Response of mesophotic colonies of Corallium rubrum to thermal anomalies

Marine heat waves (MHWs) have increased in frequency and intensity worldwide, causing mass mortality of benthic organisms and loss of biodiversity in shallow waters. The Mediterranean Sea is no exception, with shallow populations of habitat-forming octocorals facing the threat of local extinction. The mesophotic zone, which is less affected by MHWs, may be of ecological importance in conservation strategies for these species. However, our understanding of the response of mesophotic octocoral holobionts to changes in seawater temperature remains limited. To address this knowledge gap, we conducted a study on an iconic Mediterranean octocoral, the red coral C. rubrum sampled at 60 metres depth and 15 degree Celsius. We exposed the colonies to temperatures they occasionally experience (18 degree Celsius) and temperatures that could occur at the end of the century if global warming continues (21 degree Celsius). We also tested their response to extremely cold and warm temperatures (12 degree Celsius and 24 degree Celsius). Our results show a high tolerance of C. rubrum to a two-month long exposure to temperatures ranging from 12 degree Celsius to 21 degree Celsius as no colony showed signs of tissue loss, reduced feeding ability, stress-induced gene expression, or disruption of host-bacterial symbioses. At 24 degree Celsius, however, we measured a sharp decrease in the relative abundance of Spirochaetaceae, which are the predominant bacterial symbionts under healthy conditions, along with a relative increase in Vibrionaceae. Tissue loss and overexpression of the tumor necrosis factor receptor 1 gene were also observed after two weeks of exposure. In light of ongoing global warming, our study helps predict the consequences of MHWs on mesophotic coralligenous reefs and the biodiversity that depends on them.