Posidonia oceanica (L.) Delile is a seagrass, the only group of vascular plants to colonize the marine environment. Seawater is an extreme yet stable environment characterized by high salinity, alkaline pH and low availability of essential nutrients, such as nitrate and phosphate. Classical depletion experiments, membrane potential and cytosolic sodium measurements were used to characterize the high-affinity NO3−, Pi and amino acids uptake mechanisms in this species. Net uptake rates of both NO3− and Pi were reduced by more than 70% in the absence of Na+. Micromolar concentrations of NO3− depolarized mesophyll leaf cells plasma membrane. Depolarizations showed saturation kinetics (Km = 8.7 ± 1 μM NO3−), which were not observed in the absence of Na+. NO3− induced depolarizations at increasing Na+ also showed saturation kinetics (Km = 7.2 ± 2 mM Na+). Cytosolic Na+ measured in P. oceanica leaf cells (17 ± 2 mM Na+) increased by 0.4 ± 0.2 mM Na+ upon the addition of 100 μM NO3−. Na+-dependence was also observed for high-affinity l-ala and l-cys uptake and high-affinity Pi transport. All together, these results strongly suggest that NO3−, amino acids and Pi uptake in P. oceanica leaf cells are mediated by high-affinity Na+-dependent transport systems. This mechanism seems to be a key step in the process of adaptation of seagrasses to the marine environment.

The concentration of CO 2 in the atmosphere has increased over the past 200 years and is expected to continue rising in the next 50 years at a rate of 3 ppm•year −1. This increase has led to a decrease in seawater pH that has changed inorganic carbon chemical speciation, increasing the dissolved HCO − 3. Posidonia oceanica is a marine angiosperm that uses HCO − 3 as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of HCO − 3 is the diminution of cytosolic Cl − (Cl − c) in mesophyll leaf cells due to the efflux through anion channels and, probably, to intracellular compartmentalization. Since anion channels are also permeable to NO − 3 we hypothesize that high HCO − 3 , or even CO 2 , would also promote a decrease of cytosolic NO − 3 (NO − 3 c). In this work we have used NO − 3-and Cl −-selective microelectrodes for the continuous monitoring of the cytosolic concentration of both anions in P. oceanica leaf cells. Under light conditions, mesophyll leaf cells showed a NO − 3 c of 5.7 ± 0.2 mM, which rose up to 7.2 ± 0.6 mM after 30 min in the dark. The enrichment of natural seawater (NSW) with 3 mM NaHCO 3 caused both a NO − 3 c decrease of 1 ± 0.04 mM and a Cl − c decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO 2 also produced a diminution of the NO − 3 c, but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon (HCO − 3 or CO 2) in NSW would have an effect on the cytosolic anion homeostasis mechanisms in P. oceanica leaf cells. In the presence of 0.1 mM ethoxyzolamide, the plasma membrane-permeable carbonic anhydrase inhibitor, the CO 2-induced cytosolic NO − 3 diminution was much lower (0.1 ± 0.08 mM), pointing to HCO − 3 as the inorganic carbon species that causes the cytosolic NO − 3 leak. The incubation of P. oceanica leaf pieces in 3 mM HCO − 3-enriched NSW triggered a shortterm external NO − 3 net concentration increase consistent with the NO − 3 c leak. As a consequence, the cytosolic NO − 3 diminution induced in high inorganic carbon could result in both the decrease of metabolic N flux and the concomitant biomass N impoverishment in P. oceanica and, probably, in other aquatic plants.