Photo-physiology and morphology reveal divergent warming responses in northern and southern hemisphere seagrasses

A better understanding of species and population responses to thermal stress is critical to predict changes in their distribution under warming scenarios. Seagrasses are a unique group of marine plants that play fundamental roles in marine environments and provide vital ecosystem services. Nevertheless, previous studies on seagrass thermal tolerance have focused exclusively on a handful of species, with the majority of these remaining virtually unexplored. Moreover, to date, no study has compared the response to thermal stress between northern and southern hemisphere seagrasses. Here, we conducted comparative mesocosm experiments using four seagrass species from the northern (i.e. Mediterranean: Posidonia oceanica, Cymodocea nodosa) and southern (i.e. Australia: Posidonia australis and Zostera muelleri) hemisphere as representative of two different life strategies, i.e. climax (P. oceanica, P. australis) and pioneer (C. nodosa, Z. muelleri). Plants acclimatized to the mesocosm conditions at ambient temperature (i.e. 26 °C) during a 5-week period, were exposed to a simulated marine heatwave (i.e. 32 °C) for 2 weeks. Measurements of plant responses, including photo-physiology, morphology, and pigment content, were performed at the end of the warming exposure. Results showed that warming had no significant effects on photosynthetic performances of northern hemisphere seagrasses while negatively impacted their southern hemisphere counterparts. Similarly, warming favored the growth of northern hemisphere plants, but strongly inhibited the development of southern hemisphere species. Furthermore, photo-physiological and pigment content results suggested pioneer seagrasses better dealt with warming than climax species. Our study provides more insights into the field of seagrass ecology and yields potential implication for future seagrass conservation and restoration activities.