Abstract
The disruption of the coral–algae symbiosis (coral bleaching) due to rising sea surface temperatures has become an unprecedented global threat to coral reefs. Despite decades of research, our ability to manage mass bleaching events remains hampered by an incomplete mechanistic understanding of the processes involved. In this study, we induced a coral bleaching phenotype in the absence of heat and light stress by adding sugars. The sugar addition resulted in coral symbiotic breakdown accompanied by a fourfold increase of coral‐associated microbial nitrogen fixation. Concomitantly, increased N:P ratios by the coral host and algal symbionts suggest excess availability of nitrogen and a disruption of the nitrogen limitation within the coral holobiont. As nitrogen fixation is similarly stimulated in ocean warming scenarios, here we propose a refined coral bleaching model integrating the cascading effects of stimulated microbial nitrogen fixation. This model highlights the putative role of nitrogen‐fixing microbes in coral holobiont functioning and breakdown.
Elevated sea temperatures and high irradiance can cause the breakdown of the symbiosis between reef corals and their intracellular photosynthetic algae known as coral bleaching. Despite its critical relevance under global environmental change, our understanding of the underlying causes remains incomplete. Here we identify a mechanism linking the increased growth and activity of coral‐associated nitrogen‐fixing microbes (Bacteria and Archaea) with coral bleaching even in the absence of heat and light stress. Our findings emphasize the critical role of coral‐associated nitrogen‐fixing microbes as potential drivers of reef degradation, thereby providing novel management opportunities for the mitigation of climate change effects on coral reefs.