Deciphering the effects of nitrate enrichment and heat stress on the physiology of the coral Acropora kenti and the composition of its associated symbiotic and microbial communities.

Coral reefs are unarguably under increasing pressures arising from various environmental stressors. Coral survival in the face of environmental change relies heavily on nutrient exchanges between the host and the photosynthetic endosymbionts. While the functional contribution of the coral microbiome remains poorly understood, increasing evidence suggests that associated microorganisms are essential for coral resilience as they are intricately linked to nutrient cycling and energy flows in the ecosystem. Nitrogen underpins many aspects of coral holobiont functioning but the effect of its availability in its most abundant environmental form, nitrate, on the coral response to stress is equivocal: while nitrate sustains symbiont communities, it has also been reported to have adverse effects on the response to oxidative stress and to accentuate bleaching. In this study, using a crossed treatment experimental design in a mesocosm setup, we investigated the responses of the coral Acropora kenti to a nitrate enrichment of 5 µM in combination with a heat stress of 4 DHW over a period of 3 weeks. Corals’ health was monitored throughout the experiment and corals’ physiological response to the different treatments was assessed at the start of the stress and at the end of the experiment by measuring respiration rates, photosynthetic capacity, growth rates, symbiont densities, pigment and protein contents. In addition, corals were sampled to identify the composition of the associated symbiont and microbial communities using high-throughput sequencing of the genes ITS2 and 16S respectively. The heat stress treatment induced moderate to severe bleaching that was not alleviated by the increased nitrate supply. Nuances in the physiological data and the integration with the sequencing data give valuable inputs into the holobiont’s functioning by disentangling the effect of nitrate availability and heat stress on the resilience of the coral and the stability of its associated symbiotic and microbial communities.