A healthy symbiotic relationship between corals and S ymbiodiniaceae 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 metal s (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 metal s 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 F e -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 metal s (especially Fe) seems to repress growth and ability of ROS scavenging , elevat e energy metabolism and innate immunity, and alter cell adhesion capability, with implications in symbiosis disruption and coral bleaching.
Mutualistic nutritional symbioses between coral animals and photosynthetic dinoflagellates of Symbiodiniaceae are the foundation of the highly biodiverse and productive coral reef ecosystem ( Peixoto et al., 2017). A symbiotic Symbiodiniaceae provide s its coral host with vital photosynthesis product s that can meet up to 95% of the coral ’s energy requirements; in exchange, the coral host provide s inorganic nutrients and serve s as shelter (Muscatine and Porter, 1997) for the symbiont . This mutualistic relationship, however, is delicate and susceptible to the environment factors including thermal stress, high light and extreme salinity (Davy et al., 2012; Kuanui et al., 2015; Nielsen et al., 2018; Skirving et al., 2019). These stress condition s often lead to the expulsion , death, or pigment loss of the Symbiodiniaceae from the host, known as coral bleaching. Global warming and anthropogenic activities have been responsible for massive bleaching and coral degradation , and posed major threats to the precious coral reef ecosystem around the world (Hughes et al ., 2017). Reef coral s liv e in tropical nutrient -poor waters , and they can experience limitation of nutrients, including trace metals ( Measures and Vink 2000; Obata et al ., 2008 ). It has been shown that trace metal deficiency can also induce coral bleaching (Ferrier -Pages et al ., 2018). Trace metal s are essential components of electron transport chains or important cofactors for enzymes involved in various biological processes, such as chlorophyll synthesis, nitrate reduction, and photoprotection or photorepair (Twining and Baines, 2013; Andresen, et al., 2018) .