RNA-seq profiling of Fugacium kawagutii reveals strong responses in metabolic processes and symbiosis potential to deficiencies of iron and other trace metals.

ABSTRACT

A healthy symbiotic relationship between corals and Symbiodiniaceae relies on suitable temperature and adequate nutrients including trace metals. Besides global warming, trace metal deficiency has been shown to cause coral bleaching, a phenomenon responsible for extensive coral reef degradation around the world. How trace metal deficiency impacts Symbiodiniaceae and coral symbiosis is poorly understood, however. In this study, we applied RNA-seq to investigate how Fugacium kawagutii responds to the deficiency of five trace metals (Fe2+, Zn2+, Cu2+, Mn2+, Ni2+). We identified 685 to 2805 differentially expressed genes (DEGs) from these trace metal deficiency conditions, among which 372 were commonly regulated by all the five trace metals and were significantly enriched in energy metabolism (e.g. fatty acid synthesis). Furthermore, genes associated with extracellular matrix (ECM), cell surface structure and cell adhesion were impacted, suggesting that the ability of recognition and adhesion of F. kawagutii may be altered by trace metal deficiencies. In addition, among the five metals, Fe2+ deficiency exhibited the strongest influence, with Fe-rich redox elements and many antioxidant synthesis genes being markedly down-regulated, indicative of adaptive reduction of Fe demand but a compromised ability to combat oxidative stress. Overall, deficiency of trace metals (especially Fe) seems to repress growth and ability of ROS scavenging, elevate energy metabolism and innate immunity, and alter cell adhesion capability, with implications in symbiosis disruption and coral bleaching.