Human-induced climate change causes an increase in the intensity and frequency of marine heat waves, which is one of the main drivers of coral bleaching and reef degradation. However, the underlying mechanisms that cause the breakdown of the mutualistic symbiosis between the coral host and its photosynthetic symbiont are not completely understood. While reactive oxygen species (ROS) and oxidative stress have long been stated as major triggers of the symbiosis breakdown, previous studies demonstrate the pivotal role of more complex pathways within the coral holobiont. Therefore, it is essential to resolve the direct impact of thermal stress on ROS production and how this effects the photosynthetic efficiency of the symbiont and finally coral bleaching.