Thriving Across Depths: How Blue Light Shapes a Large PSI Supercomplex and Specic Photosynthetic Traits in the seagrass Posidonia oceanica

Photosynthetic organisms rely on finely tuned mechanisms to optimize photosynthesis under different light conditions. While these processes are well-characterized in land plants, the adaptive strategies of marine plants remain largely unexplored. The Mediterranean seagrass Posidonia oceanica (Alismatales), a key ecosystem engineer thriving from the surface up to 40m depth and one of the largest long-term blue carbon sinks in coastal environments. Here, we investigate how P. oceanica adjusts its photosynthetic apparatus in response to varying light spectra encountered at different seawater depths. Contrary to land plants, P. oceanica maintains a relatively high PSI/PSII ratio and a high content of the major light-harvesting complex II (LHCII), regardless of depth. Notably, the antenna size of the photosystems remains stable across depths, although we document significant depth-dependent reorganization of the thylakoid membrane ultrastructure. Moreover, we identify a novel large PSI-LHCII supercomplex (L-PSI-LHCII) in P. oceanica , characterized by additional Lhca proteins, reduced red-shifted absorption, and increased chlorophyll b content. Ultrafast spectroscopy reveals the distinct energy transfer dynamics within this complex. The presence of a similar supercomplex in other marine plants, such as Zostera marina , suggests a conserved adaptive strategy among seagrasses.