Posidonia oceanica leaf bleaching: does it affect the plant photoprotective mechanisms?

Seagrasses are vulnerable to climate change, although photoprotective mechanisms through photopigment rearrangement, have been detected to avoid photoinhibition and photooxidative stress at high temperatures. Posidonia oceanica, endemic to the Mediterranean Sea, is not an exception. Leaf bleaching has been associated to temperature but how it affects seagrass viability is still unknown. Through a depth-cross-transplantation experiment in Cyprus, we explored the role of temperature and irradiance in determining bleaching onset and analyzed changes in photopigments after transplantation. Unexpectedly, after three months, leaf bleaching did not increase in plants transplanted to a different depth from the origin (e.g. from -10 to -30). Rather, higher bleaching was found when cuttings were kept at their origin depth (i.e. -10m), likely indicating that light and temperature are not the main drivers of this phenomenon. Photopigment rearrangement at different light and temperature conditions suggested that usual photoprotective mechanisms, used by the plant to respond to environmental stressors, are preserved despite the presence of bleaching. Understanding this phenomenon has become pivotal to predict the performance of P. oceanica in a changing Mediterranean Sea.