression of these biological processes at the transcriptional level likely contributes to the observed high toxicity of Cu in S. japonica.