Nitrogen availability improves the physiological resilience of coral endosymbiont Cladocopium goreaui to high temperature.

The physiological response of symbiotic Symbiodiniaceae to high temperature is believed to result in coral bleaching. However, the potential effect of nitrogen availability on heat acclimatization of symbiotic Symbiodiniaceae is still unclear. In this study, physiological responses of Symbiodiniaceae Cladocopium goreaui to temperature and nitrogen nutrient stress conditions were investigated. Nitrogen deficiency caused significant declines in cell concentration and chlorophyll content per cell, but significant increases in nitric oxide synthase activity, caspase3 activation level, and cellular carbon content of C. goreaui at normal temperature. Algal cells under high temperature and nitrogen deficiency showed significant rises in Fv/Fm, catalase activity, and caspase3 activation level, but no significant changes in cell yield, cell size, chlorophyll content, superoxide dismutase, nitric oxide synthase activity, and cellular contents of nitrogen and carbon, in comparison with those under normal temperature and nitrogen deficiency. Growth, chlorophyll, and nitrogen contents of algal cells under the high temperature and nitrogen-replete conditions were significantly higher than those under high temperature or nitrogen deficiency alone, whereas nitric oxide synthase activity, superoxide dismutase activity, catalase activity, carbon content, and caspase3 activation level exhibited opposite trends of variation. Transcriptomic and network analyses revealed ion transport and metabolic processes mainly involved in regulating these physiological activities under different temperature and nitrogen nutrient. The totality of results shows that high temperature activates stress responses, induces antioxidant capacity of apoptosis, and limits the growth rate of C. goreaui. Adequate nitrogen nutrient can improve the resilience of this Symbiodiniaceae against heat stress through repressed apoptosis, promoted ion transport, and optimized metabolism.