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.