Nitrate enrichment exacerbates microbiome and metabolism disturbances of the coral holobiont under heat stress

Abstract

Coral reef ecosystems are facing severe deterioration due to escalating global temperatures and human-induced activities. Combined nitrate and heat stress can exacerbate coral bleaching, however, the underlying mechanism is still unclear. In the present study, we assessed the bleaching status of Acropora hyacinthus, a reef-building coral species, under high temperature and nitrate stress conditions using chemostat cultivation. We observed nitrate enrichment (9 μM) induced a significant reduction in photosystem efficiency (Fv/Fm) of Symbiodiniaceae and an increased thermal bleaching of corals under high temperature (30 °C). Nitrate exposure promoted the proliferation of Enterobacteriaceae and Vibrionaceae, which are bacterial families, potentially augmenting the coral's susceptibility to disease while exerting negligible effects on the fungal community. Alterations were observed in the metabolic pathways of both the coral hosts and Symbiodiniaceae, including down-regulated folate biosynthesis and inflammatory mediator regulation of TRP channels. Our findings indicate that nitrate enrichment under heat stress disrupts the metabolism of coral holobionts through altering bacterial communities, ultimately leading to increased coral bleaching.