Fig. 5. Proposed model for the role of nitric oxide during cnidarian bleaching. The host cell responds to algal-derived ROS (including superoxide and hydrogen peroxide) by producing NO through signaling leading to the upregulation of NOS. This signaling could involve the transcription factor NFB, an important mediator of NOS transcription in other systems. The reaction of superoxide with NO produces the reactive nitrogen species peroxynitrite (ONOO^-), with additive deleterious effects leading to cell death and bleaching.