Abstract

Seagrasses are the only group of flowering plants to colonize the sea. These plants occupy a relatively stable environment, characterised by high salinity (0.5 M NaCl), alkaline pH (8.2) and low availability of essential nutrients such as nitrate or phosphate (below 10 µM). Furthermore, one of the major challenges that face marine angiosperms is the acquisition of dissolved inorganic carbon for photosynthesis. Those conditions suggest that these plants have developed special physiological mechanisms for adaptation to the marine environment, which should be firmly fixed in their genome. As vascular plants, these species conserve the proton ATPase as the primary pump to energize plasma membrane. Using electrophysiology, mainly ion-selective microelectrodes, we have shown that the proton economy is used to maintain cytosolic sodium homeostasis and to drive the direct uptake of bicarbonate in Posidonia oceanica, a Mediterranean seagrass endemism. However, as we also reported for Zostera marina, sodium dependent mechanisms are used for the high-affinity uptake of nitrate or Pi. In the context of climate change, seagrasses face two major consequences; the rise of dissolved inorganic carbon species (CO2 and bicarbonate) and seawater acidification. Most seagrasses show non-saturated C3 photosynthesis and are able to use bicarbonate as an inorganic carbon source. In the case of P. oceanica, bicarbonate use has an important side effect on cytosolic chloride, probably by the opening the S-type anion channels, which also mediate nitrate efflux. Consequently, bicarbonate enrichment of natural seawater also evokes the on-going decrease of cytosolic nitrate in P. oceanica mesophyll leaf cells. Thus, the chronic diminution of cytosolic nitrate could impair nitrogen assimilation and would contribute to the N biomass dilution expected under elevated inorganic carbon.