Warming Reduces Pathogen Pressure on a Climate-Vulnerable Seagrass Species

Climate change is predicted to alter pathogen–host relationships and there is evidence of an increase in disease in both terrestrial and marine environments. Infection rates do not always increase linearly with temperature since thermal tolerance ranges of host and pathogens do not necessarily overlap and the host may benefit from thermal refugia of low or no disease pressure. Here, we demonstrate that climate warming may alleviate pathogen pressure in a climate-vulnerable Mediterranean seagrass, Posidonia oceanica. We tested the impact of warming on infection by Labyrinthula sp.—the causative pathogen of wasting disease—and the combined effect of elevated temperature and disease on photobiology. Infected and control shoots of P. oceanica were incubated at temperatures between 24 and 32 °C, encompassing maximum summer seawater temperatures projected for the Mediterranean Sea during the twenty-first century. Warming reduced the occurrence and severity of the disease and temperatures >28 °C inhibited cell division and growth of Labyrinthula. Photochemical efficiency was not significantly affected by short-term warming or by Labyrinthula infection. These results suggest that, unlike what has been predicted for the majority of pathogen–host relationships, warming may lead to a reduced risk of wasting disease in P. oceanica and relieve pathogen pressure from this species.